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.

August 27, 1999 -- Vol. 3 Number 35

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There is no national science just as there is
no national multiplication table; what is national
is no longer science.
-- Anton Chekhov (1860-1904)

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Contents of This Issue:

1. On the Birth and Life of Neutron Stars
2. Amino Acids in the Martian Meteorite Nakhla
3. On Stratospheric Ozone
4. Molecular Biology: Integrin Signaling
5. Orphan Nuclear Receptors and Reverse Endocrinology
6. On the Biology of Hair Follicles

In Focus: Quantum Reaction Dynamics

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1. ON THE BIRTH AND LIFE OF NEUTRON STARS
In this century, astronomy and astrophysics have contributed
greatly to what might be called the "Hall of Wonders", the
gallery of real-world spellbinding objects. Perhaps the two most
outstanding contributions to this gallery are black holes and
neutron stars. Black holes, born of the death of super-massive
stars, are in this century the most spectacular astronomical
objects known; neutron stars, born of the death of merely massive
stars, are not far behind on any gauge of the extraordinary. A
neutron star is an extremely dense and compact star that has
undergone gravitational collapse to such an extent that much of
the material has been compressed into neutrons. Such stars were
theoretically postulated in the 1930s, but it was not until 1967
that their existence was actually confirmed by observations.
Neutron stars are believed to form when a massive star exhausts
its fuel and the mass of the stellar core remaining after a
consequent supernova explosion exceeds 1.4 solar masses (the
Chandrasekhar limit). With diameters of only 10 to 15 kilometers,
intense magnetic fields [e.g., 10^(8) tesla], and extremely rapid
spin (e.g., as much as 500-700 rotations per second), young
neutron stars are evidently responsible for various intrinsic
pulsation phenomena, and thus are called "pulsars". It is also
thought that gamma-ray bursts may have neutron star origins.
Models of the structure of neutron stars have been derived from
study of the sudden changes in pulsar spin rates ("glitches").
... ... Joshua N. Winn (Massachusetts Institute of Technology,
US) presents a review of current research on neutron stars, the
author making the following points:
     1) Perhaps one problem with the "jaw-dropping" statistics
concerning pulsars is that the statistics are too extreme. Using
Earthly analogies, it is difficult to comprehend an entity with a
density of 10^(14) grams per cubic centimeter [*Note #1] in a
gravitational field that produces an acceleration of free fall
(g) 600 billion times that produced at the surface of Earth. It
is even more difficult to visualize such a dense city-size object
"as it spins furiously, squirting plasma (ionized gases) from
electric arcs near the poles of its inconceivable magnetic field
and swirling up a superfluid of neutrons in its interior." But
after 30 years of research, enough is known about pulsars to
reconstruct the outlines of their existences.
     2) The newborn neutron star, produced by a dead massive
star, possesses four salient characteristics: a) a fast rotation
rate, typically 50 times per second; b) a directed velocity in
excess of 1000 kilometers per second that may be caused by
violent asymmetry in the supernova explosion of the parent star;
c) extreme temperature due to the small radiative surface area
(e.g., temperatures of 100,000 to 1 million degrees kelvin; d) an
immense magnetic field, typically a million times stronger than
that of the Earth. (The author notes: "It is easier to write the
figure 10^(12) gauss than to ponder its zeros.")
     3) A spinning magnet generates voltages, as was demonstrated
by Michael Faraday in 1831, and a neutron star is a gargantuan
spinning magnet, the resultant voltage tearing electrons,
positrons, and ions from its surface and flinging them outward
into space along the magnetic field lines. This is the so-called
"pulsar wind", whose effects can observed in the turbulence of
gas clouds associated with supernova, the so-called "pulsar-wind
nebula."
     4) A second consequence of the intense magnetic field of the
spinning neutron star is the pulsation of the pulsar. The
magnetic poles of neutron stars emit narrow beams of radiation in
addition to the pulsar-wind, and since the magnetic axis is
offset from the spin axis, the beams execute an oscillation in
space observed from Earth as a pulsing of radiation. The
pulsation rate of neutron stars declines with their age, and the
pulse period (P) and the "spin-down rate" ("P-dot"; the time-
derivative of P) are at present the two most important
observables concerning pulsars. A plot or P-dot versus P of the
approximately 700 pulsars that have been identified has been
useful in research on these objects in the same way that the
*Hertzsprung-Russell diagram, plotting luminosity versus
temperature, has been useful in understanding the general
evolution of ordinary stars.
     5) Most pulsars slow down at a steady rate, but a few
pulsars exhibit slight and sudden "glitches" in their periods,
and these glitches have been used as the basis for studying the
structure of pulsars. It is presently believed that glitches
probably originate beneath the thin crust of a pulsar, in a dense
mantle of heavy nuclei permeated by a *superfluid of neutrons,
and that the *angular momentum of the superfluid is broken up
into discrete quantized vortices which migrate outward to the
crust as the neutron spin-rate decreases. It is believed the
migrations and surface eruptions of superfluid quantum vortices
(which transfer angular momentum to the crust) cause the sudden
changes in spin-rate which are observed as "glitches" in pulsar
behavior [*Note #2].
-----------
Editor's note: In addition to the background material below, see
the SW Focus Report "The Death of Stars" available at URL
[http://scienceweek.com/swfr002.htm]
-----------
Joshua N. Winn: The life of a neutron star.
(Sky & Telescope July 1999)
QY: Joshua N. Winn, Mass. Inst. of Technology 617-253-1000.
-----------
Text Notes:
... ... *Note #1: At such a density, the entire human species
could exist as an object the size of small marble -- with the
total mass of the entire human species retained: i.e., the marble
would weigh approximately 10^(14) grams.
... ... *Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type (luminosity vs. temperature), and is one of the most useful
diagrammatic aids in astrophysics. The course of a star's
evolution can be traced as a particular path in the H-R diagram,
with the paths of various types of stars showing significant
differences.
... ... *superfluid: In general, a "superfluid" is a fluid that
flows without any resistance (i.e., zero viscosity).
... ... *angular momentum: The momentum of a body by virtue of
its rotation and/or orbital revolution. It is a conserved
quantity, and as a consequence a body spins faster as it becomes
smaller.
... ... *Note #2: Quantum vortices were discussed in the last
issue of SW (20 Aug 99 #34) in connection with the superfluid 
behavior of liquid helium-3. Here is the material from that issue
on quantum vortices: A quantum vortex is a type of flow pattern
exhibited by superfluids under certain experimental conditions,
e.g., liquid helium in a rotating container. The term "vortex"
designates the familiar whirlpool pattern where the fluid moves
circularly around a central line and the velocity decreases in
inverse proportion to the distance from the center. A superfluid
is considered to be characterized by a macroscopic
quantum-mechanical wave function that locks the superfluid into a
*coherent state. This forces certain mathematical constraints on
the wave function, so that for a superfluid in a rotating
container, the system (the wave equation for the system) produces
a lattice of quantized vortex lines, each line the axis of a
microscopic vortex, with the entire array of vortex lines
rotating rigidly with the container. The essential idea is that
when superfluid helium is in a rotating container, the
mathematics of the system wave function are such that a set of
discrete microscopic vortex states are produced by each
particular set of boundary conditions, and these microscopic
vortex states are experimentally observable [*Note #3]. In short,
the result is a system where the "quantum world" becomes visible
on a macroscopic scale.
... ... *coherent state: In quantum physics, coherence is matter
of locking of phase differences between wave functions. The wave
functions of two or more particles are said to be coherent if the
phase difference between their wave functions remains constant.
In general, a perfectly coherent system of particles can be
described by a single macroscopic wave function.
... ... *Note #3: Below a certain rotation speed threshold, no
vortices exist, and the superfluid remains at rest while the
container rotates (the Landau state). At the threshold speed, the
first vortex appears and corresponds to the first excited
rotational state of the system. If the container continues to
accelerate, additional quantized vortices appear, and at any
given speed the vortices form a regular array that rotates with
the vessel.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99
-------------------
Related Background:
ON NEUTRON STARS
During the terminal stages of the evolution of a star, part of
the mass of the star is blown off and lost. If the remnant mass
is between 1.4 and 2 to 3 solar-masses, the star will collapse
into a neutron star, a body with a radius of only 10 to 15
kilometers, but with a core so dense that its component protons
and electrons have merged into neutrons. The average density of a
neutron star is 10^(15) grams per cubic centimeter and the weight
of an object on the surface of a neutron star would be 10^(11)
times its weight on the surface of the Earth. Neutron stars
apparently have an outer shell of iron, but it is iron like no
Earth iron, an iron of 4 orders of magnitude greater density.
Theory predicts that a neutron star should rotate very rapidly,
be extremely hot, and have an intense magnetic field. *Pulsars,
sources of pulsed radio energy, are evidently spinning neutron
stars which emit beams of radiation from their magnetic poles. A
few pulsars have been found in *binary systems, and the empirical
estimated masses of the pulsars are consistent with the masses
predicted by neutron star models. ... ... L. Bildsten and T.
Strohmayer (2 installations, US) present a review of current
research concerning neutron stars, the authors making the
following points: 1) With a density comparable to that of an
atomic nucleus, a neutron star provides an extreme environment
for fast and violent phenomena. Matter orbiting a neutron star
can have a period as short as a millisecond. When such matter
crashes into the star (i.e., is "accreted" by the star), such
matter can be moving at one-third the speed of light. In general,
because their behavior can vary over readily observable
timescales, neutron stars can be rich sources of information
about nuclear physics, general relativity, and astrophysics. 2)
Though relatively elusive, neutron stars have been detected and
studied over a broad range of electromagnetic frequencies, from
radio frequencies to *gamma rays. To date, astronomers have
identified more than 1000 of the estimated 10^(8) neutron stars
in our galaxy. New orbiting astronomical satellites have produced
recent rapid growth in our knowledge of these objects, with much
of the progress occurring in our understanding of neutron stars
that undergo sudden large energy releases. 3) Although most
neutron stars have been discovered as radio pulsars, only a small
fraction of the radiated energy of a neutron star (typically
approximately 10^(-5)) is expected to be radio emission energy.
Most of the energy instead departs as photons with energies above
10^(8) *electronvolts. 4) The precise timing of radio pulsars has
yielded astonishing astronomical discoveries, such as multiple
Earth-mass planets orbiting a neutron star, and the direct
confirmation of the loss of orbital angular momentum due to
gravitational radiation in a double neutron star binary system
(for which Russell Hulse and Joseph Taylor received the 1993
Nobel Prize in Physics). 4) The brightest accreting neutron stars
reside in binary systems and accrete matter from their
companions. These accreting neutron stars typically have
luminosities more than a thousand times that of the Sun. 5) There
is every reason to believe that new classes of neutron stars will
be discovered by continued observations from the currently
orbiting satellites combined with the international fleet of new
x-ray and gamma-ray satellites planned for launch during the next
two years.
-----------
Editor's note: In addition to the background material below, see
the SW Focus Report "The Death of Stars" available at URL
[http://scienceweek.com/swfr002.htm]
-----------
L. Bildsten and T. Strohmayer: New views of neutron stars.
(Physics Today February 1999)
QY: Lars Bildsten, Univ. of Calif. Berkeley, 510-643-8520.
-----------
Text Notes:
... ... *Pulsars: Pulsars were originally discovered at
radio wavelengths, but there are optical, gamma-ray, and x-ray
pulsars, and some of the gamma-ray pulsars are extremely powerful
gamma-ray emitters.
... ... *binary systems: Binary stars are a pair of stars
revolving around a common center of mass under the influence of
their mutual gravitational attraction, and apparently the
majority of stars in the Universe are binaries and not singlets.
In some cases the binary system is resolvable into two
components, and in other cases the presence of a second star is
inferred by perturbations in the motion or emitted radiation of
the first star. If the binaries are close enough, they may share
stellar material, and this results in a particular kind of
stellar evolution.
... ... *gamma rays: Gamma rays are radiation of high energy,
from about 10^(5) *electronvolts to more than 10^(14)
electronvolts -- radiation with the shortest wavelengths and
highest frequencies, the gamma ray region of the electromagnetic
spectrum merging into the adjacent lower energy x-ray region.
... ... *electronvolts: An  electronvolt is defined as the energy
acquired by an electron falling freely through a potential
difference of one volt, and is equal to 1.6022 x 10^(-19) joule.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 16Apr99
-------------------
Related Background:
ON NEUTRON STARS AND THE FLUID PROPERTIES OF HOT ATOMIC NUCLEI
... ... Viola and Kwiatkowski (Indiana University Bloomington,
US), in a review of a current experimental approach to
understanding the dynamics of neutron star structure, make the
following points: 1) During the catastrophic collapse of stellar
material in the core of a star destined to become a neutron star,
gaseous nuclear matter is believed to condense to the liquid
phase. A giant nucleus, or neutron star, is formed in this
nuclear phase transition. 2) In order to understand the formation
of neutron stars and black holes, it is essential to know the
conditions under which nuclear matter changes from the gas phase
to the liquid phase. These conditions are expressed, both for
nuclear matter and for ordinary matter, in terms of an equation
of state -- the thermodynamic equation that describes the phase
behavior of a substance as a function of temperature, pressure,
and composition. 3) In the case of the neutron star, the only
accessible approach is to study the reverse process, the
expansion and vaporization of heavy atomic nuclei, which approach
a neutron star's density and, in the intranuclear domain, share
many of its fluid properties. 4) The authors review their work
using ISiS (the Indiana Silicon Sphere detector), an apparatus
designed to study fragments ejected from an atomic nucleus after
the nucleus is "boiled" by the energy of a light-ion collision in
a particle accelerator. In the "soft explosions" analyzed by
ISiS, the nucleus vaporizes. By measuring the charges,
velocities, and flight paths of nuclear fragments, ISiS has
provided evidence that atomic nuclei indeed behave like droplets
of a liquid that expand as they are heated until they reach a
boiling point of approximately 10^(11) degrees Kelvin. 5) Various
tests distinguish between the extremes of boiling and shattering.
For example, a signature of phase transition (in this case,
boiling) is that fragments are emitted randomly in all
directions, rather than preferentially emitted in the direction
of the incident projectile. 6) Essentially, the result of these
experiments is that a class of collision events has been observed
that exhibits many characteristics expected for a phase
transition in nuclear matter. The major task is now to make a
quantitative connection between the data and nuclear
compressibility, in particular the nuclear compressibility of the
core of neutron stars and black holes.
QY: Vic Viola [vicv@iucf.indiana.edu]
(American Scientist October 1998) (Science-Week 4 Sep 98) 
-------------------
Related Background:
A MILLISECOND PULSAR IN AN X-RAY BINARY SYSTEM
A pulsar is a regularly pulsating source of radiation, the
pulsations believed to involve the magnetic field of a rotating
neutron star. Pulsars were originally discovered at radio
wavelengths, but they have also been detected at optical and
gamma-ray wavelengths. They can be powerful gamma-ray emitters
(gamma-ray pulsars), and there is also a class of x-ray pulsars.
The periods of pulsars range from approximately 1.5 milliseconds
to 4 seconds and can typically be measured to accuracies of one
part in 10^(10). Pulsars with periods shorter than approximately
0.01 seconds constitute the distinct class of millisecond
pulsars. Most pulsars are single stars, but binary pulsars are
known, about half of which are millisecond pulsars. The
millisecond-pulsar neutron star is believed to be rotating
hundreds of times per second, and a large number of millisecond
binary pulsars have been discovered in globular clusters. The
origin and evolution of pulsars has not been clear, but it has
been thought that millisecond radio pulsars, which are often
found in binary systems, start as ordinary pulsars, then lose
most of their magnetic field and "spin up" to millisecond periods
by the accretion of matter (and transfer of angular momentum)
from a companion star in an x-ray binary system. Until now, there
has been no direct proof of this idea in the form of predicted
coherent millisecond x-ray pulsations in the flux of an x-ray
binary. ... ... Wijnands and van der Klis (2 installations, NL
US) now report the discovery of such a pulsar, confirming
theoretical expectations. The authors suggest the source will
probably become a millisecond radio pulsar when the accretion
turns off completely. ... ... In a contiguous paper, Chakrabarty
and Morgan (Massachusetts Institute of Technology, US) report
that the orbital period of this binary system is 2 hours, and
that the system seems closely related to "black-widow"
millisecond radio pulsars, a class of pulsars that completely
consume their binary companions.
-----------
QY: Michiel van der Klis [michiel@astro.uva.nl]
QY: Deepto Chakrabarty [deepto@space.mit.edu]
(Nature 23 Jul 98 394:344,346) (Science-Week 14 Aug 98)


2. AMINO ACIDS IN THE MARTIAN METEORITE NAKHLA
During the past several years, a sharp controversy has evolved
concerning a Martian meteorite known as ALH84001, with one
research group suggesting that analysis of the meteorite
indicates the past presence of life on Mars, and other groups
contesting the data and the interpretations of the data (see the
background material below). Of significance is the fact that the
ALH84001 meteorite, discovered in Antarctica in 1984, is not the
only meteorite that has been classified as Martian. There is a
class of meteorites known as SNC meteorites (shergottites,
nakhlites, chassignites; called "snick" meteorites") which on the
basis of their composition are considered to be from Mars [*Note
#1]. Nakhlites are a rare type of *achondrite meteorite, named
after the Nakhla meteorite that fell at Nakhla in Egypt in 1911,
the first known "*fall" of this type. ... ... D.P. Glavin et al
(4 authors at 3 installations, US) now report a detailed analysis
of the amino acids in the Nakhla Martian meteorite, the authors
making the following points:
     1) Nakhla is the first Martian meteorite found to contain
carbonates and hydrous minerals apparently associated with
aqueous alteration processes on Mars. Carbonates are present in
both Nakhla and ALH84001, and they are enriched in (sup13)C
compared with carbonates on Earth, indicating that these minerals
are Martian in origin. Nakhla has a crystalline age of only 1.3 x
10^(9) years, in contrast to approximately 4.5 x 10^(9) years for
ALH84001. Based on secondary mineral assemblages, the Nakhla
parent rock was apparently exposed to aqueous solutions after it
was formed, which raises interesting questions: How long was the
era that aqueous processes may have persisted on Mars? Does
Nakhla carry evidence of organic compounds that were present on
Mars during that period?
     2) The authors report the results of amino acid analyses on
an interior piece of the Nakhla meteorite stone, the piece 2 to 3
centimeters from the fusion crust. The authors report the Nakhla
specimen contains a unique set of amino acids that are distinctly
different from the Antarctic Martian meteorites EETA79001 and
ALH84001, which contain amino acids apparently derived from
Antarctic ice meltwater.
     3) The authors report the detection in the Nakhla sample of
a suite of protein and nonprotein amino acids, using high-
performance liquid chromatography of the water- and acid-soluble
components. Aspartic and glutamic acids, glycine, alanine, beta-
alanine, and gamma-amino-butyric acid were the most abundant
amino acids detected, and were found primarily in the 6 molar
hydrochloric acid-hydrolyzed hot water extract. The
concentrations ranged from 20 to 330 parts per billion of bulk
meteorite.
     4) The authors report the amino acid distribution in Nakhla,
including the *D/L ratios (values from less than 0.1 to 0.5), is
similar to what is found in bacterially degraded organic matter,
and that the amino acids in Nakhla appear to be derived from
terrestrial organic matter that infiltrated the meteorite soon
after its fall to Earth. However, the authors suggest it is
possible that some of the amino acids are endogenous to the
meteorite.
     5) The authors suggest that the rapid amino acid
contamination of Martian meteorites after direct exposure to the
terrestrial environment has important implications for the Mars
sample-return missions and for the curation of the samples from
the time of their delivery to Earth.
-----------
D.P. Glavin et al: Amino acids in the Martian meteorite Nakhla.
(Proc. Natl. Acad. Sci. US 3 Aug 99 96:8835)
QY: Jeffrey L. Bada [jbada@ucsd.edu]
-----------
Text Notes:
... ... *Note #1: The SNC meteorites are *igneous rocks that
apparently solidified from a cooling magma near the surface of
their parent body. All but one are relatively young (less than
1.3 billion years old); the single ancient SNC meteorite found to
date was apparently formed 4.5 billion years ago. The proportions
and isotopic abundance of noble gases trapped in one shergattite
resemble the composition of the Martian atmosphere as analyzed by
the Viking landers. The current consensus is that the SNC
meteorites were ejected from Mars by impacts, and entered orbits
around the Sun before falling to Earth.
... ... *igneous rocks: In general, igneous rocks are rocks that
have congealed from a molten mass.
... ... *achondrite meteorite: A class of *stony meteorite
usually lacking the small rounded inclusions (chondrules) found
in chondrites. Achondrites comprise approximately 9 percent of
all meteorite falls. They consist principally of one or more of
the minerals plagioclase, pyroxene, and olivine. Nakhlites are
augite-olivine achondrites.
... ... *stony meteorite: A "stony meteorite" (aerolite) is a
meteorite compose primarily of silicate minerals. Approximately
95 percent of all meteorites observed to fall are stony
meteorites. They are usually difficult to distinguish from
terrestrial rocks.
... ... *fall: A meteorite that is actually been observed to fall
and is recovered at the place of impact. A meteorite that is not
observed to fall but is subsequently found and identified is
called a "find".
... ... *D/L ratios: Dextrorotatory/Levorotatory ratios of the
optical isomers of the amino acids. Nearly all biological amino
acids are chiral and levorotatory with respect to polarized
light. The presence of meteorite amino acid D/L ratios similar to
those found in terrestrial biological material can be taken as
evidence of meteorite contamination, but there have been recent
suggestions that extraterrestrial selection of amino acid optical
isomers may result from differential interaction of
extraterrestrial amino acids with circularly polarized
ultraviolet radiation (see SW 20 Aug 99 report #4).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99
-------------------
Related Background:
THE MARTIAN METEORITE MICROBES CONTROVERSY: AN UPDATE
In 1984, a 1.9 kilogram meteorite the size of a potato
(designated ALH84001) was found in Antarctica, and because of its
chemical composition the consensus is that this meteorite (and a
dozen similar meteorites) originated from the planet Mars. The
basis for the consensus is the detailed quantitative
correspondence of the trapped gases in the meteorites to Martian
atmospheric gases, and the specific distributions of oxygen
isotopes. In 1996 a group of researchers, D. McKay et al
(National Aeronautics and Space Administration Johnson Space
Center, US; Stanford University, US) reported they had concluded
that unusual characteristics of the meteorite ALH84001 can be
most reasonably interpreted as vestiges of ancient Martian
bacterial life. In particular, the authors noted the presence of
tubules 20 to 40 nanometers in diameter (called by some
"nannobacteria"), and they proposed these structures were
fossilized bacteria or parts of microorganisms. The report was
first delivered at a press conference in August 1996 (published
as a paper 9 days later) and provoked considerable media
attention and controversy when it appeared. The controversy has
continued, with many biologists objecting to the interpretation
of the rock data, and in particular objecting to the idea of
"bacteria" 20 to 40 nanometers in diameter.
... ... Allan Treiman (National Aeronautics and Space
Administration, US) presents a review and update of the ALH84001
meteorite controversy, the author making the following points: 1)
Early hopes for a fast resolution of the controversy concerning
meteorite ALH84001 have evaporated: no agreement has emerged on
whether or not the meteorite ever contained Martian life. 2)
There is no disagreement that ALH84001 formed on Mars
approximately 4.5 billion years ago, that the meteorite was
probably ejected into space approximately 16 million years ago by
an asteroid impact, that the meteorite fell in Antarctica 13,000
years ago, and that the meteorite remained in Antarctica until
found on the ice in 1984. 3) ALH84001 is an igneous rock (i.e., a
rock congealed from a molten mass) that apparently crystallized
slowly from molten lava and which contains globules of carbonate
minerals scattered along fractures. All the evidence for life is
in the carbonate globules or their rims. 4) The 4 lines of
evidence originally proposed by the McKay group were a) the
presence in the meteorite of carbon compounds (polycyclic
aromatic hydrocarbons) suggestive of decayed organic matter; b)
the presence in the meteorite of unusual small crystals of
magnetite (an iron oxide) matching identical crystals believed to
be produced only by Earth bacteria; c) the presence in the
meteorite of apparently incompatible minerals (e.g., iron-sulfide
and iron-oxide) close together whose proximity would suggest
organic action if the rock were from Earth; and d) the presence
in the meteorite of bacteria-shaped formations. 5) The author
[Treiman] suggests that a) The polycyclic aromatic hydrocarbons
may or may not be Martian, and if they are, they may or may not
be related to life. b) The magnetite crystals are indeed Martian,
but there is evidence that some of these crystals formed without
life and the origin of the others remains unclear. c) The mineral
associations in the carbonate globules do not prove life, but
also do not exclude it. d) The bacteria-shaped objects in
ALH84001 are not fossil bacteria but could be fossils of bacteria
fragments. (McKay's group now agrees that the objects are too
small to be fossil microbes.) 6) The author concludes: "McKay's
original hypothesis (as expressed in the 1996 paper) depended on
all four lines of evidence working together... The evidence has
not been verified, so the hypothesis has not succeeded... Despite
world attention, significant spending, and the work of the best
laboratories on Earth, the question [of life on Mars] is
unresolved."
-----------
Allan Treiman: Microbes in a Martian meteorite?
(Sky & Telescope April 1999)
QY: Allan Treiman [treiman@lpi.jsc.nasa.gov]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 21May99
-------------------
Related Background:
EVIDENCE THAT MARTIAN METEORITE AMINO ACIDS ARE CONTAMINANTS
As the subunits that compose protein polymers in living systems,
the detection of certain amino acids in a material is often
interpreted as indicating a possible biological origin. The
meteorite ALH84001, along with a number of other discovered
meteorites, has a composition that suggests it was apparently
ejected from the surface of Mars, and during the past year it has
been proposed that microanalysis of this meteorite indicates the
possible presence of bio-organics and biogenic fossils. This
proposal, however, has met with considerable controversy, and the
controversy is still in full force. ... ... Bada et al (4 authors
at 3 installations, US) now report that the amino acids present
in a sample of the ALH84001 meteorite appear to be terrestrial in
origin and similar to those found in the ice where the meteorite
was discovered, although the possibility remains that minute
amounts of endogenous amino acids are preserved in the meteorite.
The authors suggest that radiocarbon studies (cf. contiguous
report: Jull et al, Science 279:366 1998), coupled with their own
amino acid results, indicate that major and minor organic
constituents in the Martian meteorites are contaminants.
QY: Jeffrey L. Bada [jbada@ucsd.edu]
(Science 16 Jan 98) (Science-Week 30 Jan 98)
-------------------
Related Background:
AN ARGUMENT FOR RELIC LIFE ON MARS
... ... Gibson et al (National Aeronautics and Space
Administration Houston, US; University of Georgia, US), the
group including some of the authors of the 1996 McKay report, in
a review of the evidence for relic life on Mars, consider the
ALH84001 meteorite not only the strongest evidence for Martian
relic life, but also for the possibility of present Martian
microbial life. The authors are hopeful that in 2005 a "sample
return" mission will be launched to robotically collect Martian
rocks and soil and return them to Earth.
QY: Everett K. Gibson [egibson@ems.jsc.nasa.gov]
(Scientific American December 1997) (Science-Week 28 Nov 97)
-------------------
Related Background:
EVIDENCE AGAINST NANOFOSSILS IN MARTIAN METEORITE
The term "nanofossils" (originally spelled "nannofossils" by the
group that introduced the term) refers to elongated microscopic
forms found in the Martian meteorite ALH84001. Several groups in
the space and geology communities have proposed these forms are
fossilized bacteria, but most biologists have rejected the idea
on the basis that the forms are too small to be bacteria and
should not be classified as such. Bradley et al (3 installations,
US) now report that new analysis of material from the ALH84001
meteorite indicates the majority of the elongated microscopic
forms can be resolved as either emergent substrate layers or
magnetite whiskers, rather than biogenic nanofossils. Their
report is followed by a response from McKay et al (3
installations, US CA), some of the original proponents of the
nanofossil idea, and in their response McKay et al say the
artifact possibilities mentioned by Bradley et al are already
known to them, but are not related to their own observations.
They add that living bacteria as small as 70 nanometers in
diameter have been observed in mammalian blood, and that soil
bacteria as small as 80 nanometers have also been observed. The
references for these bacterial forms are one unpublished paper
and two recently published papers in Proc. Soc. Photo-Opt.
Instrum. Eng. 3111:420,429 (1997). It is evident that the
nanofossil controversy has not yet been resolved.
QY: J.P. Bradley, Georgia Inst. Technol. 404-894-2000; David S.
McKay [david.s.mckay@jsc.nasa.gov]
(Nature 4 Dec 97) (Science-Week 26 Dec 97)
-------------------
Related Background:
A CONTROVERSY CONCERNING MINIMUM POSSIBLE DIMENSIONS OF BACTERIA
Apart from their heuristic significance, scientific controversies
can be either amusing or irritating. In recent months, a
controversy between some geologists and many biologists has
developed, and it is apparently irritating the biologists. The
issue concerns the minimum possible dimensions of bacteria. The
geologists are led by Robert L. Folk (University of Texas, Austin
TX US), and they have proposed that certain microscopic entities
found in the Martian meteorite ALH84001 are fossils of what they
term "nannobacteria" (their own unique spelling of the prefix
nano-), which they say are similar to those found in Earth
travertine and limestone rocks, and which have dimensions of 30
to 50 nanometers. This has caused a furor among biologists, whose
understanding of bacteria and life forms in general is that the
smallest dimensions possible for a life form with a bounding
plasma membrane is about 200 nanometers. In fact no membrane-
bound bacterium with dimensions less than 340 nanometers has ever
been identified, and one can make simple calculations that a 50
nanometer bacterium would not have enough internal volume to
sustain its chemistry. Folk published papers on the subject in
several geological journals in 1996, starting the debate, and in
the Letters section of the 20 June 1997 issue of *Science* the
debate continues, and this week it is being reported in the
popular media as a "debate about life on earth". What evidently
irritates biologists is the apparent misunderstanding by these
geologists of experimental methods in biology. Characterizations
of "living" vs. "non-living" by biologists are made on the basis
of experimental laboratory replicability of an organism, and not
on the basis of the visible structure of an entity. Which means
the geologists involved need to attempt to culture their
Earth-rock entities, and which means decisions that the Martian
meteorite's so-called "nannobacteria fossils" are actually such
will require demonstration of cultured entities with those
dimensions. Biologists are not unwilling to admit the existence
of new species of life forms, of which they have already
recognized several million entities, but they argue that one does
not classify pieces of rock as a life form on the basis of
structure alone.
(New York Times 29 Jul 97) (Science-Week 1 Aug 97)


3. ON STRATOSPHERIC OZONE
Ozone [O(sub3)] is a compound formed when oxygen gas is exposed
to ultraviolet radiation. In the outer atmosphere (stratosphere),
ozone acts to shield the Earth from excessive radiation. In the
lower atmosphere (troposphere), ozone forms from combustion gases
and is a major air pollutant contributing to photochemical smog.
Since the discovery in 1985 that an ozone hole develops over the
Antarctic in late winter and early spring, intense research
efforts have clarified the roles of atmospheric transport and
chemistry in stratospheric ozone changes.
... ... D.W. Fahey and A.R. Ravishankara present a review of
current research on stratospheric ozone, the authors making the
following points:
     1) Average ozone concentrations in the polar stratosphere
show a pronounced cyclical variation over the course of the year.
In winter and early spring, ozone builds up at the poles as
ozone-rich air is transported from lower latitudes toward the
polar regions. But when transport to high latitudes slows and
solar illumination increases in late spring and summer, catalytic
ozone destruction leads to a substantial decrease (approximately
30 percent).
     2) Ozone is produced via solar ultraviolet photolysis of
oxygen and destroyed through catalytic cycles involving reactive
nitrogen, halogen (chlorine and bromine), and hydrogen species.
     3) The balance between photolytic production, transport, and
chemical destruction determines the abundance of ozone at any
particular stratospheric location. This balance is also strongly
season dependent. In addition, the relative contributions of the
3 types of catalytic destruction of ozone differ between the
summer and winter-spring seasons.
     4) During the summer, large regions of the polar
stratosphere receive uninterrupted sunlight for many weeks.
Photolysis reactions, several of which are complete ozone
destruction cycles, occur continuously under these conditions.
Total ozone concentrations therefore continuously decrease
throughout high latitudes in late spring and early summer.
     5) The authors suggest that we now understand in some detail
how the combined effects of transport, chemical ozone production,
and catalytic ozone loss control ozone during the annual cycle of
stratospheric conditions. The summer ozone decreases at high
latitudes will persist in the future because natural reactive
nitrogen rather than human-induced reactive halogen species are
primarily responsible for ozone destruction in those regions. In
contrast, the winter-spring ozone destruction will gradually
lessen in the next decades as halogen emissions steadily decrease
-- barring other changes to the stratosphere such as major
cooling of this region due to greenhouse gases.
-----------
D.W. Fahey and A.R. Ravishankara: Summer in the stratosphere.
(Science 9 Jul 99 285:208)
QY: A.R. Ravishankara [ravi@al.noaa.gov]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 27Aug99


4. MOLECULAR BIOLOGY: INTEGRIN SIGNALING
The "extracellular matrix" is a complex network of macromolecules
lying between cells where the cells form tissues. The term
"cytoskeleton" refers to the quasi-rigid macromolecular matrix
inside cells that among other things determines cell shape. The
integrins are a family of glycoprotein trans-membrane cell
membrane receptors that bind to extracellular matrix components
at the outer cell membrane surface, and interact with
cytoskeletal components at the inner membrane surface (thus
"INTEGRating" extracellular and intracellular structures).
Integrins are therefore important in cytoskeleton-extracellular
matrix interactions and in cell-cell adhesion, which makes them a
central component in the formation and structure of various
tissues.
... ... F.G. Giancotti and E. Ruoslahti (2 installations, US)
present a review of current research on integrins with a focus on
integrin signals that control various basic cellular behaviors.
The authors make the following points:
     1) Concerning the basic signaling machinery, the cytoplasmic
tails of integrins are generally short and always devoid of
enzymatic features. Integrins transduce signals by associating
with adaptor proteins that connect the integrin to the
cytoskeleton, to cytoplasmic *kinases, and to transmembrane
*growth factors. 
     2) Concerning control of the life and death of the cell by
integrins, loss of attachment to the extracellular matrix causes
apoptosis (programmed cell death) in many cell types. This
phenomenon, called "anoikis" (from the Greek word meaning
"homelessness"), may help maintain the integrity of tissues by
preventing cells that have lost contact with their surroundings
from establishing themselves at inappropriate locations.
     3) Concerning control of the cell cycle by integrins, cells
require anchorage to the extracellular matrix to proliferate.
Integrins activate growth-promoting signaling pathways that are
responsible for this anchorage requirement. Integrin signals are
also necessary for cells to proceed through the cell division
cycle, since progression through the *G1 phase of the cell cycle
requires sequential activation of the *cyclin-dependent kinases.
     4) Concerning control of cell shape, cell growth, and cell
survival by integrins, when cells come in contact with the
extracellular matrix, their usual response is to extend
*filopodia, apparently to sample the terrain. Integrins at the
tip of the filopodia bind to the extracellular matrix and
initiate the formation of *focal adhesions. *Actin-rich
*lamellipodia are then generated as the cell spreads on the
extracellular matrix. Fully developed focal adhesions and
associated *actin stress fibers then appear. These same events
occur cyclically during cell migration and produce the traction
necessary for movement.
     5) The authors conclude: Because integrins assemble large
signaling complexes and activate multiple signaling pathways,
they serve as a class of "master regulators" of cell function.
Future biochemical studies will provide a better understanding of
the integrin signaling complexes and may reveal new
organizational principles at the cell membrane... Advances in the
integrin field are also being translated into practical
applications, with three integrin-directed drugs currently
available.
-----------
F.G. Giancotti and E. Ruoslahti: Integrin signaling.
(Science 13 Aug 99 285:1028)
QY: Filippo G. Giancotti [f-giancotti@ski.mskcc.org]
-----------
Text Notes:
... ... *kinases: In biochemistry, a "kinase" is an enzyme that
catalyzes a reaction involving the transfer of phosphate groups,
the transfer usually activating another enzyme involved in a
specific function.
... ... *growth factors: Growth factors are peptide hormones that
regulate the growth of cells and tissues.
... ... *G1 phase: The "cell cycle" is the name given to the
ordered sequence of phases through which a cell passes from one
mitotic cell division to the next. The cycle is divided into 4
phases, of which G1 is the first "gap" in DNA synthesis.
... ... *cyclin: Cyclins are a group of proteins that activate
protein kinases involved in regulating the progression of cells
through the eukaryotic cell cycle.
... ... *filopodia: Dynamic extensions of the cell membrane
protruding from the surfaces of migrating cells. They grow and
retract rapidly, probably as a result of polymerization and
depolymerization of actin filaments (see below).
... ... *focal adhesions: (adhesion plaques) In general, a focal
adhesion is an intracellular attachment structure under the cell
surface where the cell attaches to the extracellular matrix in
vivo or to the substrate in cultured cells. Internal actin
filaments connect to the focal adhesion.
... ... *Actin: A family of ubiquitous structural proteins
present in all eukaryote cells. 
... ... *lamellipodia: Sheet-like extensions of the leading edges
of cells during locomotion.
... ... *actin stress fibers: In general, stress fibers are
bundles of contractile filaments resembling tiny myofibrils
(muscle fibrils). They occur in the cytoplasm of cultured
*fibroblasts and are composed of actin, *myosin, or other
cytoskeletal proteins.
... ... *fibroblasts: A type of connective tissue cell, secreting
structural proteins (e.g., collagen) that form certain tissue
components, including the extracellular matrix.
... ... *myosin: A group of proteins involved in the contractile
apparatus of both muscle and non-muscle cells. All the proteins
in this group have adenosinetriphosphatase (ATPase) activity,
i.e., they catalyze the hydrolysis of adenosinetriphosphatase to
adenosinediphosphatase (ADP).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99
-------------------
Related Background:
INTEGRIN CHANGES AND REGULATION OF NEURON ADHESION
... Cell adhesion is simply the ability of cells to remain in
association with each other. Sensory neurons are nerve cells
specifically designed to act as energy transducers, or to receive
input from energy transducers, and to produce an output that is
propagated to other nerve cells. Nerve cells in tissue culture
(and nerve cells in a developing nervous system and nerve cells
undergoing regeneration) exhibit growth of axons and dendrites,
and this growth is called "neurite outgrowth". M. L. Condic and
P. C. Letourneau (University of Utah, US; University of
Minnesota, US) report that the concentration of integrin ligand
regulates the amount of integrin receptor expressed on the
surface of sensory neurons, with the relationship being inverse
-- when ligand concentration is low, integrin receptor amount
increases -- and that ligand concentration determines surface
concentrations of integrin by changing the rate at which the
receptor is removed from the cell surface. In addition, increased
expression of integrin at the cell surface is related to
increased neuronal cell adhesion and neurite outgrowth. The
authors suggest that integrin regulation is the key process in
the development and regeneration of nerve cell axons.
QY: M. Condic [maureenc@mailman.med.utah.edu]
(Nature 23 Oct 97) (Science-Week 14 Nov 97)


5. ORPHAN NUCLEAR RECEPTORS AND REVERSE ENDOCRINOLOGY
The ongoing activities of a biological cell are essentially
controlled by cellular proteins, or more exactly by which set of
proteins are present in the cell during any specific time-frame.
The synthesis of these proteins is in turn determined by which
parts of the genome (i.e., which genes) are activated to be
transcribed into RNA for the subsequent synthesis of these
particular proteins. Transient activation of certain genes (and
inactivation of other genes) is therefore one of the most
important aspects of the regulation of cellular activities.
Hormones are chemical messengers, some of which are intimately
involved in gene activation. *Steroid hormones and *thyroid
hormones, for example, easily pass through cell membranes because
they are lipid soluble, and upon entering a target cell, the
hormone binds to and activates an intracellular receptor commonly
located within the cell nucleus. The activated receptor then
alters gene expression, i.e., turning on or off specific genes of
the nuclear DNA. As the DNA is transcribed, new *messenger RNA
(mRNA) is formed, and this *RNA leaves the nucleus and enters the
*cytosol to direct synthesis of new proteins, usually enzymes, on
the *ribosomes. The new gene products cause the physiological
responses characteristic of the hormone.
... ... S.A. Kliewer et al (3 authors at Glaxo Wellcome, US)
present a review of recent research on nuclear hormone receptors
with a focus on progress in understanding hormone signaling
through the use of "reverse endocrinology". The authors make the
following points:
     1) Nuclear receptors have critical roles in nearly every
aspect of vertebrate development and adult physiology by
transducing the effects of small lipophilic hormones into
transcriptional responses. Members of the nuclear receptor family
share several structural features, including a central highly
*conserved DNA binding domain that targets the receptor to
specific DNA sequences.
     2) The term "orphan receptor" was coined a decade ago to
describe gene products that appeared to belong to the nuclear
receptor family on the basis of sequence identity, but that
lacked identified hormones. Orphan receptors have been identified
in most metazoans, including approximately 40 encoded by distinct
genes in humans.
     3) A concomitant of orphan receptor research is the new
concept of "reverse endocrinology". Historically, new hormones
were discovered through analysis of their effects on
physiological or developmental processes, and the purified
hormone was then used to identify its partner receptor. This
classic approach established the field of endocrinology and led
to many key insights into physiology and disease. *Cloning of the
orphan receptors produced a new era in which this process is
reversed and the orphan receptors are used to search for
previously unknown hormones. The hormones themselves can then be
used as chemical tools to elucidate the biology of the signaling
pathway.
     4) The first decade of orphan nuclear receptor research has
yielded a large number of new family members and many intriguing
clues concerning their biological functions. At present, however,
ligands for only a handful of the orphans have been identified.
The authors conclude: "The next decade of reverse endocrinology
promises an explosion in our understanding of nuclear hormone
signaling pathways."
-----------
S.A. Kliewer: Orphan nuclear receptors: Shifting endocrinology
into reverse.
(Science 30 Apr 99 284:757)
QY: Steven A. Kliewer [sak15922@glaxowellcome.com]
-----------
Text Notes:
... ... *Steroid hormones: In general, a steroid hormone is any
member of a group of biological lipids having a particular
structure of 4 fused carbon rings. The sex hormones, adrenal
cortical hormones, vitamin D, and bile acids are steroids.
Certain carcinogens are also steroids.
... ... *thyroid hormones: Either of the compounds secreted by
the thyroid gland: thyroxine and triiodothyronine. They are
iodine-containing peptides that stimulate metabolic rate and that
are essential for normal growth and development.
... ... *messenger RNA (mRNA): The ribonucleic acid molecule
transcribed from DNA that carries the coded information
specifying the sequence of amino acids in a protein.
... ... *RNA leaves the nucleus: The nucleus of cells that have
nuclei is surrounded by a specialized phospholipid bilayer
membrane which separates the components of the nucleus from the
cytoplasm. This double membrane is perforated with structured
pores measuring approximately 120 nanometers in diameter, and it
is through these pores that any material exchange between nucleus
and cytoplasm occurs. The nuclear pores are not passive, but
contain active plugs that serve as selective transporters.
... ... *cytosol: In general, the "cytosol" is the soluble
fraction of cytoplasm remaining after all particulates have been
removed.
... ... *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.
... ... *conserved: In general, a "conserved sequence" is a
nucleotide sequence (and by implication a protein amino acid
sequence) that is similar or identical in the genomes of many and
often diverse organisms. Conserved sequences often code for key
functional proteins.
... ... *Cloning: (gene cloning) In this context, "cloning" is a
laboratory technique whereby a gene is isolated from its native
DNA environment and inserted into a specialized DNA fragment
(cloning vector). The vector is incorporated into a foreign host
(usually bacteria or yeast), and this allows production of the
gene and its products in large amounts for research.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99


6. ON THE BIOLOGY OF HAIR FOLLICLES
The biological functions of human hair include protection from
the elements and dispersion of sweat gland products (e.g.,
*pheromones). Hairs also detect mechanical stimuli at the surface
of the skin, the stimuli activating *neuroreceptors in the hair
follicles, and specialized cells in the hair follicles detect
surface pathogens and activate the immune system. Beyond these
involvements, in our present culture hair has significant
psychosocial importance, and patients with hair loss (*alopecia)
or excessive hair growth often suffer as a result.
... ... R. Paus and G. Cotsarelis (2 installations, DE US)
present a review of the biology of hair follicles, the authors
making the following points:
     1) Hair follicles vary considerably in size and shape,
depending on location, but they all have the same basic
structure. Rapidly proliferating matrix cells in the hair bulb
produce the hair shaft, whose bulk (the cortex) is composed of
hair-specific *intermediate filaments and associated proteins.
Pigment in the hair shaft is produced by *melanocytes
interspersed among the matrix cells. As the matrix cells
differentiate and move upward, they are compressed and funneled
into their final shape by the rigid inner-root sheath, whose
dimensions and curvature largely determine the shape of the hair.
     2) In utero, the *epithelium and underlying *mesenchyme
interact to form hair follicles. During this time, the precise
distribution of hair follicles over the surface of the body is
established, and the future *phenotype of each hair (e.g., long
scalp hair vs. short eyebrow hair) is determined. Many of the
molecular signals that control these events were first discovered
in Drosophila (fruit flies). For example, the mammalian
counterparts of various genes (e.g., hedgehog, patched, wnt,
disheveled, armadillo, engrailed, notch) -- all necessary for the
normal development of Drosophila -- are critical for the normal
formation of human hair follicles as well.
     3) Approximately 5 million hair follicles cover the human
body at birth. No additional follicles are formed after birth,
although the size of the follicles and hairs can change with
time, primarily under the influence of *androgens. The precise
spacing and distribution of the follicles are established by
genes expressed very early in the morphogenesis of the follicles.
Slightly later in development, cells containing the protein
products of *homeobox genes appear in the precise locations where
the hair follicles will form. Once the distribution of the
follicles has been established, subsequent molecular events in
the developing follicle determine the future phenotype of each
hair.
     4) Each hair follicle perpetually goes through 3 stages:
growth (anagen), involution (catagen), and rest (telogen).
Determining the molecular signals that orchestrate the transit of
the follicle between these stages is a key focus of hair
research. Although most of our current knowledge of the
substances that modulate hair growth in humans is derived from
clinical observations, studies in mice have identified some of
the molecular events associated with hair-follicle cycling. Many
*growth factors and growth factor receptors are apparently
critical for normal hair follicle development and cycling, but no
single growth factor appears to exert ultimate control over these
processes.
-----------
R. Paus and G. Cotsarelis: The biology of hair follicles.
(New England J. Med. 12 Aug 99 341:491)
QY: Ralf Paus [paus@uke.uni-hamburg.de]
-----------
Text Notes:
... ... *pheromones: In general, a "pheromone" is a type of
hormone (ectohormone) secreted by an individual and perceived by
a second individual of the same species, thereby producing a
change in the sexual or social behavior of the second individual.
... ... *neuroreceptors: In general, a "neuroreceptor" consists
of a specialized transducing terminal of an input (afferent)
nerve fiber.
... ... *alopecia: There are 3 major categories of alopecia: 1)
Androgenetic alopecia: baldness caused by "miniaturization" of
genetically predisposed follicles in the male pattern (frontal
recession and thinning in the vertex) or the female pattern (loss
of hair primarily over the crown, with sparing of frontal hair).
"Miniaturization" is a conversion of large (terminal) hairs into
small (vellus) hairs. 2) Alopecia areata: hair loss in patches,
believed to be caused by an autoimmune response to hair follicles
in the anagen stage. 3) Permanent alopecia: destruction of hair
follicles as a result of inflammation, trauma, scarring
(fibrosis), etc.
... ... *intermediate filaments: A class of durable protein
(e.g., keratin), the filaments measuring 8 to 10 nanometers in
thickness. In addition to their presence in hair, they also
comprise part of the cytoskeleton of eukaryotic cells (i.e.,
cells with internal membrane-bound organelles such as nucleus).
... ... *melanocytes: Pigment-producing cells located in the
deepest layer of the epidermis. They transfer pigment to
epidermal cells, and are therefore responsible for epidermal
pigmentation. (The "epidermis" is the most superficial layer of
the skin.)
... ... *epithelium: In animals, epithelial cells (epithelium)
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.
... ... *mesenchyme: An embryonic connective tissue from which
all other connective tissues arise. ... ... *phenotype: In
general, the term "phenotype" refers to the total appearance of
an organism as determined by the interaction during development
between its genetic constitution (genotype) and the environment.
In this context, the term is used to describe the specificity of
development of a hair type (not an organism), that development
involving an interaction between genetic determinants and
surrounding tissue (environmental) determinants.
... ... *androgens: A generic term for the male sex hormones.
... ... *homeobox genes: Homeobox is a family of similar
nucleotide sequences that encodes sequence-specific DNA-binding
proteins, and such genes are apparently essential for the
differentiation of various cell groups during embryonic
development. Homeobox sequences were first discovered in the
fruit fly, but they have homologues in many organisms, including
humans.
... ... *growth factors and growth factor receptors: Growth
factors are peptide hormones that regulate the growth of
cells and tissues, and growth factor receptors are protein
molecules on the cell surface or within the cytoplasm that bind
to growth factors and act as initial receivers of the hormone
signal.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

IN FOCUS: QUANTUM REACTION DYNAMICS
"Experiments and theory on chemical reaction dynamics aim to
study the details of reactions beyond just simple rate constants
at room temperature. Reactant and product molecules can have
different translational energies, varying angles of orientation,
and different internal quantum states. Developing experiments and
theory to study such microscopic aspects of chemical reactions,
and to probe directly the structure and lifetimes of reaction
transition states, has become a major field. The results provide
the most detailed insight into the mechanisms of chemical
reactions and are useful in a variety of other contexts... The
best theory of chemical reactions uses quantum mechanics. The
*Born-Oppenheimer approximation is usually invoked, allowing
electronic and nuclear motion to be separated, so that a
calculation reduces to two separate steps: solution of the
Schrodinger equation for the electrons with fixed positions of
the nuclei to obtain a potential energy surface, followed by
solution of the "*quantum scattering" Schrodinger equation for
the nuclei moving on this potential surface... A major recent
experimental advance has been the ability to use time-resolved
*pump-probe laser methods to study chemical processes occurring
on a time scale of femtoseconds [10^(15) sec]. Reactive
scattering theory has also been developed to help understand the
results of such experiments that can probe directly the
transition states of chemical reactions."
-----------
David C. Clary: Quantum Theory of Reaction Dynamics
(Science 20 Mar 1998 279:1879)
-----------
Text Notes:
... ... *Born-Oppenheimer approximation: In this approximation,
the motion of atomic nuclei is taken to be so much slower than
the motion of electrons, that calculations of the motions of
electrons can be made by assuming fixed positions of the nuclei.
The approximation was theoretically justified in 1927 by Max Born
(1882-1970) and J.R. Oppenheimer (1904-1967).
... ... *quantum scattering: Scattering theory is a quantitative
approach to describing collision-based interactions between
particles, and quantum scattering theory is such an approach
applied to particles in a quantum mechanical domain.
... ... *pump-probe laser: In general, in a "pump-probe laser"
technique, an ultrashort pulse of laser light excites a
chromophore and triggers the process to be investigated.
-------------------
Notes by SCIENCE-WEEK [http://scienceweek.com] 27Aug99

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