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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
June 2, 2000 -- Vol. 4 Number 22
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I thank God I was not made a dexterous manipulator;
the most important of my discoveries have been
suggested to me by my failures.
-- Humphry Davy (1778-1829)
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1. Chemical Physics:
H(sub3)+ in Chemistry and Astronomy
-----------------------------------
Ionized molecular hydrogen serves as a benchmark in quantum
chemistry, has been discovered in molecular clouds and in the
diffuse interstellar medium, and has provided a tool for
characterizing the atmosphere of Jupiter.
2. Planetary Science:
The Missing Organic Molecules of Mars
-------------------------------------
One of the puzzles about the planet Mars is the fact that the
1976 Viking Mars Mission found no evidence at all of any organic
material on the Martian surface, including no evidence of organic
material expected to result from meteorite impacts.
(Includes related background material.)
3. Materials Science:
Fundamental Quantum Mechanical Phenomena in Semiconductors
----------------------------------------------------------
The characteristics of the envelope wavefunction of quasi-
particles in semiconductors makes it possible to explore
properties and phenomena in semiconductors which are not
accessible under ordinary conditions in other systems.
(Includes related background material.)
4. Evolutionary Biology:
Experimental Evolution and Aging in Fruit Flies
-----------------------------------------------
An evolutionary experiment on the fruit fly Drosophila
confirms the theoretically expected key role of extrinsic
mortality rates in the evolution of growth, maturation,
reproduction, and aging.
5. Medical Biology:
Prospects for Neural Stem Cell Repair of Injured Spinal Cord
------------------------------------------------------------
Gene therapy and grafting techniques involving human stem cells
are being explored as possible methods for restoring function
following traumatic spinal cord injury.
(Includes related background material.)
6. Medical Biology:
On Aging and Cognitive Decline
------------------------------
The apparent continuum between normal aging, mild cognitive
impairment in old age, and Alzheimer's disease provides support
for the idea that Alzheimer's disease may be an accelerated form
of brain aging.
(Includes related background material.)
7. Brief Note:
Monarch Butterfly Magnetic Compass Paper Retracted
In Focus: On the History and Success of the Human Species
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1. CHEMICAL PHYSICS:
H(SUB3)+ IN CHEMISTRY AND ASTRONOMY
Molecular hydrogen [H(sub2)] is the normal form of hydrogen
gas found on Earth, the entity consisting of 2 hydrogen atoms
bound together by 2 shared electrons. Molecular hydrogen is also
found in so-called "molecular clouds" in interstellar space, cool
dense regions of interstellar matter within which atoms tend to
be combined into molecules. Molecular clouds are composed mainly
of molecular hydrogen at a density of 300 to 2000 molecules per
cubic centimeter.
Also found in molecular clouds is ionized molecular hydrogen
[H(sub3)+], an entity which plays an important role in diverse
fields from chemistry to astronomy. H(sub3)+ serves as a
benchmark in quantum chemistry, has been discovered in molecular
clouds and in the *diffuse interstellar medium, and has provided
a tool for characterizing the atmosphere of Jupiter.
... ... B.J. McCall and T. Oka (University of Chicago, US)
present a review of current research on H(sub3)+, the authors
making the following points:
1) The H(sub3)+ molecular ion consists of 3 protons bound by
2 electrons and can be thought of as a hydrogen molecule with an
attached extra proton. This ion is the dominant positively
charged ion in molecular hydrogen *plasmas. The entity was first
identified in 1911 by J.J. Thomson (1856-1940) using an early
form of mass spectrometry. H(sub3)+ lacks a stable electronic
state (necessary for electronic spectroscopy), and it also lacks
a permanent dipole moment (necessary for rotational
spectroscopy). The only spectroscopic probe of this ion is its
infrared rotation-vibration spectrum, which was first observed in
the laboratory in 1980 by Takeshi Oka.
2) During the past two decades, over 600 spectral lines of
H(sub3)+ in low-energy rotational-vibrational states have been
detected. Theoretical spectroscopists are now able to reproduce
this laboratory spectrum with high accuracy from first principles
and provide predictions of new lines to help guide laboratory
work. Because H(sub3)+ is the simplest polyatomic molecule, these
calculations serve as a benchmark for calculations of other
polyatomic molecules such as water.
3) At present, a controversy surrounds the recombination of
H(sub3)+ with electrons, a process which is the dominant
destruction mechanism in some plasmas. In the past three decades,
laboratory measurements of this recombination rate have differed
by 4 orders of magnitude. The situation has improved, but
discrepancies between different experiments remain, and the rate
is still uncertain within a factor of 10. In addition, the best
theoretical estimates of the recombination rate are 100 times
lower than the experimental data.
-----------
B.J. McCall and T. Oka: H(sub3)+ -- An ion with many talents.
(Science 17 Mar 00 287:1941)
QY: Benjamin J. McCall [bjmccall@uchicago.edu]
-----------
Text Notes:
... ... *diffuse interstellar medium: In general, the term
"interstellar medium" refers to the matter contained in the
region between the stars of our Galaxy, this matter constituting
approximately 10 percent of the Galactic mass. The interstellar
medium is not uniformly distributed through space, but contains
regions of high-density and low-density clouds. It is the
interstellar medium that provides the material from which new
stars are born.
... ... *plasmas: In general, in this context, a "plasma" is a
fully ionized gas consisting of ions and electrons moving freely.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
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2. PLANETARY SCIENCE:
THE MISSING ORGANIC MOLECULES OF MARS
One of the puzzles about the planet Mars is the fact that the
1976 Viking Mars Mission found no evidence at all of any organic
material on the Martian surface, including no evidence of organic
material expected to result from meteorite impacts. This result
has suggested that the surface layers of Mars (the Martian
regolith) may contain a powerful oxidant that converts all
organic molecules to carbon dioxide at a rate which is rapid
relative to the rate at which they arrive, and this idea is
currently influencing the planning of future Mars missions.
... ... S.A. Benner et al (4 authors at University of Florida
Gainesville, US) examine the idea of a potent Martian surface
oxidant in terms of what is known about the oxidation of organic
compounds generally and the nature of organics likely to come to
Mars via meteorites. The authors make the following points:
1) The authors suggest that nonvolatile salts of
benzenecarboxylic acids, and perhaps oxalic acid and acetic acid,
should be metastable intermediates of meteoritic organics under
oxidizing conditions. Salts of these organic acids would have
been largely invisible to the gas chromatography-mass
spectroscopy measurements made by the Viking Mars mission.
2) Experiments indicate that benzenehexacarbolic acid
(mellitic acid) is generated by oxidation of organic matter known
to come to Mars, is rather stable to further oxidation, and would
not have been easily detected by the Viking experiments.
3) The authors suggest that approximately 2 kilograms of
meteorite-derived mellitic acid may have been generated per
square meter of Martian surface over 3 billion years. How much
remains depends on decomposition rates under Martian conditions.
4) The authors suggest that as available data do not require
that the surface of Mars be very strongly oxidizing, some organic
molecules might be found near the surface of Mars, perhaps in
amounts sufficient to be a resource. Missions should seek these
organics and recognize that these complicate the search for
organics from entirely hypothetical Martian life.
5) The authors state: "As in any organic reaction, the
specific oxidant, specific ambient conditions, and specific
catalysts determine what intermediates will accumulate in the
oxidative degradation of organic compounds on Mars. Only by
missions to Mars can we learn these specifics to decide what has
actually happened to meteoritic organics and, by inference, to
other organics that might have come to the Martian surface."
-----------
S.A. Benner et al: The missing organic molecules of Mars.
(Proc. Natl. Acad. Sci. US 14 Mar 00 97:2425)
QY: Steven A. Benner [benner@chem.ufl.edu]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
Related Background:
MARS PATHFINDER MISSION: A SUMMARY
One year has passed since the Mars Pathfinder mission, and
various assessments of the mission have recently appeared. Jim
Bell (Cornell University, US), a member of the Mars Pathfinder
science team, reviews the mission with a particular focus on the
question of whether Pathfinder, as an instance of "faster and
cheaper" solar system exploration, also produced "better"
science. Bell makes the following points: 1) The Pathfinder
mission was a phenomenally successful demonstration of new
technologies and sheer engineering nerve -- a high-stakes gamble
that traded high risk for low cost and hit the jackpot. 2)
"Better" data means higher-quality data, not just more data, and
data that provides more definitive results than previous data.
The mission satisfies this definition of "better", and is viewed
by most scientists involved as a major scientific success in its
own right, and not merely a test flight. 3) The measurements
performed by Pathfinder fall into 5 categories: geology,
mineralogy/geochemistry, surface-material properties, atmospheric
science, planetary rotational dynamics. 4) Geology: Many of the
features observed at the landing site area are similar to
formerly flooded plains on Earth. But other processes involving
wind, volcanism, or impact could be responsible for many of the
observed landforms. A major geologic finding was that wind has
been an extremely important, and perhaps dominant, geologic
process on this part of Mars, and may have been the only major
agent of change for a huge span of time, conceivably several
billion years. 5) Mineralogy and geochemistry: The most striking
result was the finding by the Sojourner Alpha Proton X-Ray
Spectrometer that most of the rocks analyzed have a high silicon
content, higher than that of the Martian meteorite samples
gathered on Earth. If these Pathfinder site rocks are indeed
volcanic, there constituents suggest a more active subsurface
volcanic system than previously proposed. It was also found that
the dust of Martian dust storms has a high magnetic component,
and the data support the conclusion that the magnetic dust grains
consist at least partly of the iron oxide mineral maghemite. This
mineral is rare on Earth, usually forming in iron-rich water
solutions, which suggests that Martian dust may have formed in a
much wetter and perhaps warmer environment than exists today. 6)
Surface materials: The Pathfinder evidence indicates the
uppermost surface layer of soil at the site is extremely fine-
grained, like flour, consisting of particles only a few microns
in diameter. This top layer was probably formed by the gentle
settling of atmospheric dust. 7) Rotational dynamics: The data
from Pathfinder appear most consistent with Mars possessing a
relatively large metallic core, from 1300 to 2000 kilometers in
radius, which is approximately 40 to 60 percent of the overall
radius of the planet. In contrast, Earth's core occupies only 19
percent of its radius. 8) Atmospheric science: The Martian
atmosphere is complex and dynamic. For example, temperature at
the landing site increased by more than 20 degrees centigrade
after sunrise, and in the morning the air at the surface was more
than 10 degrees centigrade warmer than the air only a meter off
the ground. 9) The mission returned nearly 300 megabytes of data,
including more than 17,000 images. Only a fraction of the data
has yet been analyzed in detail or published in the peer-reviewed
literature. Analysis and interpretations are likely to continue
for some time.
QY: Jim Bell, Cornell University 607-255-2000.
(Sky and Telescope July 1998) (Science-Week 3 Jul 98)
For more information: http://scienceweek.com/swfr.htm
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3. MATERIALS SCIENCE:
FUNDAMENTAL QUANTUM MECHANICAL PHENOMENA IN SEMICONDUCTORS
If anyone needs another example of the reciprocal interaction of
basic science and technology, they might consider the doublet
quantum physics and semiconductor technology. More than 50 years
ago, quantum physics provided the original theoretical
foundations for the development and applications of
semiconductors in electronics. One of the consequences of
intensive applied research in semiconductors during the past
several decades has been the development of techniques for
precise fabrication of nanoscale condensed matter systems, and
now, completing the interaction loop, such fabricated condensed
matter systems are being used to explore fundamental questions in
quantum physics.
... ... D.S. Chemla and J Shah (2 installations, US) present a
review of quantum phenomena in semiconductors, the authors making
the following points:
1) The large dielectric constant and small effective mass in
a semiconductor allows a description of its electronic states in
terms of *envelope wavefunctions whose energy, time, and length
scales are mesoscopic, i.e., halfway between those of atomic and
those of condensed matter systems. This property makes it
possible to demonstrate and investigate many quantum mechanical,
many-body, and quantum kinetic phenomena with tabletop
experiments that would be nearly impossible in other systems.
This, along with the ability to custom-design semiconductor
nanostructures, makes semiconductors an ideal laboratory for
experimental investigations.
2) In experiments with semiconductors using the technique of
ultrafast nonlinear optical spectroscopy, results indicate that a
semiconductor behaves more like a strongly interacting system
than like an atomic system, and the authors suggest these results
provide insights into the physics of strongly interacting
systems, insights relevant to other condensed matter systems but
not easily accessible in other materials.
-----------
D.S. Chemla and J. Shah: Ultrafast dynamics of many-body
processes and fundamental quantum mechanical phenomena in
semiconductors.
(Proc. Natl. Acad. Sci. US 14 Mar 00 97:2437)
QY: Daniel S. Chemla, Univ. of Calif. Berkeley 510-642-6000.
-----------
Text Notes:
... ... *envelope wavefunctions: In quantum mechanics, a "wave
function" is a mathematical quantity analogous to the amplitude
of a wave, the quantity appearing particularly in the
Schroedinger wave equation. The most generally accepted
interpretation of the wave function is that if the wave function
is P, and |P| represents the absolute value of P, then
[|P|^(2)]dV represents the probability that a particle is located
within the volume element dV. The analogy between P and the
amplitude of a wave is entirely formal, and there is no
macroscopic physical quantity with which P can be identified. In
condensed matter physics, a current popular approach to
circumvent the quantum-level complexity of systems containing
10^(22) to 10^(23) particles per cubic centimeter is to use a
formalism which replaces real particles interacting through the
long-range Coulomb force with a smaller number of fictitious
"quasi-particles". In ultra-small engineered semiconductor
structures, quantum size effects appear whenever at least one of
the sample dimensions is of the order of or smaller than a length
scale characterizing the quasi-particles. In semiconductors,
quasiparticles, complex objects involving many atomic sites, can
be characterized by "envelope wavefunctions" whose natural
energy, time, and length scales are quite different from those of
an atomic system.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
THEORETICAL PHYSICS:
ON THE ORGANIZING PRINCIPLES OF MESOSCOPIC SYSTEMS
During the past decade, a number of researchers in many fields of
science have been drawn to the idea that mesoscopic systems,
i.e., systems in the length-scale range between angstroms and
centimeters, may be obeying a unique set of rules not apparent
from considerations of first principles. New ideas in science
always have a skeptical audience until enough experimental
evidence exists to be persuasive, but it is also true that new
ideas are more quickly brought to the attention of the science
community when they are proposed by prominent researchers. At
present, one of the chief proponents of this new approach to
mesoscopic systems is the theoretical physicist R.B. Laughlin,
who received the Nobel Prize in Physics in 1998 for his work on
the behavior of electrons in high magnetic fields at extremely
low temperatures (see related background material below).
... ... R.B. Laughlin et al (5 authors at 5 installations, US)
present an analysis of the behavior of mesoscopic systems, the
authors making the following points:
1) The authors suggest that a dogma central to much of
science is that knowledge of the underlying physical laws alone
is sufficient for the understanding of all things, even those
things that cannot be seen. But the dogma, the authors suggest,
is false. Although behavior of atoms and small molecules can be
predicted with reasonable accuracy starting from the underlying
laws of quantum mechanics, the behavior of large molecules
cannot, for the errors always eventually run out of control as
the number of atoms increases because of exponentially increasing
computer requirements. At the same time, however, very large
aggregates of particles have some astonishing properties (e.g.,
the ability to levitate magnets when they are cooled to cryogenic
temperatures) that are commonly acknowledged to be "understood".
How can this be? The authors suggest the answer is that these
properties are actually caused by collective organizing
principles that formally grow out of the microscopic rules, but
are in a real sense independent of them.
2) The authors point out that the miracles of nature
revealed by molecular biology are no less astonishing than those
found by physicists in macroscopic matter, and that their
existence leads one to question whether as-yet-undiscovered
organizing principles might be at work at the mesoscopic scale,
at least in living things. The authors state: "This is by any
measure a central philosophical controversy of modern science,
for a commonly held view is that there are no principles in
biology except for Darwinian evolution. But what if this view is
just a consequence of our inability to see? Indeed the rules of
self-organization at macroscopic length scales were not self-
evident at the time of their discovery, and were accepted as true
only after repeated confrontations with experiment left no
alternative. The existence of similar rules at the mesoscopic
scale would have profound implications for all of science, not
just biology, for noncrystalline matter often has curious and
poorly understood behavior suggestive of mesoscopic organization.
It is thus a question worth asking." The authors call the search
for the existence of mesoscopic "protectorates" (mesoscopic
domains exhibiting unique emergent principles) -- the proof or
disproof of organizing principles appropriate to the mesoscopic
domain -- the "middle way".
3) The authors suggest that the discovery of physical
principles at mesoscale lengths will reinforce the attack by
biologists on the mysteries of cellular function. In addition, a
framework for understanding mesoscopic organization "will be an
extraordinary help in the effort to create an entirely artificial
system with the complex adaptive behavior characteristic of life.
Such artificial systems should be capable of a variety of
functions that present biological systems cannot perform."
4) The authors conclude: "In any event, the applicability of
the science of mesoscale organization that we believe can be
developed will not be limited to the world between angstroms and
centimeters. Organization following similar principles may well
be manifested in astrophysics... Complex structures already have
been proposed for the exotic matter expected in neutron stars,
while ideas developed to explain mesoscopic organization on Earth
may be useful in explaining the origin of large-scale structure
in the Universe."
-----------
R.B. Laughlin et al: The middle way.
(Proc. Natl. Acad. Sci. US 4 Jan 00 97:32)
QY: Branko B. Stojkovic, Los Alamos National Laboratory, Los
Alamos, NM 87545 US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 7Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
FRACTIONAL ELECTRONS IN CONDENSED MATTER PHYSICS
Solid state physics, both experimental and theoretical, has been
undergoing a transformation during the past decade, and if one
must characterize the transformation it can perhaps be
accomplished by saying there is now an entire domain of
experimental observations in the condensed state that are best
described by a new set of rules that for the time being are
producing as many problems as solutions. What is involved is the
so-called "fractionalization" of the electron, with the quantum
theory of solids now possessing a considerable variety of
fractionated electron entities: composite fermions, composite
bosons, spinons, holons, heavy electrons, small polarons, and
entities called "quasi-particles". Since all of these entities,
including the fractional-charge electron, arise as theoretical
explanations for experimental observations, one could make an
argument that these entities are in effect virtual particles
produced by special constraints on condensed systems, but
evidently not all physicists would agree with the use of the term
"virtual", and in fact the differences may be one of semantics,
as was the early debate about the particle nature of
semiconductor holes. What is important is that the physics of the
condensed state is apparently in a fermentation stage, and we can
expect that out of this fermentation will come a new physics with
enormous practical applications. Philip W. Anderson (Princeton
University, US), in a review of the history of the physics of the
electron in condensed matter, emphasizes that study of electron
fragility in the quantum Hall regime has been one of the great
successes of the last decade, but the apparently simpler problem
of the motion of spin-charge separated electrons in one- and
two-dimensional systems is only beginning to yield to
understanding. Anderson predicts the motion problem will be a
solid problem for the next few decades.
QY: P.W. Anderson, Princeton Univ., Physics (609) 258-4400.
(Physics Today October 1997) (Science-Week 24 Oct 97)
For more information: http://scienceweek.com/swfr.htm
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4. EVOLUTIONARY BIOLOGY:
EXPERIMENTAL EVOLUTION AND AGING IN FRUIT FLIES
One of the important questions in evolutionary theory
concerns the evolutionary basis of aging (senescence). In
general, evolutionary theory predicts the impact of extrinsic
forces killing members of a population (extrinsic mortality) on
intrinsic mortality (i.e., lifespan), growth, maturation, body
size, and reproduction. According to evolutionary theory, when
extrinsic mortality rates increase, the increase lowers the
probability of survival to a given age and causes the strength of
*selection to decline faster with age, which in turn makes an
increase in intrinsic mortality "more affordable" or "less
avoidable". From this idea follows a central prediction of the
evolutionary theory of aging: Higher extrinsic mortality rates
should lead to higher intrinsic mortality rates and a decrease in
lifespan.
A method called "experimental evolution" has recently come
into use to test certain predictions of evolutionary theory. In
contrast to artificial selection, in which the experimenter
determines the trait to be selected, in experimental evolution
the experimenter creates the conditions under which a prediction
should hold and then allows the evolving population to select
those traits that solve the problem posed by the experimental
condition. This approach has been used with bacteria, algae,
certain fish, and temperature adaptation in flies. The basis for
the method is that evolution is more rapid than previously
thought, and is thus amenable to experimental manipulation.
... ... S.C. Stearns et al (4 authors at University of Basel,
CH) report an evolutionary experiment on the fruit fly
Drosophila, the experiment testing *life-history theory and the
evolutionary theory of aging. The experiment has been ongoing
since 1993. The authors report that as theory predicts, imposed
higher extrinsic mortality rates did lead to the evolution of
higher intrinsic mortality rates, to shorter lifespans, to
decreased age and size at egg-hatching (eclosion), and to a
shift in peak fecundity to earlier in life. The authors suggest
these results confirm the key role of extrinsic mortality rates
in the evolution of growth, maturation, reproduction, and aging,
and that in this case this occurred with a selection regime that
maintained selection forces on fertility throughout life while
holding population densities constant.
-----------
S.C. Stearns et al: Experimental evolution of aging, growth, and
reproduction in fruit flies.
(Proc. Natl. Acad. Sci. US 28 Mar 00 97:3309)
QY: S.C. Stearns [steve.stearns@unibas.ch]
-----------
Text Notes:
... ... *selection: In this context, the term "selection" refers
to a composite of all the forces that cause differential survival
and differential reproduction among genetic variants. When the
selective agencies are primarily those of human choice, the
process is called "artificial selection"; when the selective
agencies are not of human choice, the process is called "natural
selection".
... ... *life-history theory: In general, life-history theory is
concerned with the evolutionary adaptations within a biological
lineage that involve the timing of reproduction, fecundity,
longevity, etc.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. MEDICAL BIOLOGY:
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."
-----------
S.S.W. Han and I. Fischer: Neural stem cells and gene therapy:
Prospects for repairing the injured spinal cord.
(J. Amer. Med. Assoc. 3 May 20 283:2300)
QY: S.S.W. Han, MCP Hahnemann University School of
Medicine, Philadelphia, PA US.
-----------
Text 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.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEUROBIOLOGY: 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.
-----------
M.S. Ramer et al: Functional regeneration of sensory axons into
the adult spinal cord.
(Nature 20 Jan 00 403:312)
QY: Matt S. Ramer [matt.ramer@kcl.ac.uk]
-----------
Text 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.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
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6. MEDICAL BIOLOGY: ON AGING AND COGNITIVE DECLINE
In humans, the cognitive decline usually associated with
advanced age is mirrored by certain gross and microscopic
anatomical changes in the brain, but since the relation between
cognition and brain anatomy at any level is at present far from
clear, there are more questions than answers in any discussion of
the aging human brain in health and disease.
The term "dementia" refers to a apparent structurally
caused permanent or progressive decline in several dimensions of
intellectual function, the decline interfering substantially with
the individual's normal social or economic activity. There are
various forms of dementia produced by various causes. Alzheimer-
type dementia (Alzheimer's disease) is apparently related to what
appear to be specific cellular and histological degenerative
processes, with loss of cells from several specific brain areas,
the brain showing moderate to marked atrophy. Memory loss is the
most prominent early symptom.
... ... Bruce A. Yankner (Harvard University, US) presents an
essay on the relation of cognitive decline to aging, the author
making the following points:
1) Several evolutionary arguments have been advanced to
account for either the preservation or loss of cognitive function
with aging: a) It has been argued that dementia occurs at an age
that was rarely achieved during much of human evolution, and
therefore could not have been subject to selective evolutionary
pressure. b) It has also been suggested that the preservation of
function in old age conferred a selective advantage by creating a
class of older individuals who could care for children and thus
free the mother for other activities such as foraging or food
preparation. c) But an equally plausible argument is that the
burden of an aging population would have produced selection for
rapid cognitive and physical decline after the age of maximal
reproductive fitness. These arguments are all based on the idea
that cognitive decline is linked in some way to the aging
process.
2) Although Alzheimer's disease was first recognized as a
pathological entity in 1907, it was not until late in the 20th
century that the concomitant molecular components of the
pathologic process -- *amyloid plaques and *neurofibrillary
tangles -- were elucidated, and an intense debate continues over
which of these two lesions (or some other process) is the cause
of dementia. Proponents of the so-called "amyloid hypothesis"
argue that all genetic mutations that give rise to Alzheimer's
disease also predispose to amyloid deposition, and that amyloid
is neurotoxic. But opponents of this hypothesis point out that
amyloid plaques correlate poorly with the degree of dementia, and
that animal models of amyloid-plaque deposition do not replicate
the neurodegenerative changes in a brain with Alzheimer's
disease.
3) The author points out that in the 1970s researchers
demonstrated that dementia of the Alzheimer type is a disease
rather than an inevitable consequence of aging. Plaques and
tangles may appear in the brains of many cognitively normal aged
individuals, but the degree of pathology is usually much greater
in Alzheimer's disease. This apparently contradicts the idea
that senility is a normal process, and has provided hope to many
that Alzheimer's disease can be prevented or reversed.
4) The author points out that the apparent continuum
between normal aging, mild cognitive impairment, and Alzheimer's
disease provides support for the idea that Alzheimer's disease
may be an accelerated form of brain aging. This in turn raises an
old question: If we live long enough, will we all become
demented? Furthermore, if neurodegenerative diseases in general
represent pathological variants of the aging process, then
understanding mechanisms of normal brain aging may lead to
fundamental insights into neurodegenerative disease mechanisms.
5) The author concludes: "The 20th century has witnessed a
dramatic prolongation of lifespan, but little progress in
preventing age-related cognitive decline. The anticipated further
prolongation of human lifespan in the 21st century will be a
hollow victory unless cognitive function can also be preserved."
-----------
Bruce A. Yankner: A century of cognitive decline.
(Nature 9 Mar 00 404:125)
QY: Bruce A. Yankner, Harvard Univ. Medical School 617-432-1550.
-----------
Text Notes:
... ... *amyloid plaques: The term "amyloid" ("starch-like")
refers to a variety of polypeptide molecules defined by their
properties, particularly by their tendency to arrange in a
twisted *beta-pleated fibrillar structure. Amyloid is in general
a proteinaceous material, deposits of which have been classically
noted to occur in the brains of Alzheimer's disease and older
Down syndrome patients, and to a much lesser degree, in
association with normal aging. Amyloid material consists
primarily of a highly aggregated 42-amino acid polypeptide called
"beta-amyloid". Its precursor, *cloned in 1989, is a larger
protein called "amyloid precursor protein", fragments of which
have been reported to show both neurotoxic and neurotrophic
effects. In general, a "plaque" is a sharply defined small area,
in this context a microscopic deposit.
... ... *beta-pleated: In general, protein chains fold into
alpha-helices or beta-sheet structures. The beta-sheet is a
protein structure where the peptide is extended and stabilized
by hydrogen bonding between NH and CO groups of different
polypeptide chains or of separate regions of the same chain.
... ... *cloned: In this context, the term "cloned" refers to the
isolation and replication/amplification of the gene encoding a
particular protein in order to produce a quantity of that
protein.
... ... *neurofibrillary tangles: A neurofibril is an
intracellular filamentous structure seen with the light
microscope and composed of ultramicroscopic tubular and
filamentous protein arrays (neurotubules and microfilaments).
The function of these structures is unknown. In general, the
term "neurofibrillary tangle" refers to intraneuronal
accumulations of neurofibrils that assume twisted contorted
patterns.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON ABNORMAL PROTEIN AGGREGATION IN NEURODEGENERATION
... ... E.H Koo et al (3 authors at 3 installations, US)
present a review of current research concerning amyloid
diseases, the authors making the following points:
1) Alzheimer's disease and Parkinson's disease are the most
common forms of age-related neurodegenerative disorders. The
pathogenesis of these and other neurodegenerative diseases
remains unclear, and effective treatments are currently lacking.
However, recent studies from three diverse disciplines --
neuropathology, genetics, and biophysics -- have begun to
converge on a novel target for therapeutic attack: Ordered
protein aggregation.
2) Abnormal protein aggregation characterizes many, if not
all, neurodegenerative disorders, not just Alzheimer's disease
and Parkinson's disease, but also *Creutzfeldt-Jakob disease,
*motor neuron disease, the large group of *polyglutamine
disorders, including *Huntington's disease, as well as diseases
of peripheral tissue such as *familial amyloid polyneuropathy.
Many of these diseases were originally identified by their
histochemical staining property, hence their designation as
amyloid (starch-like). Subsequently, it became apparent that
amyloid deposits contain extremely insoluble protein fibrils that
share similar morphological features (80 to 150 angstrom
fibrils), but comprise many different proteins with no obvious
amino-acid sequence similarity.
3) The correlation and co-localization of protein fibrils
with tissue degeneration suggests that fibrillization either
contributes to cell death or is an inseparable epiphenomenon.
Recent genetic studies support the former possibility. Mutations
that cause Huntington's disease, Creutzfeldt-Jakob disease, and
familial amyloid polyneuropathy are localized to the genes
encoding the fibril-forming proteins huntington, prion-protein,
and transthyretin, respectively. In Alzheimer's disease and
Parkinson's disease as well, rare early-onset disease forms have
been linked to mutations in the fibril-forming proteins amyloid
beta-protein or its precursor amyloid precursor protein
(Alzheimer's disease), or alpha-synuclein (Parkinson's disease).
Increasing evidence thus favors the postulate that protein
fibrillization is an early and critical process in all of these
diverse diseases (although the fibril itself may not be the
culprit). The authors suggest it therefore follows that
inhibition of fibril formation could be a viable therapeutic
strategy, and that is therefore critical to develop an
understanding of the process of fibril formation at a molecular
level.
4) The authors conclude: "Recent biophysical studies have
provided important insights into the mechanisms of amyloid fibril
formation. In turn, this information may prove to be essential in
the development of new therapeutic strategies that will allow us
to critically test the amyloid hypothesis in both animal models
and in human neurodegenerative diseases."
-----------
E.H. Koo et al: Amyloid diseases: Abnormal protein aggregation in
neurodegeneration.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9989)
QY: Edward H. Koo, Univ. of Calif. San Diego 619-534-2230.
-----------
Text Notes:
... ... *beta-pleated: See main report.
... ... *cloned: See main report.
... ... *Creutzfeldt-Jakob disease: Until 30 years ago,
Creutzfeldt-Jakob disease was an obscure form of dementia
unknown to most physicians. The name is now familiar to the
medical community as the major *transmissible spongiform
encephalopathy (or prion disease) in humans, and familiar to
research scientists because of its strange causative agent
(*prions) that exhibit apparently novel modes of replication and
transmission.
... ... *transmissible spongiform encephalopathy: In general, an
encephalopathy is any disorder of the brain. In this context, the
term "spongiform" refers to the sponge-like texture or appearance
of the brain upon autopsy. In this context, the term
"transmissible" means capable of being transmitted from one
individual to another.
... ... *prions: Prions are a class of poorly understood proteins
implicated in a number of exotic human neurological diseases and
in some common animal diseases such as sheep scrapie and bovine
spongiform encephalopathy in cattle ("mad cow disease").
... ... *motor neuron disease: Motor neurons are spinal cord
nerve cells that control voluntary muscles).
... ... *polyglutamine disorders: In these disorders (e.g.,
Huntington's disease), a genetic mutation in a particular coding
region of DNA produces an unstable expanded trinucleotide repeat
(cytosine-adenosine-guanosine), resulting in a protein with an
expanded glutamate sequence. The normal and abnormal functions of
this protein (called "huntingtin") are unknown. ("Glutamine" is
the trivial name for the gamma-amide of glutamic acid.)
... ... *Huntington's disease: (Huntington's chorea) First
described by George Huntington (1850-1916), the disease attacks
specific regions of the brain (e.g., caudate nucleus and
putamen), and leads to insanity and eventual death.
... ... *familial amyloid polyneuropathy: In general, a
"polyneuropathy" is a disease process involving a number of
peripheral nerves (i.e., nerve cells and nerve fibers outside the
central nervous system, with the central nervous system
consisting of the brain and spinal cord). The term "familial" is
used for any disease that affects more members of the same family
than can be accounted for by chance (Note: the term "familial" is
commonly but incorrectly used to mean "genetic").
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 22Oct99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE MOLECULAR BASIS OF THE NEURODEGENERATIVE DISEASES
The term "neurodegenerative disorders" is loosely applied to a
group of chronic and progressive diseases of the nervous system,
all of which are characterized by selective and symmetric loss of
neurons in motor, sensory, or cognitive systems. Some
neurodegenerative diseases are extremely cell-specific, causing
loss of only one type of nerve cell, while the neuronal loss
caused by other neurodegenerative diseases is more general,
affecting a wide variety of nerve cells. In all cases, these
diseases primarily affect nerve cells and not other cells.
... ... Joseph B. Martin (Harvard University, US) presents a
review of the molecular basis of neurodegenerative disorders, the
author making the following points:
1) Delineation of the patterns of cell loss and
identification of disease-specific cellular markers have aided in
the classification of these diseases. For example:
... ... a) *Alzheimer's disease is characterized by *senile
plaques, *neurofibrillary tangles, neuronal loss, and
*acetylcholine deficiency.
... ... b) *Parkinson's disease is characterized by *Lewy bodies
and depletion of *dopamine.
... ... c) *Amyotrophic lateral sclerosis is characterized by
*cellular inclusions and swollen *motor axons.
... ... d) *Huntington's disease is characterized by loss of
*gamma-aminobutyric acid-containing neurons of the *neostriatum.
2) *Mendelian inheritance can be demonstrated in many
neurodegenerative disorders. In some diseases, such as
Huntington's disease, a family history of the disease can be
ascertained in almost every case, whereas in other diseases,
such as Alzheimer's disease, Parkinson's disease, and
amyotrophic lateral sclerosis, about 1 to 10 percent of cases
are inherited. In other conditions, such as *spinocerebellar
ataxia, the syndromes have been difficult to classify because of
clinical overlap, and variants can only be differentiated by
genotyping after the mutant genes have been identified.
3) In families with the above and other neurodegenerative
disorders, *linkage analysis, *positional cloning, and searches
for mutations in candidate genes have been extremely productive.
These efforts, which began in the 1980s with the search for the
gene that causes Huntington's disease, have led to the
identification of mutant genes in more than 50 disorders of the
nervous system.
4) The genetic anomalies that cause neurodegenerative
diseases are varied and complex. In some diseases, several genes
have been found, each of which leads to a similar clinical and
pathological syndrome, with only variations in the age at onset
and the rate of progression to suggest that there are differences
in pathogenic mechanisms. In other disorders, errors in DNA
replication resulting in an increased number of *nucleotide
triplet repeats are associated with selective patterns of
neurodegeneration.
5) In general, the various pathologies of the
neurodegenerative disorders apparently involve abnormalities in
the transport, degradation, and aggregation of proteins that lead
to cell-specific changes and ultimately to neuronal death,
probably by *apoptosis.
-----------
Joseph B. Martin: Molecular basis of the neurodegenerative
diseases.
(New England J. Med. 24 Jun 99 340:1970)
QY: Joseph B. Martin [joseph_martin@hms.harvard.edu]
-----------
Text Notes:
... ... *Alzheimer's disease: Alzheimer's disease is
characterized by the presence of large numbers of extracellular
agglomerations (plaques) and intracellular *neurofibril tangles
in the cerebral cortex of the brain. There is also a massive
neuronal cell loss. While plaques and tangles are found in normal
aging brains, they are more numerous and widespread in
Alzheimer's disease. The major protein component of the plaques
is a 39 to 43 amino acid peptide called beta-amyloid, which is
now known to be derived from a much larger protein called the
amyloid precursor protein. This latter protein has been found to
be expressed in every tissue studied.
... ... *neurofibril: A filamentous structure seen with the light
microscope and composed of ultramicroscopic tubular and
filamentous protein arrays (neurotubules and microfilaments). The
function of these structures is unknown.
... ... *senile plaques: In general, the term "plaque" refers to
a deposit. In this context, the deposits are usually
extracellular protein agglomerations.
... ... *neurofibrillary tangles: A neurofibril is a filamentous
structure seen with the light microscope and composed of
ultramicroscopic tubular and filamentous protein arrays
(neurotubules and microfilaments). The function of these
structures is unknown.
... ... *acetylcholine: In general, a neuron has input extensions
(dendrites) and a single but usually branched output extension
(axon). The junction between the terminal of a neuron's axon and
another neuron is called a "synapse". When studying the synapse,
the first neuron is called the "presynaptic" neuron, and the
second neuron is called the "postsynaptic" neuron.
Neurotransmitters are chemical substances released at the
terminals of nerve axons in response to the propagation of an
impulse to the end of that axon. The neurotransmitter substance
diffuses into the synapse, the junction between the presynaptic
nerve ending and the postsynaptic neuron, and at the membrane of
the postsynaptic neuron the transmitter substance interacts with
a receptor. Depending on the type of receptor, the result may be
an excitatory or an inhibitory effect on the postsynaptic nerve
cell. At present acetylcholine, 5 amines, 4 amino acids, 2
purines, and more than 28 peptides are known to be
neurotransmitters.
... ... *Parkinson's disease: A neurological disorder first
described by James Parkinson (1817) and associated with
degeneration of a specific small region of the brain and a
resultant loss of projection to several important brain centers.
... ... *Lewy bodies: Intracytoplasmic neuron inclusions
especially seen in Parkinson's disease.
... ... *dopamine: Dopamine is a neurotransmitter found in
several major areas of the brain, and the degeneration of
so-called dopamine neurons is apparently involved in Parkinson's
disease. Dopamine has also been implicated in the intricate
effects of the psychostimulating drugs associated with drug
abuse. The dietary precursors of dopamine are phenylalanine and
l-tyrosine.
... ... *Amyotrophic lateral sclerosis: A progressive disease of
motor neurons (spinal cord nerve cells that control voluntary
muscles). 50 percent of patients die within 3 years of the first
symptoms.
... ... *cellular inclusions: A general term for residual
entities in cytoplasm produced by metabolism; in this context,
granules or crystals not found in normal cells.
... ... *motor axons: Axons of motor neurons. They can be quite
long: a spinal cord motor neuron controlling muscles in a toe,
for example, has a cell body in the spinal cord and an axon that
runs as a single extension from the spinal cord down to the toe
musculature. Such axons usually propagate impulses at high
velocity (e.g., 100 meters per second).
... ... *Huntington's disease: (Huntington's chorea) First
described by George Huntington (1850-1916), the disease attacks
specific regions of the brain (e.g., caudate nucleus and
putamen), and leads to insanity and eventual death.
... ... *gamma-aminobutyric acid: A widely distributed brain
neurotransmitter.
... ... *neostriatum: This is a term used when considering the
two brain regions, the caudate nucleus and the putamen, as a
single anatomical entity.
... ... *Mendelian inheritance: In general, any inheritance
scenario following the classical Mendelian laws governing the
inheritance of chromosomal genes via the transmission of
chromosomes to subsequent generations, and producing inheritance
of single-chromosome-locus traits.
... ... *spinocerebellar ataxia: In general, an ataxia is an
inability to coordinate muscle activity during voluntary
movement. Spinocerebellar ataxia is the most common hereditary
ataxia. The spinocerebellar degenerative disorders are a group of
diseases involving neurons in several nervous system structures,
including the spinal cord and cerebellum.
... ... *linkage analysis: In general, an analysis of chromosomal
gene location based upon inheritance patterns.
... ... *positional cloning: In general, the identification of a
gene responsible for a disease from a knowledge of its position
in the human genome, and no assumptions about the gene product.
Inherited disease genes identified by positional cloning include
Duchenne muscular dystrophy and Huntington's disease.
... ... *nucleotide triplet repeats: (coding triplet repeats;
codon repeats) In general, a codon is the basic genetic coding
unit, a triplet of nucleotides in DNA. A codon repeat is a string
of identical codons which if expressed produce a string of
identical amino acids in a protein.
... ... *apoptosis: In general, programmed cell death produced by
control mechanisms designed to destroy defective cells.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jul99
For more information: http://scienceweek.com/swfr.htm
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7. BRIEF NOTE:
MONARCH BUTTERFLY MAGNETIC COMPASS PAPER RETRACTED
This is a cautionary tale, an example of how the
extraordinary complexity of biological organisms, systems whose
operating variables are often unknown, can lead researchers
astray. In November 1999, a research team (4 authors at the
University of Kansas Lawrence, US) reported that fall migratory
monarch butterflies, tested for their directional responses to
magnetic cues under three conditions, amagnetic, normal, and
reversed magnetic fields, showed three distinct patterns: In the
absence of a magnetic field, monarchs lacked directionality as a
group; in the normal magnetic field, monarchs oriented to the
southwest with a group pattern typical for migrants; when the
horizontal component of the magnetic field was reversed, the
butterflies oriented to the northeast. In contrast, nonmigratory
monarchs lacked directionality in the normal magnetic field. The
authors suggested the results were "a direct demonstration of
magnetic compass orientation in migratory insects."
Four months later, in March 2000, the authors retracted
their paper, noting the following: "The positive response to
magnetic fields in two experiments cannot be repeated. Further
experiments show the false positives in these tests result from a
positive [directionality of movement (taxis)] by the butterflies
to the light reflected off the clothing of the observers. We
therefore retract our report. We regret the inconvenience that
publication of this study may have caused."
The Editors of ScienceWeek extend their compliments to the
authors for their prompt retraction, an effort made in the
interest of the science community, and a salient example of good
science.
-----------
J.A. Etheredge et al: Monarch butterflies (Danaus plexippus L.)
use a magnetic compass for navigation.
(Proc. Natl. Acad. Sci. US 23 Nov 99 96:13845)
QY: Orley R. Taylor [monarch@ukans.edu]
-----------
J.A. Etheredge et al: Retraction
(Proc. Natl. Acad. Sci. US 28 Mar 00 97:3782)
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Jun00
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IN FOCUS: ON THE HISTORY AND SUCCESS OF THE HUMAN SPECIES
"The paleontological and archaeological evidence... suggests that
modern H. sapiens has populated the world, from a birthplace in
Africa or the Middle East, in the last 40,000 years or so. This
would imply that the modern human races have differentiated in
this very short time. Confirming evidence has come from molecular
studies, which find that there are only minute inter-racial
genetic differences. Several studies of human DNA have also
suggested an African origin for all human races 200,000 to
100,000 years ago. The record of human evolution seems to show an
ever-quickening pace of change. Major innovations have occurred
ever-more rapidly: bipedalism (10 to 5 million years ago;
enlarged brain (3 to 2 million years ago); stone tools (2.5
million years ago); wide geographic distribution (2 to 1.5
million years ago); fire (1.5 million years ago); art (35,000
years ago); agriculture and the beginning of global population
increase (10,000 years ago). The rate of population increase was
about 0.1 percent per annum [10,000 years ago], rising to 0.3
percent per annum in the 18th century, and [to] about 2.0 percent
per annum today. In other words, the total global human
population will more than double during the lifetime of any
individual born today. In numerical terms at least, Homo sapiens
has been spectacularly successful."
-----------
Michael J. Benton: _Vertebrate Paleontology_
(Blackwell Science, London 2000, p.388)
-------------------
SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
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The first issue of SCIENCE-WEEK appeared May 1, 1997, and it has
been published regularly each week since that date. SW is
designed to cross existing conceptual and linguistic barriers
between the scientific disciplines. In general, the biology is
written for physicists and chemists, and the physics and
chemistry are written for biologists, with an attempt to retain
some exactitude in the particular science involved in the news.
These are the aims. Undoubtedly, we are not always successful,
and for that we apologize. In any case, what we hope is that our
readers are reading out of their fields more than in their
fields, since that is the essence of this publication.
We welcome comments, suggestions, and criticisms from our
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welcome. Editorial contact: [editors@scienceweek.com].
Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner
Copyright (c) 1997-1999 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved
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