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ScienceWeek
SCIENCE-WEEK
A Weekly Digest of the News of Science
June 19, 1998
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We are a scientific civilization. That means a civilization
in which knowledge and its integrity are crucial. Science is
only a Latin word for knowledge... Knowledge is our destiny.
-- Jacob Bronowski (1908-1974)
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Contents of This Issue:
1. On Cloning for Profit
2. Cosmology: Open, Closed, or Flat Universe?
3. On the Nature of Dark Matter
4. Supramolecular Assemblies: Current and Future Research
5. Evidence for Computational Function of Neuron Dendrites
6. Origin of Human Vocal Behavior: An Anatomical Consideration
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1. ON CLONING FOR PROFIT
Whenever scientific progress has a universally evident potential
impact on what the French call the "amour propre" of the crowd,
the science involved becomes a focus of intense discussion. And
if the instance is one where profits are to be made from the
progress, then the discourse concerning the progress will most
likely not play out for some time. Such will probably be the case
concerning cloning, and sober assessments by specialists are
always useful. ... ... In a short review of the past, present,
and future of cloning as a scientific enterprise, Anderson and
Seidel (2 installations, US) make the following points: 1)
Broadly defined, cloning is asexual reproduction that results in
a genetically identical organism. To many people, cloning was
invented with the birth of Dolly. In fact, cloning has been
practiced for millennia with plants, and for decades with
mammals, and Dolly's birth followed an orderly progression of
experiments that started with cloning mammalian embryos in the
1970s. 2) The first successful mammalian cloning by nuclear
transfer, in which cells from cleavage-stage sheep embryos were
fused with unfertilized sheep eggs, was reported in 1986. 3)
Successful cloning from older embryos (and ultimately from an
adult cell, in the case of Dolly) challenged conclusions from
previous work that the nuclei of differentiated cells are unable
to support normal development. 4) Practical applications of
cloning technology are the norm in the plant field: entire cloned
forests are currently produced, forests composed of genetically
desirable cloned trees. 5) When the appropriate techniques become
sufficiently reliable, mammalian cloning will be used to multiply
unique animal genotypes for agricultural production. But the
first applications of mammalian cloning will be to produce
transgenic animals for specific medicinal purposes (e.g., the
production of sheep engineered to produce human blood-clotting
protein factor IX in their milk). 6) Despite the high-profile
successes with nuclear transfer in mammals, the basic cell
biology of the cloning process is not yet well understood. It is
now evident that an important requirement in successful nuclear
transfer is that the stage of the cell cycle for nuclear donor
and host cytoplasm be compatible, and the management of this
compatibility is currently a focus of research.
QY: Gary B. Anderson (gbanderson@ucdavis.edu)
(Science 29 May 98 280:1400) (Science-Week 19 Jun 98)
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Related Background:
CLONED TRANSGENIC CALVES FROM FETAL FIBROBLASTS
Research has been in progress for more than a decade to develop a
system for genetic modification and large-scale cloning in
cattle, an important species in agriculture, biotechnology, and
human medicine. During the past 18 months, there has been much
publicity concerning the cloning of sheep using somatic cell
donor cells, the research conducted by the Wilmut group in the
UK. ... ... Now Cibelli et al (8 authors at 3 installations, US)
report similar results (but with a different method) in cattle.
Actively dividing fetal fibroblasts were genetically modified
with a marker gene, a clonal line was selected, and the cells
were fused to enucleated mature oocytes. Out of 28 embryos
transferred to 11 recipient cows, three healthy, identical,
transgenic calves were generated. Furthermore, the life span of
near senescent donor fibroblasts could be significantly extended
by nuclear transfer. With the ability to extend the life-span of
these primary cultured cells, this system would be useful for
inducing complex genetic modification in cattle. The authors
suggest their somatic cell nuclear transfer procedure could
improve the efficiency of producing transgenic cattle and broaden
the scope of applications for transgenic cattle.
QY: James M. Robl (robl@vasci.umass.edu)
(Science 22 May 98 280:1256) (Science-Week 12 Jun 98)
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Related Background:
SHEEP CLONING RESEARCH RESULTS: QUESTIONS AND ANSWERS
In a rare public exposure of what is usually a private or at
least specialist-restricted dispute between researchers,
Sgaramella and Zinder (2 installations, IT US), in a letter to
the journal Science and in interviews with various news media,
have extensively criticized the Roslin (Scotland, UK) sheep
cloning group headed by Ian Wilmut. Sgaramella and Zinder focus
on the cloning of the sheep Dolly from an adult ovine cell, and
state there has been a lack of any confirmation of this experi-
ment, that the original experiment was poorly controlled, the
interpretations untested, corollary mitochondrial data not
provided, and so on. Sgaramella and Zinder suggest that endless
debates about cloning are less than correct in the face of both
"the scientific weaknesses of the experiment and the possible
impact on the societal credibility of science itself" by debates
based on "facts" only presumed. In a contiguous reply, Campbell
et al (including Ian Wilmut) provide details explaining the
protocols used in the original Dolly cloning, say the Dolly
cloning was an unexpected and unplanned tangent from other
research, say the fact that Dolly is a Finn Dorset ewe restricts
the origin of Dolly to a single laboratory culture existing at
the time, that corollary data have indeed been provided to third
parties, that only 11 months have passed since publication of the
results, and since the gestation period in sheep is 5 months,
there has not yet been enough time to complete similar experi-
ments and publish data. Despite this public conflict, the
apparent consensus among embryologists is that the work of the
Wilmut group will be confirmed. QY: Norton D. Zinder, Rockefeller
University, 1230 York Avenue, New York, NY 10021 US; Ian Wilmut,
Roslin Institute, Roslin, Midlothian E-125 9PS, Scotland, UK.
(Science 30 Jan 98) (Science-Week 13 Feb 98)
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Related Background:
AN ESSAY ON THE CONFUSION OVER CLONING
Ever since Ian Wilmut introduced a cloned sheep to the world, the
subject has been a subject of media frenzy and political
brouhaha. The sheep Dolly was shown to the world on February 23,
1997, and the next day President Clinton called on the National
Bioethics Advisory Commission to undertake "a thorough review of
the legal and ethical issues". Three months later, the Commission
produced a 115 page report, and last week this report was
reviewed by Richard Lewontin, a well-known biologist (Harvard
University, US). Lewontin's conclusion: "It is impossible to
understand the incoherent and unpersuasive document produced by
the National Bioethics Advisory Commission except as an attempt
to rationalize a deep cultural prejudice, but it is also
impossible to understand it without taking account of the
pervasive error that confuses the genetic state of an organism
with its total physical and psychic nature as a human being."
Lewontin suggests that modern society is in the midst of a
widespread "genomania" fomented by the press and huckster
popularizers of science, a false view of modern biology that
proposes genetic dominance over human life. QY: R. Lewontin,
Harvard Univ. (617) 495-1551.
(New York Review of Books 23 Oct 97) (Science-Week 17 Oct 97)
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Related Background:
AN IMPORTANT NEW DEVELOPMENT IN GENETIC ENGINEERING
Predicting science is always hazardous. When the world learned of
the cloning of the sheep Dolly by Ian Wilmut (Roslin Institute,
Edinburgh, Scotland UK) last February, most genetics researchers
casually stated that in 5 or 10 years we might see genetically
engineered cloned sheep. The actuality, however, is not 5 or 10
years, but 5 months. This week, in a prepublication report to the
press, Ian Wilmut and Keith Campbell introduced Polly, born two
weeks ago, a cloned sheep that has an added human gene and an
added companion marker gene in every cell of her body. The human
gene will be identified in a forthcoming paper by the research
group. Two other lambs were born a few days ago, and are expected
to also have the human gene and marker gene. Still two others
have just the marker gene. The consequences are apparently as
follows: In short order, using these techniques, laboratories
will be cloning animals with human genes to produce hormones or
other products for use in human clinical medicine. Second, cloned
animals engineered to have human genetic diseases will be used
for research into these diseases. Third, cloned, engineered
animals will be produced with specific changes in their cell
surfaces that will reduce the probability of organ rejection and
thus give a great impetus to the use of animal organs in organ
transplantation in clinical medicine. The basic procedure used by
the Wilmut group is to take skin cells from fetal sheep, grow
them in the laboratory, add new genes, at least one of which is
human, then replace the genetic material of a sheep's egg with
that of one of the fetal skin cells that have incorporated the
new genes into their genome. After the fetal skin cell's genes
take up residence in the nucleus of a sheep egg cell, the fetal
cell's genes direct the development of a baby lamb whose every
cell contains the skin cell's genes, included the human gene and
its companion marker gene, and thus the cloning process is
complete. Although making predictions is indeed hazardous, one
feels compelled to make the following: Assuming that Wilmut's
achievements are soon replicated by independent laboratories,
during the next decade there will be a deluge of applications of
cloning research to human clinical medicine, and Ian Wilmut will
receive the Nobel Prize in Physiology and Medicine before the
decade is finished.
(New York Times 25 July 97) (Science-Week 1 Aug 97)
2. COSMOLOGY: OPEN, CLOSED, OR FLAT UNIVERSE?
Marc Kamionkowski (Columbia University, US) reviews current
research in cosmology, making the following points: 1) Determin-
ation of the geometry of the universe has been a central goal of
cosmology ever since Hubble discovered its expansion 75 years
ago. 2) The central question is whether the universe is a multi-
dimensional equivalent of a 2-dimensional surface ("flat"), a
sphere ("closed"), or a saddle ("open"). The geometry, in the
context of current theory and observations, determines whether
the universe will expand forever or eventually collapse. 3) Until
now, most astronomers have pursued the geometry by attempting to
measure the mass density of the universe. According to general
relativity, if the density is equal to, larger than, or smaller
than a critical density fixed by the expansion rate, then the
universe is flat, open, or closed, respectively. 4) Another
possibility is to look directly at the predicted observational
effects of a curved (open or closed) universe versus a flat
universe, and in particular at the angular power spectrum of the
cosmic microwave background. The authors suggest that in the near
future a new generation of experiments will provide substantial
advances in these observations, enabling more definitive
statements about the geometry of the universe, and that these
results will in turn provide clues to the new particle physics
required to understand the inflation phase following the Big Bang
origin of the universe.
QY: Marc Kamionkowski (kamion@phys.columbia.edu)
(Science 29 May 98 280:1397) (Science-Week 19 Jun 98)
3. ON THE NATURE OF DARK MATTER
Joel R. Primack (University of California Santa Cruz, US)
presents a commentary on a paper by E. Gawiser and J. Silk
(University of California Berkeley, US) ((Science 29 May 98
280:1405), Primack making the following points: 1) One of the
fundamental issues facing cosmologists concerns the evidence that
observable matter in the universe makes up only a fraction of
what is needed to explain the properties of the universe. A large
portion of matter in the universe must therefore be unobserved,
or "dark matter". 2) In current cosmology, "hot" dark matter is
defined as particles that were still moving at nearly the speed
of light at about a year after the big bang. "Cold" dark matter
is defined as particles that were moving sluggishly at that time.
Neutrinos are the standard example of hot dark matter, although
other more exotic possibilities have been discussed. 3) Gawiser
and Silk (ref. cited above) conclude that of all the currently
popular cosmological models, the only one whose predictions agree
with the data on the cosmic microwave background anisotropies and
the large-scale distribution of galaxies is the cold + hot dark
matter model, with 70% of the matter cold dark, 20% hot dark, and
10% ordinary matter (baryonic). 3) There are 3 species of
neutrinos, and there are mounting astrophysical and laboratory
data suggesting that neutrinos oscillate from one species to
another, which can only happen if they have nonzero mass. As
dark-matter candidates, neutrinos are entities with masses that
may be 10^(-5) of the mass of the electron, but with an expected
density more than 8 orders of magnitude greater than the density
of electrons and protons in the universe. Neutrinos, therefore,
can provide a substantial fraction of dark matter. 4) The success
of the cold + hot dark matter model in fitting the cosmic
microwave background and galaxy distribution data indicates that
this type of model should be investigated in more detail.
QY: Joel R. Primack (joel@physics.ucsc.edu)
(Science 29 May 98 280:1398) (Science-Week 19 Jun 98)
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Related Background:
A GRAVITATIONAL DIFFUSION MODEL WITHOUT DARK MATTER
R.J. Britten (California Institute of Technology, US) presents a
model that without dark matter quantitatively describes the flat
rotation curves of galaxies and the mass-to-light ratios of
clusters of galaxies. The hypothesis is that the agent of
gravitational force is propagated as if it were scattered with a
mean free path of about 5 kiloparsecs. As a result, the force
between moderately distant masses separated by more than the mean
free path diminishes as the inverse first power of the distance,
following diffusion equations, and describes the flat rotation
curves of galaxies. The force between masses separated by < 1
kiloparsec diminishes as the inverse square of the distance. The
excess gravitational force (ratio of 1/r:1/r^2) increases with
the scale of structures from galaxies to clusters of galaxies,
but there is reduced force at great distances because of the
approximately 12 billion years available for diffusion to occur.
This model with a mean free path of about 5 kiloparsecs predicts
a maximum excess force of a few hundredfold for galactic clusters
with dimensions of a few megaparsecs. With only a single free
parameter, the predicted curve for excess gravitational force vs.
size of structures fits reasonable well with observations from
those of dwarf galaxies through galactic clusters. Under this
diffusion model, no matter is proposed in addition to the
observed baryons plus radiation, and thus the proposed density of
the universe is only a few percent of that required for closure.
The author suggests that although the model does not follow from
present calculations based on the general theory of relativity,
it is not necessarily inconsistent with the general theory
because the diffusing gravitational elements might be interpreted
as spatial curvatures (e.g., distortions of the metric inducing
distortions in adjacent regions). The author further suggests
there is much at stake because of the scale of the intellectual
investment and the subtle arguments in cosmology that make use of
the general theory of relativity, and that the challenge of a
theory of intrinsic "beauty" may not be met at this time because
"beauty" is a subtle concept.
QY: Roy J. Britten (rbritten@etna.bio.uci.edu)
(Proc. Natl. Acad. Sci. US 31 Mar 98 95:3351)
(Science-Week 1 May 98)
-------------------
Related Background:
ON THE AGGREGATION OF YOUNG GALAXIES IN A DARK-MATTER UNIVERSE
The term "semi-analytic modeling" refers to a quantitative
modeling procedure in which empirical data are used in places to
fix the values of parameters or functions, rather than deriving
these from theoretical principles. N-body simulations, which
usually require extraordinary computational resources, are
simulations involving calculations of interactions of a large
population of entities. "Dark matter", which is thought to
comprise as much as 90% or more of the mass of the universe, is
undetectable except by gravitational effects, and "cold dark
matter" refers to dark matter particles created with low velocity
dispersions in the early universe. At the present time, computer
simulations and empirical observations of galaxy clustering favor
the idea that most dark matter in the universe is cold dark
matter. ... ... Governato et al (7 authors at 3 installations, UK
DK US) report the use of a combination of theoretical techniques
(semianalytic modeling and n-body simulations) to show that large
concentrations of young galaxies (i.e., galaxies in existence
when the universe was one-tenth of its current age) should be
quite common in a universe dominated by cold dark matter, and
that such galaxy concentrations are the progenitors of the rich
galaxy clusters seen today. The authors suggest these clustering
properties of primeval galaxies will be compared with data
collected in the near future, and that the comparison will be a
test of our current understanding of galaxy formation within the
framework of a universe dominated by cold dark matter.
QY: C.S. Frenk (c.s.frenk@durham.ac.uk)
(Nature 26 Mar 98) (Science-Week 10 Apr 98)
-------------------
Related Background:
NEW EVIDENCE FOR A COSMIC ANTIGRAVITY FORCE
Type 1a supernovas are believed to be white dwarf stars that have
accreted enough matter from another star to be pushed over a mass
threshold (the Chandresekhar threshold) and into a thermonuclear
explosion. Since most supernovas of this type have similar
spectral emission curves and absolute magnitudes at maximum, they
can be used as "standard candles" for distance determinations,
i.e., their apparent luminosity becomes a measure of their
distance. At a recent astrophysics meeting (3rd International
Symposium on Sources and Detection of Dark Matter in the
Universe, 18-20 February 1998, Marina del Rey, Calif., US),
Alexei Filippenko (University of California Berkeley, US)
presented a report that 14 distant type Ia supernovas are on the
average 10% to 15% further away than expected, and these results,
coupled with previous independent observations by others on other
type 1a supernovas, are pushing astrophysicists to a consensus
that cosmic expansion is accelerating, rather than decelerating
due to gravitational forces, and that a repulsive antigravity
force may be counteracting gravity on large scales. One
theoretical result has been a renewed interest in Einstein's
"cosmological constant", an intrinsic space-time background
energy that would produce such a force, and which for many years
has been disregarded as an ad hoc theoretical improvisation.
Apparently, astrophysicists are all accepting the new supernova
data, and the question is what to make of it -- and whether some
serious theoretical reformulations are necessary.
QY: James Glanz
(Science 27 Feb 98) (Science-Week 13 Mar 98)
-------------------
Related Background:
NO EVIDENCE OF NEARBY GALACTIC OR INTERGALACTIC HYDROGEN RESERVES
Neutral hydrogen gas is the material from which galaxies and
stars are made, and its distribution is therefore of interest.
Also, neutral hydrogen may contribute to the so-called inter-
stellar "dark matter", the existence or non-existence of which
remains one of the fundamental unresolved problems of modern
astronomy, since dark matter has been proposed as the explan-
ation, among other things, for the calculated masses of galaxies
from gravitation theory being 10 to 100 times the masses apparent
from their luminosities. Since it has been suggested by a number
of astronomers that large reservoirs of neutral hydrogen are
hidden in dark intergalactic clouds or in dim galaxies, surveys
of interstellar neutral hydrogen are of some importance in this
context. Now Martin Zwaan and Ertu Sorar (University of
Groningen, NL; University of Pittsburgh, US) report that an
analysis of data from the 300-meter Arecibo radio telescope in
Puerto Rico, which can detect neutral hydrogen out to 200 million
light years, indicates there is no significant neutral hydrogen
beyond that already known to be associated with known sources --
no significant neutral hydrogen in dark clouds and none in so-
called "low-surface-brightness" or dim galaxies. Assuming the
local universe is not atypical, it will now be difficult to
propose neutral hydrogen as a candidate for a significant
contribution to dark matter, or to propose that protogalactic
"mists" still exist in our vicinity.
QY: E. Sorar, Univ. Pittsburgh (412) 624-7488
(Science 29 August 1997) (Science-Week 12 Sep 97)
4. SUPRAMOLECULAR ASSEMBLIES: CURRENT AND FUTURE RESEARCH
One has the sense that a renaissance in materials science is
underway, a significant refocusing with a potential impact at
least as great as that following the introduction of plastics
more than a century ago. At a recent materials science symposium
on "Materials for the 21st Century and Beyond" (April 29, Hunter
College New York, US), seven leading figures in the field
presented perspectives on the near future. Nobel Laureate Jean-
Marie Lehn (Louis Pasteur University Strasbourg, FR) reviewed the
work of his group in designing and creating molecules programmed
by virtue of their structure and functional groups to spontan-
eously organize themselves into larger supramolecular assemblies
held together by hydrogen bonds, metal coordination, and so on.
The interest is not so much in the mere self-assembly into large
structures, but in the fact that such self-assembled structures
exhibit a new spectrum of physical and chemical properties with
important potential practical applications. Lehn's research
involves the use of metal ions to organize and stabilize
supramolecular structures with reversible architectures, and such
structures have special redox, optical, magnetic and other
properties. Michael D. Ward (University of Minnesota Minneapolis,
US) reported on the use of molecular building blocks to construct
crystalline frameworks with preordained architectures and new
functions. Ward's structures involve sheets of organic cations
and organic anions hydrogen-bonded to each other in a hexagonal
arrays. Work by other groups has involved supramolecular
multilayers. In 1988, researchers discovered that when certain
films consisting of alternating layers of a magnetic and a non-
magnetic metal are placed in a magnetic field, the resistance of
the film changes markedly, a phenomenon known as "giant magneto-
resistance". This discovery apparently reenergized the magnetic
materials science field because of important possible applicat-
ions to information storage technology, and Stuart P. Parkin (IBM
San Jose, US) is now leading a productive research group in this
field. Ron Dagani (Chemical and Engineering News), who authors a
review of the symposium, concludes: "Parkin's lecture made it
clear that, at least in the case of magnetic multilayers, some
materials envisioned for the 21st century are already here."
(Chem. & Eng. News 8 Jun 98) (Science-Week 19 Jun 98)
-------------------
Related Background:
A SINGLE-CARBON NANOTUBE ROOM-TEMPERATURE TRANSISTOR
Carbon nanotubes are similar to fullerenes, except their shape is
tubular. They were first discovered by Sumio Iijima (NEC
Laboratories, JP) in 1991, they come in both multi-walled and
single-walled versions, and they have diameters of the order of
10 to 30 nanometers. The use of individual molecules as
functional electronic devices was first proposed by Aviram and
Ratner in 1974. Since then, molecular electronics has attracted
much interest, particularly because it could lead to conceptually
new miniaturization strategies in the electronics and computer
industry. But the realization of single-molecule devices has
remained a challenge, largely owing to difficulties in achieving
electrical contact to individual molecules. Recent advances in
nanotechnology, however, have resulted in electrical measurements
on single nano-scale molecules. ... ... Tans et al (3 authors at
Delft University of Technology, NL) report the fabrication of a
field-effect transistor -- a 3-terminal switching device -- that
consists of one semiconducting single-wall carbon nanotube
connected to 2 metal electrodes. By applying a voltage to a gate
electrode, the nanotube can be switched from a conducting to an
insulating state. The device operates at room temperature,
thereby meeting an important requirement for potential practical
applications. Electrical measurements on the nanotube transistor
indicate that its operation characteristics can be qualitatively
described by the semi-classical band-bending models currently
used for traditional semiconductor devices, an unexpected result.
The authors suggest the fabrication of this 3-terminal switching
device at the level of a single molecule represents an important
step towards molecular electronics.
QY: Cees Dekker (dekker@qt.tn.tudelft.nl)
(Nature 7 May 98 393:49) (Science-Week 29 May 98)
-------------------
Related Background:
A METHOD FOR PATTERNING OF ORIENTATED MESOSCOPIC DOMAINS
Nanotubules (not to be confused with carbon nanotubes) are simply
tubular arrangements of molecules on a nanoscale level, in other
words of dimensions of the order of nanometers. A substance is
said to be "mesoporous" if it has pores of dimensions in the
nanoscale range, and a "mesoscopic" domain, which literally means
a domain of dimensions between microscopic and macroscopic, is
the term usually applied to a domain of nanoscale dimensions.
Electro-osmosis is the movement of a liquid, in response to an
applied electric field, with respect to a fixed surrounding phase
such as a solid, the effect produced by intrinsic charge differ-
ences between the two phases. Capillary electro-osmosis is
simply
the same effect occurring when the solid phase is a capillary
tube and the liquid phase fills the lumen of the tube. Trau et al
(6 authors at 2 installations, US) describe a method for the
directed growth of nanotubules involving fluid infiltration and
capillary electro-osmotic flow produced by an applied electric
field. The method allows the formation of orientated mesoporous
channels on a non-conducting substrate with an arbitrary micro-
scopic pattern. The authors suggest their technique provides a
convenient and economic method for fabrication of patterned
nanostructured materials for sensors, actuators, and
optoelectronic devices.
QY: I.A. Aksay
(Nature 18/25 Dec 97) (Science-Week 9 Jan 98)
-------------------
Related Background:
NANOTUBULE MOLECULAR FILTRATION MEMBRANES
... Because there is considerable interest in constructing
membranes with homogenous nanoscale pores for the purpose of
separating molecular species, there is a flourishing technology
in the production of such membranes. Now Kshama B. Jirage et al
(3 authors at Colorado State University, US) report the use of
polymeric membranes containing a collection of monodisperse gold
nanotubules with inside diameters less than 1 nanometer in a
membrane-permeation experiment to achieve separation of small
molecules based on molecular size. The authors suggest that gold
nanotubule membranes, which have already been shown to have
charge-based transport selectivity, have now been shown to have
molecular size selectivity, and offer promise for the development
of highly selective membranes for chemical separations.
QY: Charles R. Martin
(Science 24 Oct 97) (Science-Report 14 Nov 97)
-------------------
Related Background:
CONDUCTIVITY ENHANCEMENT IN METAL-DOPED CARBON NANOTUBES
As we have indicated in earlier reports, carbon nanotubes are
graphite-like sheets of hexagonally bonded carbon rolled up into
tubes of nanometer dimensions. They can now be synthesized as
single-walled or multi-walled tubes with specific diameters, and
the chemists who work with these molecular structures believe
they will eventually revolutionize a number of technologies. This
week R. S. Lee et al (University of Pennsylvania US; Rice
University, Houston TX) report that doping of single-walled
carbon nanotubes with bromine and potassium produces what are
essentially synthetic metals. In a second paper by an associated
group, A. M Rao et al (various installations in US, JP) report
that examination of metal-doped nanotube bundles using Raman
scattering indicates charge transfer mechanisms are responsible
for the metal-like behavior. R. E. Smalley (Rice University, TX),
who received the Nobel Prize in Chemistry for his work with
fullerenes (spherical carbon structures related to the nanotubes)
is associated with both groups, and one of the authors of both
papers. Now that nanotube synthesis is apparently maturing, we
can expect a flood of new reports on their electrical properties
under various conditions.
(Nature 17 Jul 97) (Science-Week 25 Jul 97)
5. EVIDENCE FOR COMPUTATIONAL FUNCTION OF NEURON DENDRITES
Compared to the cells of other tissues, nerve cells exhibit
extreme variation in shape (morphology), and one of the central
problems of neurobiology is to relate the shapes of various types
of nerve cells to specific functions (see the background material
below). The auditory system of mammals is one of the better
characterized neurophysiological systems, investigated for more
than a century, and with certain parts of the system exquisitely
defined by experimental procedures. The basic function of the
auditory system of mammals is to receive and analyze input sound
vibrations, and one cardinal aspect is sound localization. In the
auditory regions of the brainstem, there are neurons that act as
"coincidence detectors" -- binaural neurons that respond
maximally when they receive simultaneous inputs from the two
ears, and these neurons are an essential part of the analytical
system responsible for sound localization by the brain.
Essentially, coincidence-detector neurons in the auditory
brainstem of mammals and birds use interaural time differences to
localize sounds, each neuron receiving many narrow-band inputs
from both ears and comparing the time of arrival of the inputs
with an accuracy of 10 to 100 microseconds. Neurons that receive
low-frequency auditory inputs (up to approximately 2 kHz) have
bipolar dendrites (see discussion of dendrites in background
material below), and each dendrite receives inputs from only one
ear. ... ... Agmon-Snir et al (3 authors at 2 installations, US)
now present a simple model that mimics the essential features of
the known electrophysiology and geometry of these bipolar
coincidence detector neurons, and they report that the model
supports the idea that dendrites improve the coincidence
detection properties of these cells, enriching the "computational
power" of these neurons beyond that expected from model neurons
lacking dendrites. The significance of this research is that it
relates in a highly quantitative manner the relation between the
modeled dendritic morphology of a particular type of nerve cell
and its function, and the authors suggest their approach might be
used as a paradigm for the study of dendritic morphology-function
relations in other types of nerve cells.
QY: Catherine E. Carr (carr@zool.umd.edu)
(Nature 21 May 98 393:268) (Science-Week 19 Jun 98)
-------------------
Related Background:
ON VISUAL CORTEX ASYMMETRICAL LATERAL DENDRITES
Neurons exist with an enormous variety of extension architect-
ures, but in general they have one axon (which may branch
extensively) and many dendrites leading from the cell body (which
may also branch extensively). Again, in general, axons conduct
nerve impulses away from the cell body, and dendrites conduct
various types of electrical changes toward the cell body. One of
the central problems of neurobiology is to relate the specific
architecture of specific neuron types to neuron function.
... ... David Ferster (Northwestern University, US) reviews the
work of Margaret Livingston on the function of the curiously
shaped Meynert cells in the visual cortex. Meynert cells are
large pyramidal neurons distinguished by a set of long basal
dendrites that project laterally in one direction for as much as
0.7 mm. Livingston proposes that the asymmetrical dendrites make
Meynert cells sensitive to visual motion, and in particular to
motion in one direction. Livingston's experiments involved
electrophysiological recordings from neurons in the visual cortex
of macaque monkeys. Ferster suggests that if Livingston's cells
are indeed Meynert cells or neurons with asymmetrical dendritic
projections, these cells would become one of the few types of
neurons in the cerebral cortex whose distinctive dendritic
morphology can be assigned a specific visual function.
QY: David Ferster (ferster@nwu.edu)
(Neuron 20:509 1998) (Nature 2 April 1998)
(Science-Week 17 Apr 98)
-------------------
Related Background:
RETROGRADE AND ANTEROGRADE CONDUCTION IN NEURON DENDRITES
Nerve cells, in biological systems which have nerve cells, come
in all shapes and sizes, with the various shapes and sizes
correlated or not yet correlated with various functions. The
generalized neuron is more or less modeled after the vertebrate
motor neuron, a nerve cell with a particular morphology and a
particular relation of its morphology to its function, but all
neurobiologists are aware of the multiplicities of nerve cell
design actually found in nature, and the multiplicities of the
way various types of nerve cell behave. In the classical
generalized neuron, the idea is that the various membrane
depolarizations and hyperpolarizations that are the inputs to the
neuron are summated as excitatory and inhibitory inputs with an
end result at the initial segment of the axon hillock just beyond
the cell body that determines whether the nerve cell fires an
action potential that will be propagated along its axon to
another neuron or to a muscle cell. This paradigm is the classic
paradigm, supremely useful as a conceptual scheme (and a scheme
that produced at least 4 Nobel Prizes), but the fact is many
neuron types, particularly in the vertebrate brain, are
exceptions to the generalization. This may be particularly true
of the dendritic arborizations of central nervous system neurons,
and one of the fundamental questions of neurobiology is to
unravel the electrophysiology of nerve cell dendrites, and in
particular to determine if and when dendrites of particular types
of neurons actually conduct action potentials. Now Wei R. Chen et
al (3 authors at 2 installations, US, DK) report that electro-
physiological observations of the rat olfactory mitral nerve cell
indicate that the action potential can be initiated either in the
soma-axon hillock or in the distal primary dendrite, and that the
initiation site is controlled by excitatory synaptic inputs to
the distal dendrite and inhibitory synaptic inputs near the cell
body. The authors suggest that mitral cells provide a model that
widens the view of how dendritic excitability contributes to
information processing in different types of neurons in the
vertebrate brain.
QY: W. R. Chen, Yale Univ. School of Medicine 203-785-2644
(Science 17 Oct 1997) (Science-Week 7 Nov 97)
-------------------
Related Background:
(from a Science-Report Focus Report)
... Neurons, however, as we have said, have certain unique
features, among them their form. In a typical neuron, a number of
processes (projecting parts) extend from the cell body, but there
is one long process, and it can be extremely long, in excess of a
meter in some cases. There is always one and only one of these
long processes per neuron, and it is called the nerve fiber or
the "axon". But at the far end of the axon, the "distal" end, the
axon may branch profusely into many fine terminals, and these
terminals, whether a few or many, connect to other neurons that
may be some distance away.
All the other fibers that extend from the neuron cell body
are called "dendrites". They are usually short, but there may be
many of them and they may be branched, so numerous and so thickly
branched that in some nerve cells the ensemble of dendrites
resembles a tree.
The axon has two essential functions: it conducts electrical
signals in one direction, and it transports chemical substances
in both directions.
The major function of the dendrites of a nerve cell is
apparently to present an extended surface area for input from
other nerve cells. A sphere, as you know, provides a minimal
surface area for a given volume. Dendritic branching is nature's
mechanism for providing a maximal membrane surface area for a
given volume of cell cytoplasm.
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.
The generalized neuron, therefore, is a biological cell
whose morphology is designed to receive many inputs, and produce
a single output, and have that single output rapidly distributed
to one or more other neurons.
That is the crux of the neuron as a unit of information
processing and transmission, and it applies to all neurons, from
those found in the simple organisms such as worms to those found
in the brains of humans...
(Science-Report 5 Sep 97)
6. ORIGIN OF HUMAN VOCAL BEHAVIOR: AN ANATOMICAL CONSIDERATION
It can be argued that language is the most important behavioral
attribute that distinguishes humans from other animals, and one
of the important problems in anthropology and human evolution is
to demarcate as narrowly as possible the time frame during which
language in humans first appeared. Such demarcations have been
based on either apparent anatomical correlates (e.g., bone and
soft tissue analysis) or apparent archeological correlates (e.g.,
analysis of apparent symbolic behavior), with no firm specific
consensus among specialists. One of the important anatomical
features related to language is the nerve supply controlling the
muscles of the tongue. The mammalian hypoglossal canal is a bony
canal that contains the trunk of nerve fibers that constitute
this nerve supply. This canal is absolutely and relatively larger
in modern humans than it is in the African apes. ... ... Kay et
al (3 authors at Duke University, US) report a study of the
cross-sectional areas of hypoglossal canals in adult skulls of
contemporary humans, African apes, and several key fossil
hominids. They propose that hypoglossal canal size in fossil
hominids may provide an indication of the motor coordination of
the tongue and reflect the evolution of speech and language. What
they report is that the hypoglossal canals of gracile
Australopithecus, and possibly Homo habilis, fall within the
range of extant African apes, and are significantly smaller than
those of modern Homo. The canals of Neanderthals and an early
"modern" Homo sapiens (Skhul 5), as well as of African and
European middle Pleistocene Homo (Kabwe and Swanscombe), fall
within the range of contemporary Homo and are significantly
larger than those of Pan troglodytes (a chimpanzee species). In
summary, the authors suggest these anatomical findings indicate
the vocal capabilities of Neanderthals were the same as those of
humans today. The authors further suggest that the vocal
abilities of Australopithecus were not advanced significantly
over those of chimpanzees, whereas those of Homo may have been
essentially modern by at least 400,000 years ago, which is
consistent with the evidence for accelerated encephalization
rates in middle Pleistocene Homo. The authors conclude: "Thus,
human vocal abilities may have appeared much earlier in time than
the first archeological evidence for symbolic behavior."
QY: Richard F. Kay (Rich.Kay@baa.mc.duke.edu)
(Proc. Natl. Acad. Sci. US 28 Apr 98 95:5417)
(Science-Week 19 Jun 98)
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