ScienceWeek

SCIENCE-WEEK - April 12, 2002 - Vol. 6 Number 15

An Email Research Digest Published Weekly Since 1997

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A hypothesis or theory is clear, decisive, and positive,
but it is believed by no one but the man who created it.
Experimental findings, on the other hand, are messy,
inexact things, which are believed by everyone except
the man who did that work.

-- Harlow Shapley (1885-1972)

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Section 1

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Contents of this Issue (Full reports in Section 2):

[(*) = includes background reports] 

Basic Sciences:

1. On Climate Modeling (*)

2. Terrestrial Atmosphere Predictability

3. On Proton Movements in Ice

4. On the Solar Magnetic Cycle (*)

5. Planar vs. Curved Pi-Electron Molecular Interactions

6. Breakdown of Fermi-Liquid Theory in a High-T Superconductor

7. Human Evolution: on Growth Processes in Teeth

8. On Polar Dinosaurs

9. On the Biosynthesis of Estrogens

10. Developmental Biology: On Gut Formation in Animals

11. On Tool-Making by Crows

12. Galen of Pergamum (129-c.216)


Praxis:

13. Bandgap Modulation of Carbon Nanotubes

14. On Nanoscale Surface Stress

15. Micelles: Sodium Dodecyl Sulfate

16. On Fluorescence Detection of DNA-Joining Reactions

17. On the Predictability of Catastrophic Events

18. On Thermoelectric Materials

19. Smoking as a Chronic Disease

20. On Mycotoxins

21. On Spinal Cord Injury (*)

22. Random Graph Models of Social Networks

23. On Terminology in the Cloning Debate

24. A New Class of Potent Antimalarial Compounds


Miscellany:

25. In Focus: On Ciguatoxins

26. SW Archive: On the Search for Extraterrestrial Intelligence

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Section 2

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1. ON CLIMATE MODELING

Leonard A. Smith (London School of Economics, US) discusses
climate modeling, the author making the following points:

1) The traditional approach to climate modeling is to build the
most complicated model that will fit inside the largest computer
available, run it once, and see what happens. This approach
yields a single "best guess" forecast. Yet even in high school
physics, we learn that an answer without "error bars" is no
answer at all. Although it is a nontrivial task to assign
relevant uncertainty estimates to imperfect models of chaotic
systems undergoing transient changes in forcing, doing so is
conceivable. One alternative to devoting all our resources to one
best guess is to use the same computer resource to perform an
ensemble of model runs. This alternative would, of course,
require the use of simpler models, and a balance between running
different initial conditions (to cope with chaos), different
model parameterizations and parameter values (to identify tuning
issues), and different model structures (to mitigate model
error). A single best guess from a complicated model run without
good uncertainty estimates is impotent, whereas a beautiful set
of ensemble statistics on too simple a model is irrelevant. How
do we go about assigning resources between these two extremes?
And how can we tell which physical phenomena of economic and
social interest our current models might be able to forecast?

2) At best, our models hold only in certain circumstances. This
is true even for our "Laws of Physics". In climate forecasting,
to make any progress, we assume the "rosy scenario" holds: a)
nothing horrible happens that takes the model beyond its range of
validity (e.g., no asteroid collides with the Earth); and b) no
small but crucial feedback mechanism is missing from our model
(i.e., our model has a range of validity). As we are forced to
assume the rosy scenario, we can never make objective probability
statements on the basis of our climate simulations. What we can
do is establish their internal consistency: we can determine for
which phenomena and on which time scales our models might reflect
reality.

Proc. Nat. Acad. Sci. 2002 99:2487

Related Background:

ON GLOBAL CLIMATE MODELS

M. Stute et al (Barnard College, US) discuss global climate
models, the authors making the following points:

1) Global climate is a result of the complex interactions
between the atmosphere, cryosphere (ice), hydrosphere (oceans),
lithosphere (land), and biosphere (life), fueled by the
nonuniform spatial distribution of incoming solar radiation. We
know from climate reconstructions using recorders such as ice
cores, ocean and lake sediment cores, tree rings, corals, cave
deposits, and ground water that the Earth's climate has seen
major changes over its history.

2) An analysis of the temperature variations patched
together from all these data reveals that climate change occurs
in cycles with characteristic periods, for example, 200 million,
100,000, or 4 to 7 years. For some of these cycles, particular
mechanisms have been identified, for example, climate forcing by
changes in the Earth's orbital parameters or internal
oscillations of the coupled ocean-atmosphere system. However,
major uncertainties remain in our understanding of the interplay
of the components of the climate system.

3) Paleoclimate reconstructions, in particular from ice
cores, also have demonstrated that climate can change over
extremely short periods of time such as a few years. Over the
last century, humans have altered the Earth's surface and the
composition of its atmosphere to the extent that these factors
measurably affect current climate conditions, and there is
concern that perhaps during one human generation we will
gradually change climate conditions or even trigger a rapid and
much more dramatic shift: we might be "poking an angry beast".

4) Major progress in our understanding of climate processes
in the past, present, and future has been made by the development
of numerical models that simulate climate at an increasing level
of detail. Recent breakthroughs in spatial coverage and temporal
resolutions of systems recording today's climate, and high-
resolution reconstruction of past climate conditions from diverse
sources using new past-climate indicators (proxies), make it
possible to validate climate models and thus improve their
reliability for future predictions.

Proc. Nat. Acad. Sci. 2001 98:10529

Related Background:

ICE-CORE EVIDENCE OF ABRUPT CLIMATE CHANGES

Records of changes in Earth's climate are particularly clear in
high-resolution ice cores, which can preserve histories of local
climate (as reflected in snowfall and temperature), regional
climate (as reflected in wind-blown dust, sea salt, etc.), and
broader climate (as reflected in trace gases deposited from the
atmosphere) -- all on a common time scale that can demonstrate
synchrony of climate changes over wide regions.

... ... Richard B. Alley (Pennsylvania State University, US)
reviews current ice-core research, the author making the
following points:

1) Dating and accumulation: On some glaciers and ice sheets,
sufficient snow falls each year to form recognizable annual
layers that are marked by seasonal variations in physical,
chemical, electrical, and isotopic properties. These variations
can be counted to determine ages of the layers, and accuracy of
the determination can be assessed by a number of ways, including
comparison to the chemically identified fallout of historically
dated volcanoes.

2) Paleothermometry: Ice cores are essentially local
paleothermometers. The classic paleothermometer is the stable
isotopic composition of water in the ice core. Natural waters
typically contain a fraction of 1 percent of isotopically heavy
water molecules, and the vapor pressure of this heavy water is
less than ordinary or "light" water. The result is that as an air
mass is cooled and precipitates, it preferentially loses heavy
water and must increasingly precipitate light water. Both
empirically and theoretically, isotopic composition of
precipitation and site temperature are strongly correlated in
time and space.

3) Aerosols: Anything in the atmosphere can eventually end
up in an ice core. Some materials are reversibly deposited, but
most materials remain in the ice unchanged. Although details of
the air-snow transfer process are complex and not yet completely
elucidated, large changes in concentrations of most materials in
ice can with confidence be said to reflect changes in the
atmospheric loading of these materials.

4) Gases: Trapped gases in ice-core bubbles are highly
reliable records of atmospheric composition, as indicated by
comparisons among cores from different ice sheets, and comparison
with instrumental records and the air in the *firn above the
bubble-trapping depth. The slight differences between bubble and
air composition caused by gravitational and thermal effects are
well understood and recognizable.

5) Geographic coverage: The ice-core record of abrupt
climate changes is clearest in Greenland. Although no other
record is available that spans the same time interval with
equally high time resolution, it appears that ice cores from the
Canadian arctic islands, high mountains in South America, and
Antarctica also contain indications of the same abrupt changes.
Dating is considered secure for some of the Antarctic ice cores.

6) The author suggests that as the world slid in and out of
the last ice age, the general cooling and warming trends were
punctuated by abrupt changes, and climate shifts up to half as
large as the entire difference between ice age and modern
conditions occurred over hemispheric or broader regions in mere
years to decades. Such abrupt changes have been absent during the
few key millennia when agriculture and industry have arisen.

7) In summary, ice-core records indicate that climate
changes in the past have been large, rapid, and synchronous over
broad areas extending into low latitudes, with less variability
over historical times. These ice-core records come from high
mountain glaciers and the polar regions, including small ice caps
and the large ice sheets of Greenland and Antarctica.

Proc. Nat. Acad. Sci. 2000 97:1331

Notes:

*firn: The term "firn" refers to the transitional layer
between snow and glacier ice. The layer consists of snow that has
melted during one summer melt season, the layer in the process of
transforming to glacier ice as the temperature decreases.

Related Background:

ON GLOBAL CLIMATE CHANGE

Environmental change involves jumps, fluctuations, and trends,
the environment changing through the operation of the internal
machinery of the *ecosphere (biosphere), and through the external
agencies of cosmic and geological forces. Evidence of past
environmental change, almost always incomplete, derives from
geochemical, physical, biological, historical, and instrumental
sources. In recent years, high-speed computers have allowed
researchers to manipulate complicated and reasonably realistic
models of environmental change, with modelling particularly
useful for studying changes in *sedimentary basins,
biogeochemical cycles, and climate. General circulation models,
run with appropriate boundary conditions, predict climates of the
past, and these predicted climates can be compared with
paleoclimatic indicators.

... ... R.B. Alley et al (3 authors 3 installations, US) present
a review of current research on global climate change, the
authors making the following points:

1) Prediction of climate change requires observational
constraints on the current climate state, knowledge of the way
the coupled air-ocean-ice-earth-life system behaves, and
information on changing forcings such as solar variability.
Studies of past climate are also required to focus model-building
efforts on climate components that are likely to change, and to
allow testing of the ability of models to predict time-evolution
of the system.

2) The last few million years have been generally cold and
icy compared with the previous hundred million years but have
alternated between warmer and colder conditions. These
alternations have been linked to changes over tens of thousands
of years in the seasonal and latitudinal distribution of sunlight
on Earth caused by features of Earth's orbit. Globally
synchronous climate change despite some hemispheric asynchrony of
the forcing is explained at least in part by lowering carbon
dioxide during colder times in response to changes in ocean
chemistry. We live in one of the warmer times of these orbital
cycles; the coolest times brought glaciation to nearly one-third
of the modern land area.

3) Studies of past climate changes indicate that the Earth
system has experienced greater and more rapid changes over larger
areas that was generally believed possible, with jumping between
fundamentally different modes of operation in as little as a few
years. Most of the last 100,000 years or longer has been
characterized by large and abrupt regional-to-global climate
changes, and agriculture and industry have developed during
anomalously stable climatic conditions. New high-resolution
analysis of sediment cores indicates these past changes have been
caused by "*band jumps" between modes of operation of the climate
system. Recurrence of such band jumps is possible and might be
affected by human activities.

Proc. Nat. Acad. Sci. 1999 96:9987

Notes:

*ecosphere (biosphere): In general, the term "biosphere"
refers to the portion of the planet capable of supporting life.
It ranges from elevations of approximately 10,000 meters above
sea level to the deep ocean, and a few hundred meters below the
surface of the soil. The biosphere consists of the hydrosphere,
the lower atmosphere (troposphere), and the surface of the
*lithosphere, all three regions inhabited by metabolically active
organisms.

*lithosphere: In current geology, the lithosphere is the
approximately 100 kilometer rigid upper layer of the crust and
upper mantle of the Earth.

*sedimentary basins: The term "sedimentary basin" refers
to a subsiding area of the Earth's crust, which permits the net
accumulation of sediment, i.e., material derived from pre-
existing rock, from biogenic sources, or precipitated by chemical
processes.

*band jumps: In this context, the term "band jump" refers
to an abrupt change from one range of variation to another.

Related Background:

ON THE POSSIBILITY OF RAPID CLIMATE CHANGE

Over the course of geologic history, the environment on Earth has
been far from static. Geologic evidence suggests that 600 million
years ago the atmosphere lacked sufficient oxygen to support
animal life. More recently, as indicated by sediments recording
conditions over the past 500,000 years, the climate of the planet
varied between at least two different states. The record from the
past 150,000 years is particularly well-preserved, offering
details concerning repeated climate changes. Between
approximately 131,000 and 114,000 years ago, a warm period
similar to the climate of today occurred. This was followed by
what is called the "Wisconsin ice age", which ended approximately
12,000 years ago when the current relatively warm *Holocene
period began.

... ... Kendrick Taylor (Desert Research Institute,
US) presents a review of the research of a large project to
develop a climate record for the past 110,000 years, the author
making the following points:

1) The layerings of glacial ice record seasonal variations
of temperature, snowfall, concentrations of atmospheric gases,
and atmospheric circulation patterns. In general, the weight of
accumulating snow compresses the snow below it, trapping
atmospheric gases, dust, and chemicals, and a deep ice core thus
provides a sequential record amenable to analysis.

2) The author reports that by examining ice cores from
Greenland, he and his colleagues have determined that climate
changes large enough to have extensive impacts on our society
have occurred in a time-frame of less than 10 years. The author
suggests that the climate of Earth could change significantly
during a lifetime, that we are still a long way from being able
to predict such a change, but we are getting closer to an
understanding of how it might occur. A pressing concern is
whether anthropogenic changes in the atmosphere of the planet
might perturb climate stability.

3) The author points out that climate is the result of the
exchange of heat and mass between the land, ocean, atmosphere,
ice sheets, and space. As long as changes to the land, ocean,
atmosphere, and ice sheets stay below certain thresholds, climate
changes will occur slowly. But climate will change rapidly if
those thresholds are crossed. *Greenhouse warming, for example,
by altering ocean circulation and the flow of tropical heat to
the North Atlantic, could lead to rapid cooling in eastern North
America, Europe and Scandinavia. Altered ocean circulation could
lead to much larger changes. We have no experience predicting
climate switches between stable modes.

4) The author suggests human ingenuity would most likely
allow us to adapt to a rapid change in climate, but we would pay
a larger price than our civilization has ever known. The author
poses a scenario: "Imagine the economic and social cost of
moving, in a 20-year period, most of our agricultural activities
500 miles south of their current locations. Imagine the social
cost and famine if agriculture could not be relocated quickly
enough."

5) Although we do not know the critical level of greenhouse
gas concentration that would trigger a rapid climate change, we
do know that reducing the rate of greenhouse emissions would help
in two ways. First, the atmospheric concentration of greenhouse
gases would increase more slowly. Second, numerical models
predict that the climate threshold will occur at a higher
concentration of greenhouse gases if the concentration of
greenhouses increases slowly.

6) The author suggests it will be another 20 years before
the climate changes that are predicted to be associated with the
greenhouse effect becomes large enough to be unambiguously
differentiated from naturally occurring variations in climate.
As a society we have the choice of ignoring the warning signs or
taking some action.

American Scientist 1999 87:320


Notes:

*Holocene period: The most recent epoch of the geologic
time scale, from approximately 10,000 years ago to the present.

*Greenhouse warming: The physical basis of the so-called
"greenhouse effect" is essentially simple: carbon dioxide gas is
transparent to visible light but relatively opaque to infrared
radiation. The same is true of glass. Relatively high-energy
visible light radiation from the sun passes inward through the
atmosphere, warms the surface of the Earth, which then radiates
lower energy in the form of infrared radiation (heat) back to the
atmosphere. But if the atmosphere has a concentration of infrared
impenetrable gases such as carbon dioxide, the infrared radiation
cannot pass out, and the surface of the Earth underlying the
atmosphere cannot cool, and the surface of the Earth thus will
continue to grow hotter.

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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2. TERRESTRIAL ATMOSPHERE PREDICTABILITY

M. Ghil and A.W. Robertson (University of California Los Angeles,
US) discuss atmosphere predictability, the authors making the
following points.

1) The atmosphere is one of the most complex physical systems
known to humanity. In fact, we have approximately 10^(5)
observations of the atmosphere every day, which makes it probably
the best-observed macroscopic physical system there is. Despite
-- or because of? -- this detailed knowledge, our ability to
predict even large-scale atmospheric motions, as seen on a global
weather map or a hemispheric satellite picture, is limited to a
few days. The purpose of studying low-frequency, or
intraseasonal, atmospheric variability is to find out which
features of this variability are predictable for longer time
spans, of weeks to months.

2) Thirty years ago, E.N. Lorenz provided some approximate limits
to atmospheric predictability. The details -- in space and time
-- of atmospheric flow fields are lost after approximately 10
days. Certain gross flow features recur, however, after times of
the order of 10 to 50 days, giving hope for their prediction.
Over the last two decades, numerous attempts have been made to
predict these recurrent features. The attempts have involved, on
the one hand, systematic improvements in numerical weather
prediction by increasing the spatial resolution and physical
faithfulness in the detailed models used for this prediction. On
the other hand, theoretical attempts motivated by the same goal 
have involved the study of the large-scale atmospheric motions
phase space and the inhomogeneities therein.

Proc. Nat. Acad. Sci. 2001 99:2493

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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3. ON PROTON MOVEMENTS IN ICE

E. Katoh et al (National Institute of Advanced Industrial Science
and Technology, JP) discuss proton movements in ice, the authors
making the following points:

1) Protons can move in the hydrogen-bonded network of water
molecules by transferring within a hydrogen bond and then jumping
into another hydrogen bond by molecular rotation. Although this
diffusion model specific to ice was proposed in the middle of the
20th century, its process has eluded experimental investigation.
The protonic diffusion coefficient estimated for ambient-pressure
ice at 263 kelvins is on the order of 10^(-20) square meters per
second, which is 4 to 5 orders of magnitude less than the
molecular diffusion coefficient, and the fast molecular diffusion
interferes with observation of the slow protonic diffusion.
Dielectric properties and electrical conductivity, which can be
related to protonic diffusion coefficients, have been measured
for pure and doped ices instead.

2) Theoretical studies have consistently predicted the presence
of a superionic (or superprotonic) phase characterized by a fast
protonic diffusion with a coefficient of approximately 10^(-8)
square meters per second at extremely high temperatures and
pressures. The superionic state is predicted to appear in the
phase diagram at approximately 1000 kelvins and 20 gigapascals
(GPa) and to develop to higher temperatures ranging from 2000 to
4000 kelvins above 100 GPa. In the superionic phase, the protons
are believed to move quickly by jumping successively between
their neighboring occupation sites in a crystal lattice
consisting solely of oxygen atoms. Such a superionic phase can be
characterized as a partially melted state and can be compared
with an ionic fluid or a fully melted state in which neutral or
ionized water molecules diffuse freely. The superionic phase of
ice may play a crucial role in the generation of magnetic fields
in giant planets.

Science 2002 295:1264

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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4. ON THE SOLAR MAGNETIC CYCLE

Y. Asaoka et al (University of Tokyo, JP) discuss solar physics,
the authors making the following points:

1) Much of the real underlying physics of the Sun is related to
the 22 year solar magnetic cycle with recurrent positive and
negative phases. The magnetic field polarity reverses when the
solar activity is maximum, the global magnetic field profile
reversing in the heliosphere. The most recent field reversal
occurred in the beginning of the year 2000. The solar modulation
of cosmic rays is caused by an expanding solar wind, which
spreads out the locally irregular magnetic field and therefore
modifies the energy spectra of the cosmic rays entering the
heliosphere. The positive and negative particles drift in
opposite directions, taking different routes to arrive at the
Earth in the heliospheric magnetic field. The charge-sign
dependence is therefore a natural consequence on top of the
common time dependent change in the overall modulation. Thus is
explained alternate appearances of "flat" and "peaked" periods in
neutron monitor data around solar minima.

2) Recent work also indicates that the drift produces non-
negligible effects between the positive and negative particles
even during high solar activity. This view is supported by
measurements of temporal variation in cosmic-ray ratios, such as
electrons to helium nuclei, and electrons to protons, where the
largest variation is associated with the reversal of the solar
magnetic field. Among various cosmic ray pairs, antiprotons and
protons are ideal for understanding drift effects related to the
change in overall modulation level because they differ only in
charge sign.

Phys. Rev. Lett. 2002 88:051101

Related Background:

ASTRONOMY: ON THE SUNSPOT CYCLE

The bright surface layer of the Sun is called the
"photosphere", a region a few hundred kilometers thick at a
temperature that ranges from 5770 kelvins at its innermost part
to 4400 kelvins at its outermost part, the latter the Sun's
temperature minimum. The term "sunspot" refers to a dark area on
the photosphere that is cooler than its surroundings and
associated with strong magnetic fields (on the order of 0.4
tesla). Sunspots generally appear in pairs or groups, the leading
and following spots with opposite magnetic polarities. Sunspot
sizes vary from small pores approximately 300 kilometers in
diameter to groups of sunspots spanning more than 100,000
kilometers. The largest sunspots usually last the longest, up to
6 months; small spots may last for less than an hour. For the
most part, sunspots are confined to belts above and below the
solar equator.

Since the Sun is not a solid, different parts at the surface
rotate at different rates. The term "solar dynamo" refers to the
action within the Sun whereby the kinetic energy of the hot and
highly ionized gas of the solar interior is converted into the
magnetic field that gives rise to solar activity. The consensus
model, due to H.W. Babcock, is that magnetic field lines under
the photosphere run from pole to pole (the "poloidal field") and
are twisted parallel to the solar equator (the "toroidal field")
by the differential rotation of polar and equatorial regions

The so-called "sunspot cycle" (solar cycle) is a variation
in the number of sunspots and other forms of solar activity with
an average period of approximately 11 years. In each successive
cycle, the north and south magnetic polarities of the Sun are
reversed, producing a 22-year magnetic cycle. The 11-year
periodicity of the sunspot cycle is believed to arise through the
action of the solar dynamo.

... ... Douglas Gough (University of Cambridge, UK) presents a
commentary on current research on the sunspot cycle, the author
making the following points:

1) The author points out that minor physical changes in the
Sun often lead to extreme solar magnetic activity that can affect
the Earth, e.g., by disrupting radio communications and
influencing the weather. Although the sunspot cycle is of
considerable interest, we are far from understanding its origin
and dynamics.

2) The author points out that we are currently experiencing
a peak in the solar cycle and therefore in the number of
sunspots. It is generally believed that the underlying cause of
the sunspot cycle is the interaction between the rotation of the
Sun and the "dynamo" responsible for the Sun's magnetism. In this
model, the dynamo effect creates a magnetic field from the
electric currents caused by convection and large-scale shearing
motions within the Sun. But the outer regions of the Sun tend to
rotate faster near the equator and slower near the poles, and
this results in strong magnetic fields bursting through the
photosphere. These magnetic fields inhibit the convective
transport of heat, permitting material to cool at the surface,
and producing the visibly darker regions called "sunspots".

3) The author points out that none of the current dynamo
models explain the small observed variation in the luminosity of
the Sun that also follows the sunspot cycle. These small changes
in luminosity (no larger than 0.1 percent) apparently derive from
the release of stored energy somewhere within the Sun. Thus, by
studying the small changes in the radius of the Sun, we might
learn something about the source of this extra energy, and also
ultimately learn something about the process that causes the
luminosity change.

4) Recent work (M. Emilio et al: Astrophys. J. 543:1007
2000) based on sensitive satellite observations demonstrates
evidence that the energy responsible for variations in the Sun's
radius and luminosity does not come from the inner depths of the
Sun but rather from the outer layers. The author (Gough) states:
"This observation is certainly not the first claimed detection of
a small variation in the Sun's radius, but it may be the first to
survive the test of time."

Nature 2001 410:313

Related Background:

PRECISION MEASUREMENTS OF BRIGHT RINGS AROUND SUNSPOTS

A "sunspot" is a dark area of the solar surface. The center of
the spot, called the "umbra", is darker than the outer border,
which is called the "penumbra". The average sunspot is
approximately twice the diameter of the Earth and may last for
several weeks. Sunspots tend to form in pairs or groups, and a
large group may contain up to 100 spots and may last as long as 2
months. Sunspots appear dark because they are cooler than the
photosphere (the visible surface of the Sun or a star). The
temperature at the center of a typical sunspot is approximately
4240 kelvins, while the solar photosphere is at approximately
6000 kelvins. Temperatures of the order of 4000 kelvins, however,
are significant: a sunspot emits enough radiation so that a
single sunspot on its own in the absence of the remainder of the
Sun would glow a brilliant orange-red and would be brighter that
the full Moon. Analysis of the *Zeeman effect in sunspots
indicates that the magnetic field in a typical sunspot is
approximately 1000 times stronger than the average magnetic field
of the Sun, and one theory is that this powerful localized
magnetic field inhibits gas motion below the photosphere, with
the result that rising gas cannot deliver its heat to the
surface. Thus, the area cools and a sunspot is the result.
Infrared observations of sunspots have suggested that the heat
that does not emerge through the sunspot is deflected and
produces a slight increase in the temperature of the photosphere
around the sunspot, but so far these measurements have not been
precise and the slight increase has not been confirmed. The other
major theory of sunspots proposes that the removal of energy from
the sunspot location is the result of enhanced hydromagnetic wave
radiation associated with so-called "*plage fields". Of the two
theories, the first theory is currently favored.

... ... M.P. Rast et al (6 authors at 2 installations, US) now
report high-photometric-precision observations of bright rings
around 8 sunspots. The authors report the rings are approximately
10 kelvins warmer than the surrounding photosphere and
extend at least one sunspot radius out from the penumbra.
Approximately 10 percent of the radiative energy missing from the
sunspots is apparently emitted through these bright rings. The
authors conclude: "Thus, isolated sunspots are seen to be
commonly surrounded by a ring of enhanced radiation, the origin
of which is probably not bright vertical magnetic elements (plage
field), but the re-emergence of heat blocked by magnetic
inhibition of convective transport in the spot itself."

Nature 1999 401:678

Notes:

*Zeeman effect: (Zeeman splitting) The splitting of a
spectral line due to a magnetic field. Named after Peter Zeeman
(1865-1943). The effect is widely used for the determination of
magnetic fields in astronomical objects, especially concerning
the Sun and sunspots. In general, the Zeeman effect occurs when
atoms emit or absorb radiation in the presence of a magnetic
field: the field modifies the energy configuration of the atom
with the result that a spectral line is split into 2, 3, or more
closely spaced components. The spacing of the components is a
measure of the magnetic field strength.

*plage fields: A "plage" is a brighter, hotter patch in
the *chromosphere of the Sun, and a region of particularly strong
magnetic field.

*chromosphere: The region of the Sun's atmosphere
directly above its photosphere. Visible only immediately before
or after a total solar eclipse.

Related Background:

ASTROPHYSICS: THE PHYSICS OF THE SUN AND TERRESTRIAL CLIMATE

The Sun, a *main-sequence star 1.4 million kilometers in
diameter, is composed predominantly of hydrogen and helium
(approximately 70 percent hydrogen by mass, 28 percent helium by
mass, and 2 percent heavier elements by mass) and it generates
its energy via nuclear fusion processes, particularly via the
*proton-proton chain reaction. As a result, the Sun is losing
mass at a rate of approximately 4 million metric tons per second.

The generation of energy occurs in the "central core", which
has a temperature of approximately 15 million kelvins, is
approximately 400,000 kilometers in diameter, and contains
approximately 60 percent of the mass of the Sun in 2 percent of
its volume.

Outside the core is the "radiative zone", an envelope of
unevolved material through which energy from the core is
diffusively transported by successive absorption and emission of
radiation in collisions between atomic particles. It has been
estimated that it may take from 1 million years to as long as 10
to 20 million years for the energy generated in the core to reach
the surface.

The radiative zone extends to within 200,000 kilometers of
the surface. In the surface layer (the "convective zone"), where
the temperature is only 1 million kelvins, convection is
the most important mode of energy transport.

... ... Eugene N. Parker (University of Chicago, US) presents a
review of the physics of the Sun, the author making the following
points:

1) The Sun is essentially a thermonuclear core enclosed in
an opaque shroud that insulates the high temperature of the core
from the cold Universe outside. The core is brighter than 10
supernovas at maximum light, but the enclosing shroud turns back
all but one part in 2 x 10^(11) of the thermal radiation. The
outward journey of the energy from the core takes approximately 1
million years, which illustrates the immense opacity and thermal
capacity of the shroud.

2) Approximately 10^(-5) of the outflowing energy from the
core of the Sun is diverted into magnetic fields that produce a
variety of exotic effects, including *coronal mass ejection,
*solar flares, the million degree corona, the *solar wind, and x-
ray emission. These phenomena are of interest to the physicist
because they represent unanticipated manifestations of classical
physics, extrapolations to astronomical scales of basic
principles traditionally studied in terrestrial laboratories.

3) The total luminosity of the Sun varies with time, and
systematic monitoring of several Sun-type stars during the past 4
decades reveals magnetic activity cycles comparable to that of
the Sun. The luminosities of some of those stars have been
monitored for approximately 15 years, and the data show
approximately the same variation as the magnetic activity.

4) The Earth contains a great deal of information about past
solar activity. The rate of production of carbon-14 depends
directly on the intensity of *cosmic rays, and such rays are
partially excluded from the Solar System by the outward sweep of
magnetic fields in the solar wind. Thus the cosmic ray intensity
and carbon-14 production vary oppositely to the general level of
solar activity.

5) The carbon-14 record indicates that over the last 70
centuries the Sun has been without normal activity for 10
centuries and hyperactive for 8 centuries. The other 52 centuries
were variable but more or less normal. The most recent quiescent
period was from 1645 to 1715, the period called the "Maunder
Minimum". The 12th century "Medieval Maximum" is the most recent
epoch of hyperactivity. The empirical relation between the total
luminosity and magnetic activity, based on many Sun-type stars,
suggests that the Sun was fainter during the *Maunder Minimum by
0.4 +- 0.2 percent, and perhaps brighter by a comparable amount
during the Medieval Maximum. The mean annual temperature in the
northern temperate zone was lower than normal by 1 to 2 degrees
centigrade during the Maunder Minimum and higher by 1 to 2
degrees centigrade during the Medieval Maximum. The fractional
change in temperature is comparable to the fractional change in
solar brightness, with the implication that the Sun is the driver
of the climate. The consequences for agriculture were severe
during both periods, the Maunder Minimum being disastrous in
northern Europe and China, and the Medieval Maximum disastrous in
the semi-arid regions. These periods of abnormal activity of the
Sun are without explanation, as are the variations within the so-
called "normal centuries".

6) The general level of solar activity doubled or tripled
from 1900 to 1950, an estimate based on sunspot numbers and on
the intensity of geomagnetic activity. This increase suggests an
increase in solar luminosity by perhaps one part in 2000, and the
author suggests it is interesting to note that the mean
temperature in the northern temperate zone, as well as the
surface sea water temperatures, rose during the same period.
"Warmer seas, of course, reduce the rate at which atmospheric
carbon dioxide is absorbed into the oceans. It appears that the
global warming since 1950 is in part a consequence of the
continuing increase in solar brightness, seriously aggravated by
the extravagant burning of fossil fuel. So the mystery of the
variations in the total luminosity of the Sun is part of the
complicated picture of global warming."
[Editor's note: See report #3 in this issue for another approach
to millennial-scale climate changes.]

Physics Today June 2000

Notes:

*main-sequence star: The Main Sequence is a region on the
*Hertzsprung-Russell diagram where most stars lie, including our
own Sun. The evolution of a star can be diagrammed as a movement
along the Main Sequence and an eventual branching off the Main
Sequence to regions associated with various types of old stars.

*Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type, and is perhaps the most useful diagrammatic aid in
astrophysics. It allows the portrayal of the evolution of a star
as occurring along various paths in the diagram.

*proton-proton chain reaction: A chain of nuclear
reactions inside a star that converts hydrogen to helium, with
the associated release of energy. In the reaction, 4 hydrogen
nuclei (protons) fuse to form one nucleus of helium, with the
production of a number of intermediate nuclei such as deuterium
and isotopes of lithium, beryllium, and boron. The proton-proton
reaction is the most important stellar reaction at temperatures
below 18 million kelvins, and thus operates chiefly in
stars of less than 2 solar masses.

*coronal mass ejection: The corona is the Sun's faint
outer atmosphere, where the temperature is 2 million degrees
kelvin or more, the corona consisting of a low-density hot gas
that glows with a pale white color.

*solar flares: A solar flare is a sudden release of
energy in the corona of the Sun, the phenomenon usually lasting
up to several hours (in rare cases, up to more than a day).

*solar wind: The solar wind is the steady flow of charged
particles, consisting primarily of protons and electrons, from
the solar corona into interplanetary space. The solar-wind
particles have energies high enough to enable the particles to
escape the Sun's gravitational field, but the wind is influenced
by the Sun's magnetic field, and the particles can be trapped by
planetary magnetic fields.

*cosmic rays: Highly energetic particles moving at close
to the speed of light and continuously bombarding the Earth's
atmosphere from all directions. The energies of the particles are
enormous and range from 10^(8) to over 10^(19) electronvolts.

*Maunder Minimum: Named after the astronomer Edward W.
Maunder (1851-1928), who first noted the absence of reports of
sunspots in the period 1645 to 1715.

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5. ON PLANAR VS. CURVED PI-ELECTRON MOLECULAR INTERACTIONS

S. Mizyed et al (Memorial University of New Foundland, CA)
discuss pi-electron interactions, the authors making the
following points:

1) Organic chemists learned long ago that large planar polycyclic
aromatic compounds with electron-rich faces, like those of
benzene, form strong complexes with picric acid and other planar
partners that proffer electron-deficient pi-electron faces. On
the other hand, the geodesic polyarenes with curved pi-electron
faces that have recently become available are geometrically ill-
disposed to complex with planar partners. Indeed, corannulene
forms only a weak complex with picric acid and with other planar
electron-deficient pi-systems.

2) For optimal face-to-face contact, the partner for a geodesic
polyarene such as corannulene should have a geometrically
complementary curved surface. in addition, to provide a strong
intermolecular attraction, the interacting faces of the two
partners should also be electronically complementary: i.e., if
one surface is electron rich, the other surface should be
electron deficient, just as in the case for two planar partners.

3) For planar aromatic compounds such as benzene, the two pi-
electron faces necessarily exhibit identical electronic character
as a consequence of molecular symmetry. For geodesic polyarenes
such as corannulene, however, the electronic character of the
concave and convex pi-electron faces must be different. In
principle, they cannot be the same: one pi-electron face must be
more electron-rich than the other, although which face is which
may not appear intuitively obvious. Even theoretical calculations
give different answers to this question, depending on the level
of theory employed. Semiempirical calculations predict a more
negative electrostatic potential on the convex pi-electron
surface than on the concave pi-electron surface of corannulene
and of other geodesic polyarenes, whereas higher level density
functional calculations predict precisely the opposite
properties.

4) The authors report the first examples of complexation between
derivatives of a geodesic polyarene and spherical C-60 fullerene.

J. Am. Chem. Soc. 2001 123:12770

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6. BREAKDOWN OF FERMI-LIQUID THEORY IN A HIGH-T SUPERCONDUCTOR

R.W. Hill et al (University of Toronto, CA) discuss Fermi-liquid
theory, the authors making the following points:

1) Landau's Fermi-liquid theory is the definitive theory of
electrons in metals, or more generally of fermions in condensed
matter, and the theory is a major landmark of 20th century
physics. For example, this theory is the necessary foundation for
the theory of superconductivity formulated by Bardeen, Cooper,
and Schrieffer. In essence, the theory says that even in the
presence of interactions, the low-energy excitations of a system
of mobile fermions can still be described in terms of well-
defined fermionic particles, called "quasiparticles", with charge
(e), spin 1/2, and a mass (m*), the latter being renormalized by
interactions. The Weidmann-Franz law is one of the basic
properties of a Fermi liquid, reflecting the fact that the
ability of a quasiparticle to transport heat is the same as its
ability to transport charge, provided it cannot lose energy
through collisions. Reported as an empirical observation by
Wiedmann and Franz in 1853, the law states that the heat
conductivity and the electrical conductivity of a metal are
related by a universal constant.

2) High-temperature superconductors have long been thought to
fall outside the realm of Fermi-liquid theory, as suggested by
several anomalous properties, but this has yet to be demonstrated
conclusively. The authors report an experimental test of the
Wiedmann-Franz law in the normal state of a copper-oxide
superconductor, the results revealing that the elementary
excitations that carry heat in this material are not fermions.
The authors suggest this is compelling evidence for the breakdown
of Fermi-liquid theory in high-temperature superconductors.

Nature 2001 414:711

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7. HUMAN EVOLUTION: ON GROWTH PROCESSES IN TEETH

C. Dean et al (University College London, UK) discuss human
evolution, the authors making the following points:

1) A modern human-like sequence of dental development as a proxy
for the pace of life history is regarded as one of the diagnostic
hallmarks of our own genus Homo. Brain size, age at first
reproduction, lifespan, and other life-history traits correlate
tightly with dental development. Ameloblasts secrete enamel
matrix in a circadian manner, and the resulting daily enamel
increments can be used as a chronometer of tooth growth and
dental development. Occasionally, these increments can be imaged
in fossil hominins on naturally fractured tooth surfaces with
scanning electron microscopy, with confocal microscopy of the
subsurface enamel, or more predictably with polarizing light
microscopy of ground sections.

2) The authors report differences in enamel growth that
demonstrate the earliest fossils attributed to Homo do not
resemble modern humans in their development. The authors used
daily incremental markings in enamel to calculate rates of enamel
formation in 13 fossil hominins, and identified differences in
this key determinant of tooth formation time. Neither
australopiths nor fossils currently attributed to early Homo
shared the slow trajectory of enamel growth typical of modern
humans. Rather, both resembled modern and fossil African apes.
The authors reconstructed tooth formation times in australopiths,
in the approximately 1.5-million-year-old skeleton from
Nariokotome (Kenya), and in another Homo erectus specimen,
Sangiran S&-37 from Java. Apparent tooth formation times in these
specimens were shorter than in modern humans. The authors suggest
it therefore seems likely that truly modern dental development
emerged relatively late in human evolution.

... ... In a commentary on this work, Jacopo Moggi-Cecchi
(University of Firenze, IT) states: "The new results support the
idea that specific morphological characters (such as brain size,
enamel thickness, or bipedalism), as observed or inferred in
fossil specimens, cannot be interpreted in isolation to indicate
affinities with modern humanity."

Nature 2001 414:597,628

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8. ON POLAR DINOSAURS

T.H. Rich et al (Museum Victoria, AU) discuss polar dinosaurs,
the authors making the following points:

1) In 1960, footprints from Spitzbergen demonstrated that non-
avian dinosaurs had once lived at polar latitudes. Initially,
this intriguing find remained an essentially isolated discovery,
but during the past 20 years, much information about polar
dinosaurs has been unearthed. The late development of knowledge
in this field was largely a matter of logistics: The fossil
remains of most polar dinosaurs are to be found today at high
latitudes and often in remote areas. There discovery and
collection are therefore more costly than is generally the case
for comparable lower latitude fossils.

2) Despite these drawbacks, the study of polar dinosaurs provides
potentially unique insights into their physiological adaptations
because they may have been exposed to extreme conditions not
experienced elsewhere. These conditions cannot be assumed to have
been the same as at comparable latitudes today, however.
Establishing just what the climate was like in polar latitudes is
critical for the accurate interpretation of polar dinosaur finds.

3) It has been proposed that the inclination of Earth's
rotational axis may have been substantially different during the
Mesozoic Era (245 million years ago to 65 million years ago) than
it is today, resulting in warmer climates and less variable day
length through the year at high latitudes. In contrast,
theoretical investigations suggest that except for the regular
and well-understood variation by a few degrees that takes place
on the scale of tens of thousands of years, the inclination of
the Earth's axis has remained much the same relative to the plane
of the ecliptic. In the latter case, polar dinosaurs and their
associated biota would have had to contend with the same extremes
of day length through the year that characterize comparable
latitudes today.

Science 2002 295:979

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9. ON THE BIOSYNTHESIS OF ESTROGENS

C.J. Gruber et al (University of Vienna, AT) discuss the
biosynthesis of estrogens, the authors making the following
points:

1) The naturally occurring estrogens 17beta-estradiol, estrone,
and estriol are C(sub18) steroids derived from cholesterol. After
binding to lipoprotein receptors, cholesterol is taken up by
steroidogenic cells, stored, and moved to the intracellular sites
of steroid synthesis. This intracellular movement is facilitated
by the intracellular cytoskeleton and by intracellular carrier
proteins such as the sterol carrier protein-2. Different steroids
are formed by reduction of the number of carbon atoms from 27 to
18. The rate-limiting step in steroid production is the transfer
of cholesterol from the cytosol to the inner membrane of the
mitochondrion, where the cytochrome P450 enzymes that catalyze
the cleavage of the side chain of cholesterol are located. The
steroidogenic acute regulatory protein is an indispensable
component in this transfer process, and mutations of this protein
result in a severe inability to synthesize steroids and are
therefore potentially lethal.

2) Aromatization is the last step in estrogen formation. This
reaction is catalyzed by the P450 aromatase monooxygenase enzyme
complex present in the smooth endoplasmic reticulum and functions
as a dimethylase. In three consecutive hydroxylating reactions,
estrone and estradiol are formed from their obligatory precursors
androstenedione and testosterone, respectively. The final
hydroxylating step in aromatization does not require enzymatic
action and is not product sensitive.

3) Several plant components have structural and functional
similarities to estrogens and are therefore referred to as
"phytoestrogens". Genistein and daidzein are isoflavonoids found
in soybeans and clover. Green tea and various legumes contain the
lignans enterolactone and enterodiol. Genistein inhibits
steroidogenic enzymes as well as tyrosine kinase enzymes and may
have antioxidant activity. Some epidemiologic data suggest that
diets rich in phytoestrogens protect against breast cancer,
prostate cancer, colon cancer, cardiovascular disease, and
osteoporosis.

New Engl. J. Med. 2002 346:340

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10. DEVELOPMENTAL BIOLOGY: ON GUT FORMATION IN ANIMALS

B. Fuss and M. Hoch (University of Bonn, DE) discuss gut
formation, the authors making the following points:

1) The morphological processes involved in the development of the
gastrointestinal tract of animals are highly similar. In mouse
and chicken embryos, gut formation is initiated by the formation
of two open-ended tubes at opposite sites of the embryo. The
tubes are generated by the invagination of the endodermal layer
in an anterior-ventral position and later in a posterior-ventral
region. The gut tubes then grow and extend toward each other
until they meet and fuse around the yolk stalk. In the fruit fly
Drosophila, gut formation is also initiated with gastrulation by
the invagination of cells of the anterior-ventral and posterior-
dorsal region of the embryo to give rise to the foregut and
hindgut primordial tubes, respectively. The midgut forms in
between these tubes by fusion of an anterior and a posterior
primordium. As the gut tubes form, visceral mesoderm is recruited
to surround the invaginating gut epithelia.

2) The primitive gut tube of vertebrates and invertebrates is
initially regionalized along the anterior-posterior axis into
three broad domains: the foregut, the midgut, and the hindgut.
Ultimately, these domains are further subdivided, and derived
organs such as the lungs, pancreas, or liver in vertebrates, and
the proventriculus or the Malpighian tubules in Drosophila, are
specified. The similarity of the morphological processes during
gut formation is paralleled by the function of evolutionarily
conserved molecular regulators of gastrointestinal development.

Current Biology 2002 12:171

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11. ON TOOL-MAKING BY CROWS

In this context, a "stepped tool" is a tapered tool whose
tapering involves a series of steps that sequentially narrow the
short-axis diameter to make the tool end in a point.

... ... G.R. Hunt et al (University of Auckland, NZ) discuss
tool-making by crows, the authors making the following points:

1) New Caledonian crows fashion tapered tools from either the
left or the right edge of the long narrow leaves of pandanus
trees or screw pines, the crows using the tools to extract
invertebrates in rainforest vegetation. Although right-handedness
is thought to be uniquely human, the authors demonstrate that
crows from different localities display a widespread laterality
in making their tools, indicating that this behavior is unlikely
to be attributable to local social traditions or ecological
factors. The authors state that to their knowledge this is the
first demonstration of a species-level laterality in manipulatory
skills outside humans.

2) The use of left or right leaf-edges by crows depends in part
on the direction in which the leaves spiral. Clockwise- spiraling
leaves provide easier access to left edges, and anti-clockwise
spiraling provide easier access to right edges. This access
effect was overridden, however, by an island-wide preference for
manufacturing tools from left edges.

3) It has been proposed that right-handedness in humans may be a
consequence of the evolution of language, which is also
predominantly left-hemispheric. The authors suggest their results
favor the more general possibility that species-level
lateralization is an adaptation for the efficient neural
programming of complex sequential processing, of which language
and right-handedness in humans, and stepped-tool manufacture in
crows are examples.

Nature 2001 414:707

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12. ON GALEN OF PERGAMUM (129-C.216)

Vivian Nutton (University College London, UK) discusses Galen,
the author making the following points:

1) Few medical men have exercised as much influence for as long
as Galen of Pergamum. His ideas dominated medicine in the
Byzantine world from AD 300 onward, and through the medium of
translation in the world of Islam and in Western Europe from the
12th to the 17th centuries. As Yunani (Greek) medicine, Galen's
ideas still form one of the learned traditions of medicine in the
modern Muslim world. Yet by repeatedly presenting Galen's
conclusions rather than the empirical evidence and procedures on
which they were based, his followers unwittingly helped to create
the common view of Galen as bookish, dogmatic, authoritarian, and
as a stumbling-block to medical progress until the Renaissance.
During the past 30 years, scholars aided by the rediscovery of
many of Galen's works in Arabic translation have begun the
process of rehabilitation.

2) Galen achieved his authority through abundant energy, massive
self-confidence, enormous learning, near impeccable logic, and
cogent rhetoric, allied to remarkable practical skills as an
experimenter, observer, and clinician. His career was unusual for
a doctor in the ancient world. Taking up medicine at the age of
17, he spent a decade in medical studies, including 4 or 5 years
at Alexandria, the greatest medical center of antiquity. In AD
157, Galen returned to his native Pergamum (Bergama, western
Turkey), where among other duties he cared for the health of a
troop of gladiators. He soon moved on again, arriving in Rome for
the first time in AD 162, where he quickly established a
reputation for public dissection of animals. Even though half of
his total output has been lost over the centuries, Galen's
surviving works in Greek, some 115 titles, constitute
approximately 10 percent of all that remains of Greek literature
from before AD 300.

Science 2002 295:800

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13. BANDGAP MODULATION OF CARBON NANOTUBES

J. Lee et al (Seoul National University, KR) discuss carbon
nanotubes, the authors making the following points:

1) Technical and economic difficulties in further miniaturizing
silicon-based transistors with present fabrication technologies
have motivated a strong effort to develop alternative electronic
devices, including devices based on single molecules. Carbon
nanotubes have been successfully used for nanometer-sized devices
such as diodes, transistors, and random access memory cells, with
such nanotube devices usually very long compared to silicon-based
transistors.

2) The authors report a method for dividing a semiconductor
nanotube into multiple *quantum dots with lengths of
approximately 10 nanometers by inserting Gd@C(sub82) *endohedral
fullerenes. The spatial modulation of the nanotube electronic
*bandgap is observed with a low-temperature scanning tunneling
microscope. The authors report that a bandgap of approximately
0.5 electronvolts is narrowed down to 01. eV at sites where
endohedral metallofullerenes are inserted. The authors suggest
this change in bandgap can be explained by local elastic strain
and charge transfer at metallofullerene sites. The authors
suggest this technique for fabricating an array of quantum dots
could be used for nano-electronics and nano-optoelectronics.

Nature 2002 415:1005

Notes:

*quantum dots: In general, the term "quantum dot" refers to an
artificial atom. As realized in the laboratory, quantum dots are
small electrically conducting regions, typically less than 1
micron in diameter, that contain from one to a few thousand
electrons. Because of the small volume, the electron energies
within the dot are quantized, and the behavior of the quantum dot
is intermediate between that of an atom and that of a classical
macroscopic object.

*endohedral: "In-the-cage"; describing a chemical entity
contained within a molecular cage.

*bandgap: In general, a forbidden energy band. In this context, a
"band" is a closely spaced group of energy levels in atoms, in
particular a range of energies that electrons can have in a
solid. Each band represents a large number of allowed quantum
states. The outermost electrons of the atoms form the "valence
band" of the solid. In order for electrons to move through a
solid, there must exist empty quantum states with the same
energy, and this can occur only in an unfilled band, the
"conduction band". In general, so-called "metals" are good
conductors because the partly filled conduction band overlaps
with a filled valence band, and vacant energy states in the
conduction band are thus readily available to electrons. In
"insulators", the conduction band and valence band are separated
by a wide forbidden band, and electrons do not have enough energy
to jump from one band to another. In intrinsic "semiconductors",
the forbidden gap is narrow, and at normal temperatures some
electrons at the top of the valence band can move by thermal
agitation into the conduction band. In a so-called "doped"
semiconductor, the doping impurities essentially create one or
more thin separate conduction bands in the forbidden band. In
this context, the "gap" refers to the gap between energy bands,
i.e., from the upper boundary of the valence band to the lower
boundary of the conduction band.

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14. ON NANOSCALE SURFACE STRESS

J.B. Hannon et al (IBM Watson Research Center, US) discuss
nanoscale surface stress, the authors making the following
points:

1) The problems of nanoscale fabrication has produced renewed
interest in self-organization and self-assembly at surfaces, and
surface stress is expected to play a critical role in the
fabrication of devices with highly uniform and predictable
electrical, magnetic, or optical properties. For example,
theories of elastic relaxation at surfaces predict the formation
and stabilization of periodic structures with well-defined
equilibrium sizes. In these analyses, the equilibrium feature
size is determined by balancing the elastic energy gain
associated with relaxation at the phase boundary against the
energy cost of creating the boundary. It has been more difficult
to establish the link between stress and morphology in
experiments, because the key kinetic or thermodynamic parameters
necessary for quantitative interpretation are often unknown.
Furthermore, the extended periodic structures that many theories
predict can be difficult to produce and equilibrate
experimentally, which further complicates a quantitative
analysis.

2) The authors present a detailed experimental and theoretical
analysis of tunable and reversible size selection near the first-
order structural phase transition of the Si(111) surface. The
authors demonstrate how temperature can be used to tune the size
of domains during a surface phase transition. From analysis of
the measured stable domain sizes, the authors determine key
material parameters and clarify the close relationship between
nucleation and thermodynamic size selection. More generally, the
model developed by the authors describes nanoscale self-assembly
processes in contact with a reservoir (e.g., liquid- or vapor-
phase epitaxy).

Science 2002 295:299

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15. MICELLES: SODIUM DODECYL SULFATE

In general, "amphiphiles" are molecules with parts (groups)
having diverse affinities for different solvents. For example,
polar groups have an affinity for water, while hydrocarbon groups
have an affinity for oils. Most detergents are amphiphiles,
molecules with a polar head and a long hydrocarbon tail. In this
context, however, possible solvent interactions are only one
aspect of amphiphilic character. The important consideration is
that amphiphiles tend to self-organize: groups of amphiphilic
molecules will form stable domains of polar interactions and
nonpolar interactions. For example, amphiphiles may form
"micelles", spherical or cylindrical arrangements with an
interior forming one interaction domain while the surface forms
another interaction domain.

... ... Y. Rharbi and M.A. Winnik (University of Toronto, CA)
discuss micelles, the authors making the following points:

1) Sodium dodecyl sulfate is one of the most important
surfactants in common use. When one talks about surfactants in
general or teaches students about micelle formation, this
substance is almost always the first example that comes to mind.
Sodium dodecyl sulfate self=assembles to form micelles when its
concentration in water exceeds its critical micelle concentration
of 8.3 millimolar. Since it is an ionic surfactant, many of its
micelle properties change when salts are added to the aqueous
solution. For example, the critical micelle concentration
decreases in the presence of salt and the micelle size increases.

2) Chemical relaxation experiments have demonstrated that these
micelles exhibit two relaxation times. The most unresolved
question about sodium dodecyl sulfate micelles concerns the
influence of salt on the slow relaxation process. The fast
process involves association and dissociation of individual
surfactant monomers to and from the micelle, and this process can
lead to a change in micelle size while the number of micelles
remains constant. The slow process has been attributed to
relaxation of the micelle distribution through a sequence of
association and dissociation events involving surfactant monomers
and aggregates of surfactant molecules, including the formation
and breakdown of entire micelles. Newer models consider micelle-
micelle interactions involving micelle fusion followed by micelle
breakdown, but it has not been to discriminate between various
proposed mechanisms.

J. Am. Chem. Soc. 2002 124:2082

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16. ON FLUORESCENCE DETECTION OF DNA-JOINING REACTIONS

Various biochemical applications of fluorescence involve two
fluorochromes, one of which absorbs light emitted by the other.
The initially emitted light is absorbed and thus "quenched" by
the second compound. Since such quenching can only occur if the
two fluorochromes are in proximity, the process can be used as an
indicator of such proximity. The term "quench" has other meanings
in other contexts.

... ... S. Sando and E.T. Kool (Stanford University, US) discuss
detection of DNA-joining reactions, the authors making the
following points:

1) A number of strategies for detection of nucleic acid reactions
involve a change in fluorescence intensity or emission
wavelength. Fluorescence-changing methods have the distinct
advantage that unbound probe molecules can easily be
distinguished from those bound to the desired target. Such
approaches can be used either in solution or on solid supports,
whereas static methods often cannot be used in solution, and
typically require careful washing methods on solid supports.
Approaches that rely on simple intensity variation produced by
changes in quenching have the further advantage of freeing more
spectral ranges so that multiple simultaneous probing can be
achieved.

2) To date, the number of different quenching-approaches to
nucleic acid sensing is limited. Perhaps the most well-developed
approach is that of "molecular beacons", which consist of
hairpin-forming DNAs labeled in the stem with fluorophore and
quencher. Binding of the DNA molecule to a complementary sequence
results in opening of the hairpin and moving of the quencher away
from the emitting fluorophore. Beacons can be used in solution or
in solid-support approaches, but their fluorescence change
depends on solution conditions (e.g., temperature and ionic
strength), and so one must monitor conditions carefully.
Moreover, methods that rely on DNA hybridization alone are
usually not as sequence selective as some recently developed DNA-
sensing methods such as enzymatic approaches or some non-
enzymatic autoligation methods.

J. Am. Chem. Soc. 2002 124:2096

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17. ON THE PREDICTABILITY OF CATASTROPHIC EVENTS

Didier Sornette (University of California Los Angeles, US)
discusses the predictability of catastrophic events, the author
making the following points:

1) What do a high-pressure tank on a rocket, a seismic fault, and
a busy stock market have in common? Recent research suggests they
can all be described in much the same basic physical terms -- as
self-organizing systems that develop similar patterns over many
scales, from the very small to the very large. All three systems
have the potential for extreme behavior: rupture, quake, or
crash, respectively. A central property of such complex systems
is the possible occurrence of coherent large-scale collective
behaviors with a very rich structure, behaviors resulting from
the repeated nonlinear interactions among constituents: in such
systems, the whole turns out to be much more than the sum of its
parts.

2) The author proposes that catastrophic events are often
"outliers" with statistically different properties than the rest
of the population and result from mechanisms involving amplifying
critical cascades. The author describes a unifying approach for
modeling and predicting these catastrophic events or "ruptures",
i.e., for modeling and predicting sudden transitions from a
quiescent state to a crisis. Such ruptures involve interactions
between structures at many different scales. Possible
applications of the model and potential for predictions include
the rupture of composite materials, great earthquakes,
turbulence, abrupt changes of weather regimes, financial crashes,
and the events of human birth.

Proc. Nat. Acad. Sci. 2002 99:2522

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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18. ON THERMOELECTRIC MATERIALS

Brian C. Sales (Oak Ridge National Laboratory, US) discusses
thermoelectric materials, the author making the following points:

1) In the early 1950s, most semiconductor research focused on
using semiconductors not in integrated circuits but in
thermoelectric modules for home refrigeration. This never became
practical because of poor cooling efficiency. New materials and
new synthesis techniques have now reawakened interest in the use
of semiconductors in refrigeration and power generation, and some
of the promising new thermoelectric structures contain carefully
arranged films or clusters on nanometer length scales.

2) Thermoelectric devices are extremely simple, have no moving
parts, and involve no greenhouse gases. The devices use two types
of semiconductor "legs" that are connected in series: negatively
charged electrons carry electric current in the n-type leg,
whereas positively charged holes carry the current in the p-type
leg.

3) Thermoelectric refrigeration with semiconductor devices is
possible because electrons and holes carry heat as well as
electric charge. An external battery forces the hot electrons and
holes away from the cold side of the device, resulting in
cooling. In some multistage thermoelectric modules, temperatures
as low as 160 kelvins can be achieved. Today, spot cooling of
electronics is the primary application for thermoelectric
refrigerators.

4) If heat is applied to only one side of the device, a voltage
develops across the (n) and (p) legs that can be used to convert
part of the heat into electrical power. NASA has used this
principle to provide hundreds of watts of electrical power for
deep space probes such as Voyager I and II and the Cassini
mission to Saturn.

5) The major problem with thermoelectric devices is poor
efficiency. The efficiency of a thermoelectric module is
fundamentally limited by the material properties of the n- and p-
type semiconductors, regardless of how cleverly the module is
engineered.

Science 2002 295:1248

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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19. ON SMOKING AS A CHRONIC DISEASE

Nancy A. Rigotti (Harvard University, US) discusses smoking as a
chronic disease, the author making the following points:

1) Tobacco use is the leading preventable cause of death in the
US, responsible for more than 400,000 deaths annually, or 1 or
every 5 deaths. Half of regular smokers die prematurely of a
tobacco-related disease, and the potential health benefits of
smoking cessation are substantial. Cessation reduces the risk of
tobacco-related diseases, slows the progression of established
tobacco-related diseases, and increases life expectancy, even
when smokers stop smoking after the age of 65 or after the
development of a tobacco-related disease.

2) An estimated 70 percent of smokers see a physician each year,
providing physicians with substantial opportunity to influence
smoking behavior. However, many patients continue to smoke
despite knowing about or experiencing the health consequences of
tobacco use. Some who try to quit repeatedly fail. Most
mistakenly believe that stopping smoking requires only willpower
and are unaware that effective treatments are available. Tobacco
use has complex physiological and psychological determinants, and
changing any behavior is a gradual process. Smoking is best
regarded as a chronic disease that requires a long-term
management strategy, rather than a quick fix.

3) Currently, 23.5 percent of US adults (25.7 percent of men and
21.5 percent of women) smoke cigarettes. Nearly all smokers
acknowledge that tobacco use is harmful to health but
underestimate the magnitude of their own risk. Few smokers know
the full spectrum of health risks, and for many smokers, the risk
of future disease does not outweigh the current perceived
benefits of smoking or barriers to cessation. Nevertheless, 70
percent of smokers report they want to quit. Approximately one-
third of smokers try to stop smoking each year, but only 20
percent of these smokers seek help. Fewer than 10 percent of
smokers who attempt to quit on their own are successful over the
long term. The chief physiological obstacle to quitting is the
addictive nature of nicotine.

New Engl. J. Med. 2002 346:506

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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20. ON MYCOTOXINS

Ruth A. Etzel (George Washington University, US) discusses
mycotoxins, the author making the following points:

1) Mycotoxins, chemicals produced by fungi, may have evolved to
serve as a chemical defense system against insects,
microorganisms, nematodes, grazing animals, and humans.
Approximately 400 known mycotoxins exist. They can benefit humans
by their use as antibiotics (e.g., penicillins),
immunosuppressants (e.g., cyclosporine), and in control of
postpartum hemorrhage and migraine headaches (e.g., ergot
alkaloids). However, mycotoxins are also capable of producing
illness and death in humans and animals.

2) Exposure to mycotoxins may occur through ingestion,
inhalation, and skin exposure. The mycotoxins were discovered
when epidemics of illness were traced to ingestion of moldy food.
Massive mycotoxin contamination of food resulting in outbreaks of
illness occurs only rarely today, primarily in developing
countries. The major concern in developed countries are the long-
term effects of ingesting food contaminated with low levels of
mycotoxins. Although ergot alkaloids are a usual focus because of
their historical importance, currently the most commonly
encountered mycotoxins in animal feed and human foods are
aflatoxins, fumonisins, and deoxynivalenol (vomitoxin).

3) Aflatoxins, produced by Aspergillus flavus and A. parasiticus,
are common contaminants of peanuts, soybeans, grains, and cassava
(a root), especially in tropical areas. In the 1960s, aflatoxins
were found to be potent carcinogens in animals.

4) The fumonisins are a group of mycotoxins isolated from corn
contaminated with Fusarium moniliforme, F. proliferatum, and A.
ochracens. They were discovered in 1988 following the 1970
outbreak of equine leukoencephalomalacia in South Africa.

5) Vomitoxin is a trichothecene and a frequent contaminant of
wheat and corn. In China, from 1961 to 1985, multiple outbreaks
of vomiting illness were attributed to consumption of vomitoxin-
contaminated grain. In 1997 to 1998 in the US, approximately 1700
children became ill with vomiting, nausea, headache, and
abdominal cramps linked to eating burritos, although levels of
vomitoxin in the burritos were less than 1 part per million.

J. Am. Med. Assoc. 2002 287:425

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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21. ON SPINAL CORD INJURY

Martin E. Scwab (Swiss Federal Institute of Technology, CH)
discusses repair of spinal cord injury, the author making the
following points:

1) The human spinal cord is a finger=thick strand of nervous
tissue tightly enclosed in the bony vertebrae of the spinal
column. The spinal cord receives sensory information from the
skin, the muscles, the joints, and other tissues of the body, and
it transmits this information in the form of electrical impulses
to the brain, along millions of nerve fibers grouped together in
bundles. The motor commands that are subsequently generated in
the brain are sent to the spinal cord along fast-conducting nerve
fibers, which terminate in local spinal motor circuits. From
here, the electrical impulses that will direct coordinated muscle
contraction reach the muscles via the peripheral nerves.

2) A sharp blow to the spinal column can cause dislocation of
individual vertebrae and severe damage to the spinal cord,
including its complete severance. Clinically, the result of an
incomplete or complete spinal cord lesion is either paraplegia
(paralysis of the lower body) or quadriplegia (paralysis of the
body from the neck down, depending on whether the injury was
sustained in the thoracic/lumbar region or in the neck region of
the spinal column, respectively.

3) Destruction of the spinal cord can be compared to a bomb
exploding in a computer center, and repairing the spinal cord is
as complicated as trying to rebuild all of the computer
connections. In the last few years, there has been encouraging
progress in animal models, with sufficient regeneration of the
damaged spinal cord to enable some recovery of motor ability.
Some of the new strategies to repair spinal cord injuries, either
alone or in combination, offer the possibility of clinical
effective therapies for paraplegic and quadriplegic patients in
the not too distant future.

Science 2002 295:1029

Related Background:

ON REGENERATING THE DAMAGED CENTRAL NERVOUS SYSTEM

The ability to regenerate at least certain parts of the
organism is found in all living systems, including plants and
animals, unicellular and multicellular. With higher organisms,
however, for example with mammals, the process of regeneration
involves many constraints. Of great concern in clinical medicine
are injuries to the nervous system, injuries which are often
permanently debilitating because of poor or absent regeneration
of neural tissue. Important advances have recently been made in
our understanding of nervous system injury and regeneration, and
there are now indications that significant breakthroughs will
occur in the near future.

What happens when a nerve cell is injured? Consider the case
when the *axon of the nerve cell is severed. When a *peripheral
nerve fiber is cut, certain events follow in different parts of
the neuron. The distal segment of the nerve fiber, the part on
the far end of the cut, undergoes degeneration, which begins
slowly, requiring days to be completed, and involves the separate
parts of the nerve fiber differently. The axon gradually breaks
up and the segments are digested and absorbed. If there is a
*myelin sheath, it is gradually transformed into a chain of lipid
droplets, the larger of which may in the early stages contain
degenerating fragments of the axon. The fragments of the axon
disappear in a few days; parts of the degenerating myelin sheath,
in the form of droplets, may persist for six months or more. When
a nerve fiber is cut, the parts of the neuron from the break
toward the cell body (the proximal parts) also show
characteristic changes. The cell body undergoes evident changes
in *endoplasmic reticulum and *ribosomes (chromatic changes in
Nissl substance). This changes reaches its peak in 7 to 15 days,
after which there may be recovery, or complete degeneration if
there is too much damage. If the cell body completely
degenerates, the nerve fiber between the cell body and the cut
undergoes degeneration (Wallerian degeneration) just as the
distal segment does. But if the cell body survives, only a small
amount of destruction of the proximal segment occurs, and that
near the cut. Since this is a peripheral nerve, what happen then
is that from each axon near its cut end a number of small sprouts
grow out in all directions. Some of the sprouts grow in the
direction of the former distal axon segment and grow into the
connective tissue matrix that has formed scar tissue. The
haphazard arrangement of connective tissue fibers influences the
amoeboid growing tips of the nerve sprouts. Not all of the fibers
get across the scar, but a few do, and even fewer manage to
regain the original neural pathway.

The above is a description of mammalian peripheral nerve
degeneration and regeneration, the process first described at the
beginning of the 20th century. For most of the 20th century,
there was a clear dogma in neurobiology: It was believed that in
the mammalian central nervous system, including in humans, the
nerve fibers of the brain and spinal cord were incapable of
regeneration sufficient to restore function. A most important
corollary of this dogma was that this incapability of sufficient
regeneration (or any regeneration at all) was intrinsic to
central nervous system nerve cells. In 1980, that corollary dogma
was overturned, and it is now understood that the regenerative
capacity of the central nervous system is not intrinsic to
central nervous system nerve cells, but depends on the
circumstances of damage and the immediate environment of the
nerve cells. Regeneration can occur in the damaged central
nervous system, and this new understanding has caused
considerable excitement in the neurobiological and medical
communities.

... ... P.J. Horner and F.H. Gage (Salk Institute, US) present an
extensive review of regeneration in the damaged central nervous
system, the authors making the following points:

1) The authors point out that in contrast to fish, amphibia,
and the mammalian peripheral nerves and developing central
nerves, adult central mammalian neurons do not regrow functional
axons after damage. This inability of adult central nervous
system neurons to regrow after injury cannot be entirely
attributed to intrinsic differences between adult central nervous
system neurons and all other neurons, since it has been known
since the early years of the 20th century that adult central
nervous system neurons could regrow in a permissive environment.
In 1980, P.M Richardson et al replicated the early studies with
new methods that definitely confirmed that adult central nervous
system neurons have regenerative capabilities. This finding
revealed that the failure of central nervous system neurons to
regenerate was not an intrinsic deficit of the neuron, but rather
a characteristic feature of the damaged environment that either
did not support or prevented regeneration. In the past 20 years,
progress has been made in identifying the elements that are
responsible for the differences between the adult central nervous
system and peripheral nervous system environments, and in the
past few years the molecular and cellular bases of regenerative
compared with non-regenerative responses are beginning to be
revealed.

2) The authors suggest that regeneration strategies
developed from these new discoveries will be applicable to many
central nervous system disorders. Spinal cord injury could be the
most approachable, owing to the well-defined loss of cells and
axons and the relative lack of consequent chronic pathology.
Genetic disorders that result in aberrant axonal pathfinding or
neuronal cell loss may also be amenable to regeneration.
Degenerative diseases where a defined cell type is lost (e.g.,
Parkinson's disease, Alzheimer's disease, amyotrophic lateral
sclerosis) are also good targets, but may be more challenging
because of the potential for continued cell loss or axonal
degeneration. Finally, regeneration strategies may also be
applied to less well-defined disorders where diffuse cell and
axonal loss can occur, such as cerebrovascular disease, tumor,
and infection of the central nervous system.

3) Concerning recent work, an increasing number of studies
have demonstrated that an adult cut axon in the central nervous
system can be induced to regrow by either increasing the
permissive cues or decreasing the non-permissive cues of the
existing environment. Furthermore, a growing list of reports
indicate that one strategy or another can induce some level of
functional recovery following damage. The authors (Horner and
Gage), however, point out that it is not sufficient to
demonstrate axon elongation and behavioral improvement after
injury to conclude that authentic functional regeneration is
responsible for the outcome. There are many mechanisms that may
account for observed functional recovery that do not require
regeneration, and these non-regenerative mechanisms are common in
most experimental models of traumatic injury and need to be
excluded before invoking functional regeneration as the cause of
repair and recovery. The reason for sorting out the authentic
mechanisms of functional recovery is that without understanding
the underlying basis of regeneration, little progress can be made
beyond the phenomenological observation of recovery from injury.

4) The authors conclude: "Despite the progress in the last
century of research on regeneration... *Cajal's [1928] flowery
decree, as translated by Raoul May, still resonates: 'Once the
development was ended, the founts of growth and regeneration of
the axons and *dendrites dried up irrevocably. In the adult
centers the nerve paths are something fixed, ended, and
immutable. Everything may die, nothing may be regenerated. It is
for the science of the future to change, if possible, this harsh
decree.' The decree is lifted; the solution remains elusive."

Nature 2000 407:963

Notes:

*axon: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. In some types of nerve cells,
axons are extensively branched into a multitude of fine fibers
that make contact (synapses) with other nerve cells.

*peripheral nerve fiber: In mammals, neural tissue
comprising the brain and spinal cord is called the "central
nervous system", while neural tissue outside the brain and spinal
cord is called the "peripheral nervous system". The dichotomy is
more than formal, since anatomical, functional, and in this
context regeneration differences are significant.

*myelin sheath: High signal propagation velocities in
motor and sensory neurons in vertebrates are achieved by
association of the nerve fiber with an enfolding "myelin sheath".
The myelin sheath consists of concentric layers of electrically
insulating lipid material (myelin), but the sheath is
periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. 

*endoplasmic reticulum: The term "endoplasmic reticulum"
refers to a complex system of intracellular flattened sacs, and
it is the site of many important syntheses, including the
production of new surface membrane and the intracellular
transport of various biochemical entities.

*ribosomes: A ribosome (not to be confused with riboZYME)
is a small particle, a complex of various ribonucleic acid
component subunits and proteins that functions as the site of
protein synthesis.

*Cajal: Santiago Ramon y Cajal (1852-1934), one of the
founders of microscopic neuroanatomy, was awarded the 1906 Nobel
Prize in Physiology or Medicine for establishing the neuron as
the fundamental unit of the nervous system.

*dendrites: The general input extensions of nerve cells
are called "dendrites", and they may be extensively branched. In
general, dendrites are considered to receive input and axons to
propagate output, but the electrical architecture of most neurons
is complicated, and in many types of nerve cells activation of
the axon produces electrical activity that not only propagates
down the axon but also propagates backward through the cell body
and dendrites.

Related Background:

PROSPECTS FOR NEURAL STEM CELL REPAIR OF INJURED SPINAL CORD

What has happened in vertebrate evolution is that the brain has
evolved from a mere head cluster of nerve cells (a head ganglion)
of the spinal array of ganglia (the spinal cord) to a burgeoned
structure that dominates the spinal cord almost completely.
In terms of both function and anatomy, the human spinal cord can
thus be viewed as a "service" extension of the commanding brain,
the two together constituting the "central nervous system", and
like in the brain, traumatic injury to the spinal cord is usually
irreversible: brain and spinal nerve cells and nerve fibers
usually do not regenerate when damaged. Since many nerve cells
and nerve fibers in the spinal cord are essential to the control
of various voluntary and involuntary muscles of the body below
the head, traumatic injury to the spinal cord can be devastating
in its consequences. An acceleration of research into possible
mechanisms of neuronal regeneration has occurred during the past
several decades, and there is now some hope for applications of
this research to the treatment and repair of spinal cord
injuries.

... ... S.S.W. Han and I. Fischer (Hahnemann University School of
Medicine, US) present a review of current research in this field,
the authors making the following points:

1) Recent observations that several regions of the
mammalian central nervous system do continue to produce neurons
throughout life suggests there are prospects for repairing an
injured spinal cord. Researchers have developed efficient
methods for culturing the neural *stem cells of rodents,
genetically modifying these cells to produce therapeutic genes,
and then transplanting these cells into animal models of brain
diseases. These same gene therapy and grafting techniques are
being explored as possible methods for restoring function
following traumatic spinal cord injury.

2) In the developing embryo, *epithelial cells of the
*neural tube generate a variety of precursor cells that migrate
and *differentiate into neurons, *astrocytes, and
*oligodendrocytes. Central nervous system stem cells have now
been discovered in the human central nervous system and appear
to behave similarly to their rodent counterparts, and these stem
cells could potentially be used to promote the generation of new
nerve cells (neurogenesis) following injury and disease.

3) Transplantation studies have demonstrated that neural
stem cells have the capacity to differentiate in response to the
environment into which they are reintroduced and to integrate
appropriately with the host tissue. Neural stem cells can be
isolated from different areas and propagated for long periods in
culture without losing their ability for varied differentiations
(their "multipotentiality"). When transplanted back into the
central nervous system, these stem cells have the capacity to
migrate, to integrate with the host tissue, and to respond to
local cues for differentiation.

4) The authors conclude: "Transplantation of neural stem
cells and precursor cells together with gene therapy offers
great promise for spinal cord repair. Specific research goals
include improving neuronal survival, promoting functional
recovery through *axonal regeneration, compensating for
*demyelination, and replacing lost cells. Many issues will need
to be resolved before stem cells can be considered for use in
human subjects, but continued basic research on the properties
of these cells and development of appropriate animal models of
repair will pave the way for successful clinical applications."

J. Amer. Med. Assoc. 2000 283:2300

Notes:

*stem cells: In general, a stem cell is any precursor
cell, a form prior to cell differentiation. E.g., stem cells in
bone marrow that give rise to blood cells.

*epithelial cells: In animals, "epithelial cells" compose
the cell layers that form the interface between a tissue and the
external environment, for example, the cells of the skin, the
lining of the intestinal tract, and the lung airway passages.

*neural tube: The term "neural tube" refers to the early
embryonic structure (an actual hollow tube of cells formed by the
infolding and closing of a long sheet of cells) that subsequently
gives rise to the entire brain and spinal cord.

*differentiate: In this context, the term
"differentiation" refers to developmental cell specialization
(morphology and biochemistry) resulting from activation of
specific parts of the cell genome. E.g., the differentiation of a
stem cell into a nerve cell.

*astrocytes: (astroglial cell) Neuroglia are non-neuronal
cellular elements of the central and peripheral nervous systems,
and astroglia (astrocytes) are a type of neuroglia. In general,
neuroglia are thought to have important metabolic functions.

*oligodendrocytes: (oligodendroglia) Glial cells
characterized by sheet-like processes that are wrapped around
individual neuron axons to form the myelin sheath of nerve
fibers in the central nervous system. (The myelin sheath of a
nerve fiber is effectively a periodically interrupted insulation
which increases the propagation velocity of nerve impulses. See
note on "demyelination" below.)

*axonal regeneration: In general, nerve cells have a
single long extension (the "axon") that propagates the electrical
output (the action potential) of the cell. In some types of nerve
cells, axons are extensively branched into a multitude of fine
fibers that make contact (synapses) with other nerve cells.

*demyelination: (demyelinization) A number of
neurodegenerative diseases involve progressive demyelination of
various myelinated nerve fibers. High signal propagation
velocities in motor and sensory neurons in vertebrates are
achieved by association of the nerve fiber with an enfolding
sheath called myelin. The myelin sheath consists of concentric
layers of electrically insulating lipid material, but the sheath
is periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. 

Related Background:

FUNCTIONAL REGENERATION OF SENSORY AXONS IN ADULT SPINAL CORD

In vertebrates, the spinal cord is continuous with the
brain, and the two together constitute what is called the
"central nervous system". In addition to other functional
involvements, the spinal cord, and the nerves extending from and
leading into the spinal cord ("spinal nerves"), comprise neuronal
circuits that among other things mediate a number of fast
responses to environmental changes. For example, if you
inadvertently pick up a hot object, the grasping muscles in your
hand may relax and the object drop even before the sensation of
extreme heat or pain reaches your brain and your conscious
perception. This is an example of a "spinal cord reflex", a fast
automatic response to certain types of stimuli, the response
requiring only nerve fibers and nerve cells in the spinal nerves
and spinal cord. In addition to processing such reflexes, the
spinal cord also is the site for integration of nerve impulses
that originate locally in the spinal cord or that arrive from the
periphery and brain. Of great importance is that the spinal cord
is the "highway" traveled by sensory nerve impulses carrying
sensory information to the brain, and by motor nerve impulses
originating in the brain and destined for voluntary muscles via
the spinal nerves. In humans, there are 31 pairs of spinal nerves
arranged with bilateral symmetry to serve the two sides of the
body.

Sensory input to the spinal cord (and to nerve cells in the
spinal cord) occurs via sensory neurons with a special
morphology. Ordinary neurons have a cell body with short (often
arborized) extensions (dendrites) to receive input, and a long
extension (axon) to propagate output away from the cell body to
either another neuron or to a muscle cell. But most sensory
neurons conveying input to the spinal cord are quite different:
such neurons have a long input extension, as much as 1 meter long
in humans, that propagates nerve impulses at high speed _toward_
the cell body, and a short or long (depending on the specific
type of sensory nerve cell) output extension into the spinal cord
from the sensory neuron cell body located just outside the spinal
cord.

Spinal nerves are "mixed nerves", containing both input
(afferent) nerve fibers and output (efferent) nerve fibers. In
humans and other higher vertebrates, the anatomy is such that
near the spinal cord, just before joining it, each spinal nerve
bifurcates into a "dorsal root" and a "ventral root" (in humans,
posterior root and anterior root, respectively). The ventral root
contains output nerve fibers to "effector cells" (in muscles,
glands, etc.), while the dorsal root contains input nerve fibers
propagating peripheral sensory information to the central nervous
system. Each dorsal root, as seen in gross morphology, has a
bulge which contains the numerous cell bodies of the sensory
nerve fibers, and each of these bulges is called a "dorsal root
ganglion".

When the human spinal cord is injured by physical trauma
(as in an automobile accident), one of common consequences is a
traction-caused ripping of the spinal nerves (spinal nerve roots)
out of the spinal cord at a particular location in the spinal
cord axis ("spinal root avulsion"). Root avulsion usually
produces complete paralysis of those regions of the body
controlled by those particular spinal nerves, with loss of local
motor control and loss of local sensation. Natural repair of
severed connections between the spinal cord and spinal nerves
does not occur in humans, but in the past decade there has been
much progress in understanding the mechanisms of nerve fiber
regeneration, and there is now some hope of defining
interventions that may possibly provoke regeneration in cases of
human spinal nerve avulsion.

... ... M.S. Ramer et al (3 authors at 2 installations, UK) now
report evidence of functional regeneration of sensory axons in
adult mammalian spinal cord. The authors point out that the
arrest of dorsal root axonal regeneration at the transition zone
between the peripheral and central nervous system (e.g., between
the spinal cord and the spinal nerves) has been repeatedly
described since the early 20th century. The authors report their
work indicates that with *neurotrophic support to damaged sensory
neuron axons, this regenerative barrier is surmountable. In adult
rats with experimentally injured dorsal roots, *intrathecal
treatment with *nerve growth factor, *neurotrophin-3, and
*glial-cell-line-derived neurotrophic factor, resulted in
selective regrowth of damaged axons across the dorsal root entry
zone and into the spinal cord, where neurons that ordinarily
receive sensory input (dorsal horn neurons) were found to be
synaptically driven by peripheral nerve stimulation in treated
animals, demonstrating functional reconnection. In behavioral
studies, rats treated with nerve growth factor and glial-
cell-line derived neurotrophic factor recovered sensitivity to
noxious heat and pressure. The authors report that the observed
effects of neurotrophic factors corresponded to their known
actions on distinct subpopulations of sensory neurons. The
authors suggest that neurotrophic factor intervention may serve
as a viable treatment in promoting recovery from root avulsion
injuries. The authors further suggest that apart from dorsal root
injuries, once the nature of traumatic injuries in general in the
human central nervous are better understood, neurotrophic
treatment may have vast therapeutic potential for such tissue
damage.

Nature 2000 403:312

Notes:

*neurotrophic treatment: (treatment with neurotrophins)
In general, neurotrophins are chemical entities apparently
essential for the viability of nerve cells. These substances are
polypeptides of 200 to 300 amino acids, and a number of different
neurotrophins have been identified.

*intrathecal treatment: In general, treatment involving
injection into a local area surrounding the spinal cord:
injection beneath one or more of the protective sheaths that
cover the spinal cord.

*nerve growth factor: A type of neurotrophin. The various
neurotrophins can be differentiated on the basis of tissue
specificities. Nerve growth factor has apparent specificity for
dorsal root ganglion cells.

*neurotrophin-3: A specific type of neurotrophin: 257
amino acids, molecular weight 29.32 kilodaltons.

*glial-cell-line-derived neurotrophic factor: Glial cells
are cells of the central and peripheral nervous system that
metabolically support neurons. Such cells also produce the
multiple membrane layers called myelin and enfold nerve cell
axons with it. The glial cells are found everywhere in the brain
and spinal cord, and one result of a localized injury to the
central nervous system is a local proliferation of glial cells
to form a scar matrix. In this context, the term
"glial-cell-line" refers to a line of laboratory cultured glial
cells.

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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22. RANDOM GRAPH MODELS OF SOCIAL NETWORKS

In mathematics, a "graph", in the context of the field known as
"graph theory", is a mathematical object composed of points known
as "vertices" or "nodes" and lines connecting some (possibly
empty) subset of them, known as "edges".

A "random graph", in this context, is a graph in which properties
such as the number of nodes, edges, and connections between them
are determined in some random way.

... ... M.E. Newman et al (Columbia University, US) discuss
social networks, the authors making the following points:

1) A social network is a set of people or groups of people,
"actors" in the jargon of the field, with some pattern of
interactions or "ties" between them. Friendships among a group of
individuals, business relationships between companies, and
intermarriages between families are all examples of social
networks that have been studied in the past. Network analysis has
a long history in sociology, the literature on the topic
stretching back at least half a century to the pioneering work of
Rapoport, Harary, and others in the 1940s and 1950s. Typically,
network studies in sociology have been data-oriented, involving
empirical investigation of real-world networks, the investigation
often followed by graph theoretical analysis aimed at determining
the centrality or influence of the various actors.

2) Most recently, after a surge in interest in network structure
among mathematicians and physicists, partly as a result of
research on the Internet and on the World Wide Web, another body
of research has investigated the statistical properties of
networks and methods for modeling networks either analytically or
numerically. One important and fundamental result that has
emerged from these studies concerns the numbers of ties that
actors have to other actors, their so-called "degrees". It has
been found that in many networks, the distribution of actors'
degrees is highly skewed, with a small number of actors having an
unusually large number of ties. Simulations and analytical work
have suggested that this skewness could have an impact on the way
in which communities operate, including the way information
travels through the network and the robustness of networks to
removal of actors.

3) The authors describe some new exactly solvable models of the
structure of social networks, the models based on random graphs
with arbitrary degree distribution. The authors provide models
both for simple unipartite networks, such as acquaintance
networks, and for bipartite networks, such as affiliation
networks. The authors compare the predictions of their models to
data for a number of real-world social networks and find that in
some cases the models are in remarkable agreement with the data,
whereas in other cases the agreement is poorer, perhaps
indicating the presence of additional social structure in the
network not captured by the random graph.

Proc. Nat. Acad. Sci. 2002 99:2566

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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23. ON TERMINOLOGY IN THE CLONING DEBATE

B. Vogelstein et al (Johns Hopkins University, US) discuss
current terminology concerning cloning, the authors making the
following points:

1) Scientists rely on a dialect of specialized terminology to
communicate precise descriptions of scientific phenomena to each
other. In general, that practice has served the scientific
community well, and novel terms are generally created when needed
to document new findings, behaviors, structures, or principles.
However, when the scientific shorthand makes it way to the
nonscientific public, there is a potential for precise meaning to
be lost or misunderstood and for the terminology to become
associated with research or applications for which it is
inappropriate.

2) In scientific parlance, "cloning" is a broadly used shorthand
term that refers to producing a copy of some biological entity --
a gene, an organism, a cell -- an objective that in many cases
can be achieved by means other than the technique known as
somatic cell nuclear transfer. Bacteria clone themselves by
repeated fission. Plants reproduce clonally via sexual means and
by vegetative regeneration.

3) Much confusion has arisen in the public, in that cloning seems
to have become almost synonymous with somatic cell nuclear
transfer, a procedure that can be used for many different
purposes. However, only one of these purposes involves an
intention to create a clone of the entire organism (for example,
a human being). Unfortunately, legislation currently under
consideration by the US Congress does not adequately distinguish
cloning in general from human cloning, and this legislation would
prohibit a wide range of experimental procedures that in the near
future might become both medically useful and morally acceptable.

4) The authors offer the following tabulation of crucial
differences between nuclear transplantation in general and human
reproductive cloning in particular:

Scientific Usage of the Term "Cloning":

End product: In nuclear transplantation (NT), cells growing in a
petri dish; in human reproductive cloning (HRC), a human being.

Purpose: In NT, to treat a specific disease involving tissue
degeneration; in HRC, to replace or duplicate a human.

Time frame: In NT, a few weeks, (growth in culture); in HRC, 9
months.

Surrogate mother needed?: In NT, no; in HRC, yes.

Sentient human created?: In NT, no; in HRC, yes.

Ethical implications: In NT, similar to all embryonic cell
research; in HRC, highly complex issues.

Medical implications: in NT, similar to any cell-based therapy;
in HRC, safety and long-term efficacy concerns.

Science 2002 295:1237

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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24. A NEW CLASS OF POTENT ANTIMALARIAL COMPOUNDS

K. Wengelnik et al (University of Montpellier, FR) discuss
antimalarials, the authors making the following points:

1) The widespread resistance of malaria parasites to most common
antimalarials, and cross-resistance to structurally unrelated
drugs, emphasize the need for new chemotherapeutic targets. When
parasitizing red blood cells, the malaria parasite invests
heavily in membrane biogenesis, making phospholipid biosynthesis
essential for intra-erythrocytic parasite development, whereas
these activities are absent from mature uninfected erythrocytes.
Phosphatidylcholine is the major phospholipid present in infected
erythrocytes. It is mainly synthesized from plasma-derived
choline by the parasite enzymatic machinery, and thus provides an
attractive target for antimalarial chemotherapy.

2) The authors report that compounds that inhibit
phosphatidylcholine biosynthesis de novo from choline are potent
antimalarial drugs. The lead compound [*Note #1] investigated
potently inhibited in vitro growth of the human malarial
parasites Plasmodium falciparum and P. vivax, and was 1000-fold
less toxic to mammalian cell lines. A radioactive derivative
specifically accumulated in infected erythrocytes to levels
several hundred-fold higher than in the surrounding medium, and
very low dose therapy with this compound completely cured monkeys
infected with P. falciparum and P. cynomolgi.

Notes:

*Note #1: The lead compound (G25):
1,16-hexadecamethylenebis(N-methylpyrrolidinium) dibromide.

Science 2002 295:1311

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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25. IN FOCUS: ON CIGUATOXINS

Dinoflagellates are single-celled organisms found in most
aquatic environments, and they form a major part of the modern
plankton. Many genera of dinoflagellates are sensitive to such
conditions as water salinity and nutrients, and some genera are
characteristic of latitudinal oceanic temperature zones. These
organisms are less than 1 millimeter in size. They have been
classified as both plants and animals, since some species contain
chlorophyll. They have flagella that provide them with
locomotion, and they move in response to light.

-----------------------------------------------

"More than 20,000 people in subtropical and tropical regions
suffer annually from ciguatera, which is the most widespread
human poisoning caused by the consumption of seafood. The disease
is characterized by gastrointestinal, neurological, and
cardiovascular disturbances, which often persist for months or
years, and in severe cases, paralysis, coma, and death may occur.
More than 400 species of fish can be vectors of the ciguatera
toxins, which are produced by a marine dinoflagellate,
Gambierdiscus toxicus, living on macro-algae. These neurotoxins,
designated as ciguatoxins, are far more dangerous [acute toxicity
on mice median lethal dose (LD-50) = 0.25 to approximately 4
micrograms per kilogram] than the structurally related red-tide
toxins, brevetoxins (LD-50 > 100 micrograms per kilogram).
Because reef fish are increasingly exported to other areas, and
because ciguateric fish look, taste, and smell normal, ciguatera
may become a worldwide health problem. Currently, there are no
rapid and reliable methods of detecting ciguatoxins at fisheries.
Furthermore, the content of ciguatoxins in fish is extremely low,
which has hampered the isolation, detailed biological studies,
and most importantly, preparation of the anti-ciguatoxin
antibodies for detecting these toxins."

M. Hirama et al: Science 2001 294:1904

Copyright (c) 1997-2002 ScienceWeek http://www.scienceweek.com

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26. FROM THE SW ARCHIVE:

PROSPECTS FOR THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE

The conjured image is poignant: intelligent life sprinkled
throughout our Galaxy, each sprinkle separated from the others by
1000 light years, each sprinkle searching for the others with
radio transmitters and receivers, small robotic spacecraft sent
beeping into empty space between the stars, the beeping like a
faint bleating in the dark as the sprinkles search for each
other. Of course, the conjured image may be wrong: there may be
intelligent life dense in the Galaxy; or we may be alone. It does
not matter. For the human species on this planet Earth, the quest
is part of our destiny, part of what we do as a species, and it
will go on as long as we remain civilized.

... ... J.C. Tarter and C.F. Chyba (SETI Institute, US) present a
review of current and future efforts in the search for
extraterrestrial intelligence, the authors making the following
points:

1) During the past 40 years, researchers have conducted
searches for radio signals from an extraterrestrial technology,
sent spacecraft to all but one of the planets in our Solar
System, and expanded our knowledge of the conditions in which
living systems can survive. The public perception is that we have
looked extensively for signs of life elsewhere. But in reality,
we have hardly begun to search. Assuming our current,
comparatively robust space program continues, by 2050 we may
finally know whether there is, or ever was, life elsewhere in our
Solar System. At a minimum, we will have thoroughly explored the
most likely candidates, a task not yet accomplished. We will have
discovered whether life exists on Jupiter's moon Europa, or on
Mars. And we will have undertaken the systematic exobiological
exploration of planetary systems around other stars, seeking
traces of life in the spectra of planetary atmospheres. These
surveys will be complemented by expanded searches for intelligent
signals.

2) The authors point out that although the current language
is that of a "search for extraterrestrial intelligence" (SETI),
what is being sought is evidence of extraterrestrial
technologies. Until now, researchers have concentrated on only
one specific technology -- radio transmissions at wavelengths
with weak natural backgrounds and little absorption. No verified
evidence of a distant technology has been found, but the null
result may have more to do with limitations in range and
sensitivity than with actual lack of civilization. The most
distant star probed directly is still less than 1 percent of the
distance across our Galaxy.

3) The authors conclude: "If by 2050 we have found no
evidence of an extraterrestrial technology, it may be because
technical intelligence almost never evolves, or because technical
civilizations rapidly bring about their own destruction, or
because we have not yet conducted an adequate search using the
right strategy. If humankind is still here in 2050 and still
capable of doing SETI searches, it will mean that our technology
has not yet been our own undoing -- a hopeful sign for life
generally. By then we may begin considering the active
transmission of a signal for someone else to find, at which point
we will have to tackle the difficult questions of who will speak
for Earth and what they will say."

Scientific American December 1999

Related Background:

ON CARBON IN THE UNIVERSE

Carbon is a major factor in the evolutionary scheme of the
Universe because of its abundance and its ability to form complex
chemical entities. It is apparently also a key element in the
evolution of prebiotic molecules. The different forms of cosmic
carbon range from carbon atoms and carbon-bearing molecules to
complex solid-state carbonaceous structures, and evidence
gathered during the past decade has considerably enhanced our
understanding of the physical and chemical properties of carbon
materials in space.

... ...  Th. Henning and F. Salama (2 installations, DE US)
present a detailed review of the subject, the authors making the
following points:

1) More than 75 percent of the 118 *interstellar and
circumstellar molecules identified to date are carbon-bearing
molecules, and one component of interstellar dust is evidently
carbonaceous. The cosmic evolution of carbon from the
interstellar medium into *protoplanetary disks and
*planetesimals, and finally into habitable bodies, is intrinsic
to the study of the origin of life.

2) Carbon plays an important role in the physical evolution of
the interstellar medium because it is the main supplier of free
electrons in diffuse interstellar clouds, thus contributing to
the heating of interstellar gas.

3) The observation of unidentified ubiquitous molecular and
solid-state features in astronomical spectra, and the realization
that these features are linked to carbonaceous materials, have
resulted in major scientific progress in the past decade.
Laboratory and theoretical studies stimulated by these
astronomical observations have led to a better understanding of
the various forms of cosmic carbon such as polycyclic aromatic
hydrocarbons, carbon-chain molecules, carbon clusters, and
carbonaceous solids. These investigations have also led to the
detection of novel forms of carbon and laid the foundations for
the chemistry of *fullerenes.

4) The authors present the following categorization of carbon in
space:

... a) Carbon-rich circumstellar envelopes around *red giant and
*asymptotic giant branch (AGB) stars: CO, C(sub2)H(sub2), complex
hydrocarbons, gas-phase polycyclic aromatic hydrocarbons.

... b) Diffuse interstellar medium: C+, simple diatomic
molecules, gas-phase polycyclic aromatic hydrocarbons and carbon
chains.

... c) Dense interstellar medium: CO, complex hydrocarbons.

... d) Interstellar material in primitive meteorites: polycyclic
aromatic hydrocarbons.

5) The authors suggest that the widespread distribution of
complex organics in the interstellar medium has profound
implications for our understanding of a) the chemical complexity
of the interstellar medium, b) the evolution of prebiotic
molecules, c) the impact of this evolution on the origin and
evolution of life on early Earth through the exogenous delivery
(by cometary encounters and meteoritic bombardments) of prebiotic
organics.

Science 1998 282:2204

Notes:

*interstellar and circumstellar molecules: In this
context, an interstellar molecule is any molecule that occurs
naturally in clouds of gas and dust in space. In general, a
circumstellar molecule is any molecule that occurs in gas and
dust surrounding a star.

*protoplanetary disks: These are dust disks surrounding
young stars; it is from these disks that planets presumably form.

*planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.

*fullerenes: Fullerenes are large molecules composed
entirely of carbon, with the chemical formula C(sub n), where n
is any even number from 32 to over 100. They apparently have the
structure of a hollow spheroidal cage with a surface network of
carbon atoms connected in hexagonal and pentagonal rings.

*red giant: A red giant star is a star in a late
stage of evolution. Having exhausted the hydrogen fuel in its
core, the star is burning elements heavier than hydrogen. It has
a surface temperature of less than 4700 degrees Kelvin and a
diameter 10 to 100 times that of the Sun.

*asymptotic giant branch (AGB) stars: These are stars
that occupy a strip in the *Hertzsprung-Russell diagram that is
almost parallel to and just above what is called the "giant
branch" off the *Main Sequence. Stars evolve from the horizontal
H-R branch to the asymptotic giant branch when they have
exhausted the helium in their cores and are instead burning
helium in a shell.

*Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type, and is perhaps the most useful diagrammatic aid in
astrophysics. It allows the portrayal of the evolution of a star
as occurring along various paths in the diagram.

*Main Sequence: The Main Sequence is a region on the
Hertzsprung-Russell diagram where most stars lie, including our
own Sun. The evolution of a star can be diagrammed as a movement
along the Main Sequence and an eventual branching off the Main
Sequence to regions associated with various types of old stars.

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In the text, the affiliation following the names of authors in 
sources with more than one author is the affiliation of the lead 
author.

ScienceWeek copyright extends only to material originated by
ScienceWeek. Other copyrights may obtain for other material.

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