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ScienceWeek
SCIENCE-WEEK
A Weekly Email Digest of the News of Science
A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.
October 26, 2001 -- Vol. 5 Number 43
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When a science appears to be slowing down and, despite
the efforts of many energetic individuals, comes to a
dead stop, the fault is often to be found in a certain
basic concept that treats the subject too conventionally.
Or the fault may lie in a terminology which, once
introduced, is unconditionally approved and adopted by
the great majority, and which is discarded with reluctance
even by independent thinkers, and only as individuals
in isolated cases.
-- Johann Wolfgang von Goethe (1749-1832)
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. Xenon NMR Spectroscopy and Protein Conformation Changes
2. Template Assembly of Metallosalen-DNA Conjugates
3. On Spider Silk
4. A New View of Life's History on Earth
5. On Classifications in Evolutionary Biology
6. Evolution of the Mammalian Brain
7. Selector Genes and Body Plan Development
8. Retinal Ganglion Cells: Independent Encoding
9. On Reproduction Errors
10. Paleolithic Demography
11. Helicobacter Pylori and Cancer
12. Chemokines and the Molecular Basis of Cancer Metastasis
13. In Focus: On the Development of Science
14. SW Archive: On the Discovery of the Stratosphere
15. Sources
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Section 2
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1. XENON NMR SPECTROSCOPY AND PROTEIN CONFORMATION CHANGES
S.M. Rubin et al (Lawrence Berkeley National Laboratory, US)
discuss studies of conformational changes in proteins by xenon-
*NMR spectroscopy. Since the observation of a xenon-binding site
in the interior of myoglobin in 1965, the interaction of proteins
with xenon have been studied with the goal of using this inert
gas as a biomolecular probe. Xenon is relatively small and
hydrophobic, and it binds weakly to the nonpolar interiors of
many proteins with little perturbation of the structure of the
protein. The sensitivity of the xenon *chemical shift to its
local environment has motivated magnetic resonance studies of
xenon in biological systems. More recently, the intense xenon-129
NMR signals attainable with *optical pumping techniques have been
used to probe cavities in *lyophilized *lysoenzyme and
*lipoxygenase, detect blood oxygenation levels, and identify
xenon binding sites in a lipid transfer protein by the *spin-
polarization induced *nuclear Overhauser effect. The authors
report the dependence of the xenon-129 chemical shift on the
specific native conformation of a maltose-binding protein from
the bacterium Escherichia coli. This protein is a *periplasmic
protein in *gram-negative bacteria that plays a role in active
transport and serves as an initial receptor for *chemotaxis. The
authors suggest that the ability to discriminate protein
conformation via the xenon-129 chemical shift indicates the
potential use of xenon-129 NMR for direct assessment of protein
functional states and ligand binding events.
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JACS 2001 123:8616
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Notes:
... ... *NMR spectroscopy: "Nuclear magnetic resonance" refers to
the absorption of electromagnetic radiation at a characteristic
precise frequency by a nucleus with a nonzero magnetic moment in
an external magnetic field. The phenomenon occurs if the nucleus
has nonzero spin, in which case it behaves as a small magnet in
an external magnetic field. In quantum mechanics, electrons,
protons, and neutrons have an intrinsic angular momentum known as
"spin", and a magnetic moment parallel or antiparallel to that
angular momentum. When electrons are combined together to form an
atom or ion, there is a resultant angular momentum which is a
combination of the intrinsic spin of the electrons and the
angular momentum due to their motion about the nucleus, and this
is the "spin" of the atom or ion. Atoms or ions with non-zero
spin are magnetic atoms or ions. NMR spectroscopy is the main
application of NMR, the technique widely used for chemical
analysis and structure determination.
... ... *chemical shift: In this context, a change in the normal
wavelength of absorption or emission due to a change in chemical
environment.
... ... *optical pumping: In general, the use of a specific-
frequency light source to change energy distributions in atomic
or molecular systems from equilibrium distributions to
nonequilibrium distributions.
... ... *lyophilized: In this context, a "lyophilized" protein is
a protein maintained in a freeze-dried state, i.e., a state in
which a vacuum has been used to remove water by sublimation.
... ... *lysoenzyme: a protein catalyst that breaks down cells
and/or macromolecules.
... ... *lipoxygenase: In general, any member of a group of
enzymes that catalyze the oxidation of polyunsaturated fatty
acids.
... ... *spin-polarization: A phenomenon in which the unpaired
electron spin on one atom or part of a molecule is transferred to
another atom by interaction between the unpaired and paired
electrons.
... ... *nuclear Overhauser effect: In nuclear magnetic resonance
spectroscopy, a phenomenon in which saturation of electron
resonance by the application of a radio frequency field causes a
marked increase in nuclear polarization and in the produced NMR
signal.
... ... *periplasmic protein: The "periplasmic space" is the
space between the outer and inner membranes of gram-negative
bacteria.
... ... *gram-negative bacteria: Most bacteria can be classified
into two types, depending on the chemistry of their outer coat,
which chemistry determines whether a bacterium will admit certain
dyes into the interior. The classification, according to the
differential staining technique, is "gram-negative" vs.
"gram-positive", named after the bacteriologist H.C. Gram
(1853-1938). Gram-positive bacteria take up a crystal violet
stain and turn purple, while gram-negative bacteria exclude the
crystal violet and counterstain instead with stains such as
safranin, eosin red, or brilliant green. As might be expected,
since the technique differentiates the outer coats of bacteria,
some antibiotics are effective against one type and not the other
type, and vice versa. In general, gram-positive bacteria have a
structure consisting of a cytoplasmic core, a plasma membrane,
and a rigid external capsule. Gram-negative bacteria, however,
have two plasma membranes between the inner cytoplasmic core and
the external capsule: the two plasma membranes are separated by a
"periplasmic space" packed with various enzymes.
... ... *chemotaxis: In general, the term "chemotaxis" refers to
any movement of an organism in response to chemical concentration
gradients.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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2. TEMPLATE ASSEMBLY OF METALLOSALEN-DNA CONJUGATES
J.L. Czlapinski and T.L. Sheppard (Northwestern University, US)
discuss assembly of metallosalen-DNA conjugates. "Salens", which
are constructed from two salicylaldehydes and a diamine, serve as
ligands for a broad range of metal ions. Many metallosalens are
compatible with aqueous conditions and have demonstrated utility
as DNA cleavage reagents and as versatile catalysts for
*enantioselective transformations. *Watson-Crick base pairing
organizes *DNA duplex formation necessary for genetic information
storage in biological systems. DNA and RNA templates also direct
the specific binding of nucleotide substrates during diverse
enzyme-catalyzed reactions in replication, transcription, and
DNA-repair pathways. Recently, nucleic acid recognition
properties have been extended to non-biological systems, where
DNA base pairing has been used to drive the template-directed
chemical *ligation of *oligonucleotides, and the assembly of
nanostructures and novel materials. The focus of the authors is
application of the versatility of nucleic acid base pairing for
the addressable synthesis of bioconjugates in aqueous solution.
Metallosalen-DNA is an ideal system to demonstrate the concept of
nucleic acid template-directed molecular synthesis. The authors
suggest that template-directed synthesis of metallosalen-DNA
conjugates offers a unique approach to a new class of metal-DNA
hybrids. Metal=DNA conjugates have previously been used as probes
of DNA structure and electron transfer, as "chemical nucleases"
for targeted nucleic acid cleavage, and as scaffolds for metal-
mediated base pairing motifs. Thus, metallosalen-DNA may offer a
new bioconjugate platform for DNA-organized materials, nucleic
acid cleavage and detection strategies, and in vitro evolution of
novel *ribozymes and deoxyribozymes.
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JACS 2001 123:8618
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Notes:
... ... *enantioselective: In chemistry, an enantiomer is a
compound whose structure is not superimposable on its mirror
image, the compound being one of a pair of optical isomers, each
of which interacts differently with polarized light (i.e., shows
optical activity). A mixture of two optical isomers in equal
amounts is called a "racemic mixture", and racemic mixtures do
not show optical activity. A reactant or process that produces an
enantiomeric excess is simply a reactant or process that produces
or selects one enantiomer in excess over the other enantiomer.
... ... *Watson-Crick base pairing: The common nucleic acid bases
adenine, guanine, thymine, cytosine, and uracil pair selectively
with each other in nucleic acids. In "Watson-Crick base pairing",
adenine pairs with thymine, and uracil or guanine pair with
cytosine.
... ... *DNA duplex: In this context, a "duplex" is a double-
stranded molecule with complementary base-pairing.
... ... *ligation: The process whereby a 5'-3' phosphodiester
bond is formed between two DNA fragments.
... ... *oligonucleotides: A short length of single-stranded
polynucleotide chain usually less than 30 residues long.
... ... *ribozymes and deoxyribozymes: Nucleic acids that act as
catalysts (i.e., enzymes). Not to be confused with riboSOMES,
which are large structures involved in protein synthesis.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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3. ON SPIDER SILK
F. Vollrath and D.P. Knight (University of Oxford, UK) discuss
the chemistry and biology of spider silk. Spider and insect silks
have the same basic building blocks: proteins made largely from
non-essential amino acids. Although they evolved independently in
a wide range of invertebrates, all but the fecal fibers of
beetles are extrusions of specialist silk glands. The current
production rate and price of silk are similar to that of top-
quality man-made fibers such as Kevlar and Twaron. However, silk
polymers are poised for a possible comeback, albeit with a twist:
artificial fibers spun from *dope solutions composed of
genetically modified natural proteins, designer proteins, or
protein-plastic blends could be the new "techno-silks". Parent
silk gene-motifs to create genetically modified feedstocks for
artificial dope solutions are likely to be taken from the golden
silk spider Nephila clavipes, rather than from the moth Bombyx
mori, which has classically been the main silk provider. This
preference is largely because spider silk is much tougher than
the stiff but brittle cocoon threads of the moth, reflecting the
fact that spider threads have evolved to arrest "aerial missiles"
rather than form a protective cradle. Silk has been an *arachnid
construction material for approximately 400 million years, of
which half has seen web-engineering responding to the demanding
specifications of an aerial arms race with jumping and flying
prey. Accordingly, advanced web spiders have evolved an armory of
highly specialized silks. Spider dragline silks, the main
structural web silk, which are also used for the spider's
lifeline, are exceptionally strong and extensible, making them
very tough fibers. Indeed, their toughness equals that of
commercial polyaramid (aromatic nylon) filaments, which
themselves are benchmarks of modern polymer fiber technology.
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NAT 2001 410:541
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Notes:
... ... *dope solutions: In this context, a "dope solution" is an
aqueous solution of the protein molecules used to make the fiber.
... ... *arachnid: In general, the class that includes scorpions,
spiders, and mites.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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Related Background:
ON SELF-ORGANIZING POLYMERS
Although there has been much publicity recently concerning
potential applications of new inorganic materials in
nanotechnology and optoelectronics, many chemists believe that
self-organizing organic polymers hold the greatest promise for
future important discoveries and applications. Writing in a
recent review, Paul Calvert of the University of Arizona (US)
pointed out that "the known polymers are only a small set from a
vast array of possible chain molecules. In fact, we are mostly
interested in small, simple chain units and don't really expect
behavior outside the envelope we know. Sometimes, such as with
Kevlar, we are surprised."
Kevlar (poly-para-phenylene terephthalamide), the miracle
polymer that came into existence 30 years ago, is five times
stronger per unit weight than steel. It is used in bullet-proof
vests, belts for radial tires, cables, reinforced composites for
aircraft panels and boat hulls, flame-resistant garments, and
sports equipment. Chemically, Kevlar is an "aramid polymer",
which means an aromatic nylon (a nylon containing benzene rings).
It is also a liquid-crystalline polymer.
Liquid crystals can be considered a 4th phase of matter, a
state qualitatively different from the ordinary 3 phases, gas,
liquid, and solid. Liquid crystals flow like a liquid, but there
is order in at least one dimension in the arrangement of the
molecules. "Nematic crystals" are liquid crystals with long
molecules all aligned in the same direction. "Cholesteric" and
"smectic" liquid crystals have molecules arranged in distinct
layers: in cholesteric crystals, the axes of the molecules are
parallel to the plane of the layers; in smectic crystals, the
axes of the molecules are perpendicular to the plane of the
layers. A liquid-crystal polymer is a polymer with a self-
organized liquid crystal structure that combines strength with
lightness.
The term "polymer" derives from the Greek "polymeros" and
means "many parts". The individual units of polymers are called
"monomers", and most common polymers are composed of regular
repetitions of one or more monomers. There is no formal
restriction on the composition of a polymer: for example,
asbestos is an inorganic polymer. Although polymeric substances
have been known and used for thousands of years, the first
polymeric materials to attract recorded scientific interest were
silk and cobwebs: in 1665 Robert Hooke (1635-1703) suggested that
the products of the silkworm and spider could be imitated by
drawing a suitable glue-like substance out into a thread. This is
essentially the process used today in industry to manufacture
synthetic polymer fibers such as rayon.
The explosive growth of the plastics industry in the 19th
and 20th centuries was based on two developments: the discovery
of the principles of polymer chemistry and the extraction of the
monomers required for polymer synthesis as products of petroleum
refining. It was not until the 1920s that it was understood that
the various plastics being produced in industry consisted of
linear molecular chains rather than disorderly conglomerates of
small molecules.
A simple linear polymer is a chain molecule composed of
monomers with two reactive sites (bifunctional monomers), with
monofunctional terminal units. If more than one bifunctional
monomer is present, the chain is known as a "copolymer". A
copolymer in which a number of units of the same monomer are
located adjacent to one another (in "blocks" of monomers) is
called a "block copolymer". A "diblock copolymer" is composed of
two types of monomers (e.g., A and B), and may be depicted thus:
AAAAAABBBBBAAAAAABBBBBAAAAAAA.
In 1999, Edwin L. Thomas of the Massachusetts Institute of
Technology US) reviewed current research on the self-assembly of
polymers and their possible future applications. The structure of
biological macromolecules is typically determined by sequences of
different monomer units (e.g., a variety of amino acids in
proteins). In contrast, polymer chemists have long used large
blocks of identical monomer units to design the microstructure of
polymer materials, and recent work on such self-assembled block
copolymers has produced materials exhibiting intricate
3-dimensional periodic structures. These novel interconnected
nanoscale patterns are providing opportunities in a variety of
technological areas from separation membranes to photonic
crystals.
Thomas pointed out that the synthesis of polymers that can
form topologically connected periodic patterns can result in
useful materials such as a superefficient membrane of selective
permeability (perm-selective membrane). Also, periodic
dielectrics made from such polymers will be useful in certain
optical applications, and self-assembled optical components based
on block copolymers are now being realized.
In general, the term "polymer conformation" refers to the
higher order structure of polymers, and one goal of synthetic
polymer chemistry is to define the conditions producing various
types of conformations, since in many cases both the physical
properties and reactivity of polymers is conformation dependent.
In polymer chemistry, "quasi-equivalent" building blocks are
chemically identical subunits that self-control their shape by
switching between different conformational states during the
process of self-assembly.
In 1998, a research group (V. Percec et al) in the US, UK,
and Denmark reported a general method for the control of polymer
conformation through the self-assembly of quasi-equivalent
branched side-groups attached to flexible backbones, with the
production of either spherical or cylindrical polymers controlled
by the degree of polymerization. The research team suggested
their strategy will provide new approaches to the rational design
of organized supramolecular materials, expanding the synthetic
and technological uses of branched building blocks.
Many polymer chemists are now looking to biological
macromolecules for ideas concerning new polymers. One could call
it payback time: In the early years of biochemistry, in the 1930s
and 1940s, biochemists looked at polymer chemistry for ideas that
would help them understand biological macromolecules; and more
than a half century later, polymer chemists are in turn looking
at the chemistry of biological macromolecules for ideas that will
help them synthesize new useful materials.
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Paul Calvert: in _The New Chemistry_, 2001, ed. Nina Hall
SCI 1999 286:1307
NAT 8 Jan 98
SW 23 Jan 98
PRAX 2001 16 Jul
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4. A NEW VIEW OF LIFE'S HISTORY ON EARTH
Mark Newman (Sante Fe Institute, US) discusses a new analysis of
the extant fossil record. For many decades the discovery and
description of a previously unknown fossil species was the
calling card that gained new paleontologists professional
acceptance. However, in approximately the last quarter of a
century, many of the most intriguing new results in paleontology
have come not from field studies, but from the compilation and
analysis of large-scale databases of fossil species. These
databases have provided quantitative views of the pattern and
size of *mass extinction events, the rate at which new species
have appeared, and the number of species on the planet through
time, the so-called "standing diversity". Now a new effort by J.
Alroy et al (2001) has resulted in the creation of a new database
that catalogs fossils at the level of individual collections, and
preliminary analysis of this new database reveals interesting
results, calling into question some fundamental ideas about the
history of life on Earth. In particular, new methods of analysis
that eliminate biases inherent in previous studies now raise
doubts about the long-held belief that biodiversity has increased
dramatically in the last 250 million years: it may in fact be
that biodiversity has been roughly constant, although no firm
verdict has been reached yet on this point.
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PNAS 2001 98:5955,6261
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Notes:
... ... *mass extinction: The available evolutionary record on
Earth provides evidence of recurrent mass extinctions of
biological species. Apparently, environmental catastrophes,
occurring for various reasons, have suddenly removed many groups
with a resultant collapse of ecosystems. Eventually new forms
appear and evolution continues, but the periods of mass
extinction apparently are a major factor in the various patterns
of evolution.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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5. ON CLASSIFICATIONS IN EVOLUTIONARY BIOLOGY
Graham Budd (Uppsala University, SE) discusses systematics
(taxonomy) in evolutionary biology. Fossils become awkward if one
imagines that our systematic groupings of extant biota somehow
reflect the evolutionary process in its entirety -- a task that
such groupings are ill-equipped to perform. The ultimate (and
erroneous) view of evolution provided by systematic groupings is
that an ancestor of more advanced forms somehow appeared with the
features of all its descendants, and simply went on to abandon
unnecessary lungs or fins as required. This preformationist model
is patently absurd, but subconscious acceptance of its premises
continues to have a surprisingly widespread and pernicious
effect. Our perception of systematics exerts a profound influence
on our view of evolution. The low-tech study of morphology is not
currently a popular discipline, but this does not render it
unimportant. Morphology represents not just the product of the
genes, but also the mysterious interface between the genetic
material and the environment -- a prism through which the glare
of selective pressure on the genome is refracted. If we wish, for
example, to understand the evolution of the genome, we must also
consider what morphology has been doing, probably in ways that we
hardly suspect. If so, the secrets of the fossil record should be
of interest in even the most up-to-date molecular laboratories.
The classification of *trilobites might yet find its place in the
lexicon of the genomic revolution.
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NAT 2001 412:487
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Notes:
... ... *trilobites: An extinct class of marine arthropods.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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6. EVOLUTION OF THE MAMMALIAN BRAIN
J.H. Kaas and C.E. Collins (Vanderbilt University, US) discuss
problems in current research concerning the evolution of the
mammalian brain. The brains of mammals vary greatly in size,
shape, internal organization, and functional capabilities. The
human brain is much larger than that of a *mouse lemur, for
example, and includes subdivisions and connections not found in a
lemur. Researchers interested in how these differences evolved
have only a few sources of information. A useful if fragmentary
historical record is available from "endocasts" of fossilized
skulls: occasionally, during fossilization, the space inside the
skull fills with silt, which then hardens, producing a cast
(endocast) of the internal structures. This record can be very
accurate, but it mainly reveals the sizes and shapes of brains
and the locations of large brain features such as grooves
(fissures). Endocasts, however, provide few clues to internal
organization. Thus, theories about brain evolution have also
depended on comparative studies of the brains of living mammals.
The general premise of comparative studies is that physical
features that are found in several species have been retained
from their common ancestor, and that features that are shared
only by closely related species must have evolved recently. The
challenge for those using this approach is to get reliable and
comparable observations from the brains of a suitable range of
living species.
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NAT 2001 411:141
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Notes:
... ... *mouse lemur: A type of lemur. Lemurs are a class of 15
species of mostly arboreal prosimians, small primates with long
tails and fox-like faces. The mouse lemur, measuring
approximately 5 inches in length, is probably the smallest
primate. Another lemur, the dwarf lemur, is about the same size.
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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Related Background:
IN FOCUS: ON HUMAN FOSSILS AND HUMAN BRAINS
[Editor's note: Prehistoric peoples ranging back to the dawn of
the human species have left us their bones, and from these bones
(when we can find them) we attempt to construct an image of our
ancestors. It is an enormously difficult and murky undertaking,
and the results perhaps most ambiguous when we try to establish
an idea of the perished brains of human fossils. Skulls are
measured to determine the volume of the brain, endocasts are made
to establish the inner surfaces of the skull and perhaps the
convolutions of the brain once protected by that skull, and there
is much controversy in anthropology concerning interpretations of
data. Here is a commentary by a brain neuroanatomist on the
difficulties of knowing the brains of ancient peoples, his words
nearly 40 years old but still relevant.]
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"The results of our inquiries into the brains of fossil men are
somewhat meager: we cannot deduce any details about their mental
life, whether they believed in God, whether they could speak or
not, or how they felt about the world around them... That the
brain increases in size as we go from the Australopithecinae to
modern man -- or to the Upper Paleolithics, for that matter --
is quite obvious and, of course, gratifying. But the meaning of
the increase is again not quite clear because, as we all know,
brain size as such is a very poor indicator of mental ability.
This has been shown best perhaps by [Karl] Pearson [1857-1936]
(1925) some years ago. In his series, very gifted persons, such
as Leon Gambetta [1838-1832], Anatole France [1844-1924], or
Franz Joseph Gall [1758-1828], had very small brains, of about
1100 grams. Other equally gifted persons had very large brains;
thus [George Gordon] Byron [1788-1824] and Dr. [Samuel] Johnson
[1709-1784] had brains of about 2000 grams. And, of course, some
very ordinary persons had equally large brains. So brain size was
certainly not very important, and the correlation between brain
size and mental capacity was insignificant. But whether this
argument can be extended to an evolutionary series is again
another matter. For one thing, we know far too little about the
bodily proportions of fossil forms. Obviously, the brain stands
in a certain relation to the rest of the body, and this rest is
still largely hidden from us. Brain size as such is none too
meaningful. Moreover, mere size completely leaves out of account
the inner structure of the brain, which may be different in
different forms and which may determine to a great extent what
the brain can do."
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Gerhardt von Bonin: _The Evolution of the Human Brain_
(University of Chicago Press, 1963, p.76)
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SW 2001 16 Feb
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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7. SELECTOR GENES AND BODY PLAN DEVELOPMENT
M. Affolter and R. Mann (University of Basel, CH) discuss body-
plan gene determinants. For the many cases in which organism
body-plan differences are inherited, variations have been traced
to specific genes, first in the fruit fly Drosophila melanogaster
and later in vertebrates. Most of these genes encode
"transcription factors" conserved throughout evolution, specific
proteins that control the development of "morphogenetic fields",
which are discrete sets of cells that give rise to specific
structures in the adult. Such genes are generally referred to as
"selectors", because they select distinct developmental pathways
that ultimately give rise to structures such as eyes, antennae,
legs, or wings in the fly. Although these selector genes are
known to encode transcription factors, exactly how they
orchestrate the development of morphogenetic fields remains
unclear. To build a leg or a wing, cells need to know not only
which structure they are making, but also where they are located
within that structure. A large body of elegant research has
provided an understanding of how a cell knows where it is.
Specifically, the generation of positional information within
morphogenetic fields is controlled by a small evolutionarily
conserved set of intercellular signaling pathways. Of importance
is that for each of these pathways, one of the last steps is the
activation of a pathway-specific transcription factor (the
signaling effector). These same pathways are used repeatedly
during development to provide positional information to most, if
not all, of the structures in developing animals.
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SCI 2001 292:1080
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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8. RETINAL GANGLION CELLS: INDEPENDENT ENCODING
The neural portion of the retina is a multilayered outgrowth of
the brain that processes visual data extensively before
transmitting nerve impulses to the thalamus in the brain, which
then relays nerve impulses to the primary visual cortex. Three
distinct layers of retinal neurons are separated by two zones
where synaptic contacts are made. The three layers of retinal
neurons, in the order in which they process visual input, are the
"photoreceptor layer", the "bipolar cell layer", and the
"ganglion cell layer".
... ... S. Nirenberg et al (University of California Los Angeles,
US) discuss the neurophysiology of retinal ganglion cells. A
principal goal in vision research is to understand how visual
stimuli are encoded in the activity of the retinal ganglion
cells, since these cells provide all the information about the
visual world that the brain receives. Correlated firing among
neurons is widespread in the visual system. Neighboring neurons,
in regions from retina to cortex, tend to fire together more
often than would be expected by chance, but the importance of
this correlated firing for encoding is unclear and controversial.
The authors report a study of correlated neuron firing in the
isolated mouse retina. Natural stimuli were presented and
responses of retinal output cells (ganglion cells) recorded.
Information theoretic techniques were then used to measure the
amount of information about the stimuli that can be obtained from
the cells under two conditions: a) when their correlated firing
is taken into account, and b) when their correlated firing is
ignored. The authors find that more than 90 percent of the
information about the stimuli can be obtained from the cells when
their correlated firing is ignored, and the authors suggest this
indicates that retinal ganglion cells act largely independently
to encode information, which greatly simplifies the problem of
decoding their activity.
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NAT 2001 411:698
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9. ON REPRODUCTION ERRORS
Jack Cohen (University of Warwick, UK) discusses reproduction
errors in biological organisms. It is important to emphasize that
in general two parents produce only two parents: approximately 14
of every 16 starlings die without breeding; a female frog lays
approximately 10,000 eggs, of which approximately 2 breed; and
all but approximately 2 of 40 million cod eggs contribute to food
chains and not to the future of their species. During the past
several decades, reproductive biology has altered its view of
reproductive errors. We used to believe that reproduction is
basically replication with few mistakes. We now realize that even
bacterial replication generates many mistakes, requiring a whole
suite of enzymes to repair them -- a process that does not
usually recover the original DNA sequence. This important
reversal of our paradigm of biological reproduction seems not yet
to have been generally noticed. University population-genetics
courses are still wedded to Mendelian peas and laboratory
Drosophila models, and most modern evolutionary and population-
genetic models are grounded in few-mutant-allele thinking, often
with a neutralist viewpoint. The author suggests that now that we
have information about the genetic ecology of wild populations,
and we can see how far from nature were the early theories of
population genetics, it is time to grasp the complexity of real
life instead of using easy arithmetic and fairy stories to
describe how reproduction works. Perhaps this proneness to
mistakes in reproduction is evidence that well-designed efficient
biology is out of reach of the natural selection of undirected
genetic change. We could go further and claim that terrestrial
life has achieved only barely enough effective multiplication to
keep biology going.
-----------
NAT 2001 411:529
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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10. PALEOLITHIC DEMOGRAPHY
Mary C. Stiner (University of Arizona, US) discusses current
research in *paleolithic demography. All Paleolithic *hominids
lived by hunting and collecting wild foods, an aspect of
existence that began to disappear only with the emergence of the
farming and herding societies of the *Neolithic approximately
10,000 or less years ago. What are the roots of this remarkable
economic transformation? The answer lies in equally revolutionary
changes that occurred within certain stone age cultures several
thousand years before. In 1968, Lewis R. Binford noted what
appeared to be substantial diversification of human diets in
middle- and high-latitude Europe at the end of the Paleolithic,
approximately 12,000 to 8000 years ago. Rapid diversification in
hunting, food processing, and food storage equipment generally
accompanied dietary shifts, symptoms of intensified use of
habitats, and fuller exploitation of the potential foodstuffs
they contained. Some of this behavior was directed to grinding,
drying, and storing nuts, but it also involved small animals. In
1969, Kent Flannery pushed these observations further with his
"Broad Spectrum Revolution" hypothesis, proposing that the
emergence of the Neolithic culture in western Asia was prefaced
by increases in dietary breadth in foraging societies just before
the period. Flannery argued that subsistence diversification,
mainly by adding new species to the diet, raised the carrying
capacity of an environment increasingly constrained by climate
instability at the end of the Pleistocene. Today, 32 years later,
and contrary to the results of interim studies, the data on small
game use in southern Europe and western Asia support Flannery's
"Broad Spectrum Revolution" hypothesis of expanding dietary
breadth in response to demographic packing during the late
Pleistocene.
-----------
PNAS 2001 98:6993
-----------
Notes:
... ... *paleolithic: The term "Paleolithic" (Old Stone Age) is
essentially an archeological term applied to Eurasia with
approximate time-frame segments as follows:
Upper Paleolithic: from 40,000 to 8000 years ago.
Middle Paleolithic: from 200,000 to 40,000 years ago.
Lower Paleolithic: from 2.5 million to 200,000 years ago.
... ... *hominids: The terms "hominoid", "hominid", and "hominin"
are not interchangeable, but their classification criteria are
variously in a state of flux. In general, the hominoids are a
primate superfamily; the hominid family is currently considered
to comprise both the great ape lineages and human lineages within
the hominoid superfamily; the "homininae" comprise both the human
lineages and the African ape lineages within the hominids, and
the "hominini" comprising only the human lineages.
... ... *Neolithic: (New Stone Age) Characterized by first
domestication of animals, cultivation of plants, production of
sophisticated stone tools, etc. Considered to have begun
approximately 10,000 years ago.
-----------
SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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11. HELICOBACTER PYLORI AND CANCER
N. Uemura et al (Kure Kyosai Hospital, Kure City, JP) discuss
Helicobacter pylori infection and the development of gastric
cancer. Since the discovery of the bacterium H. pylori in 1983,
the diagnosis and treatment of upper gastrointestinal disease
have changed greatly. A higher risk of the development of gastric
cancer has been reported in subjects with positive serological
tests for H. pylori. The World Health Organization and
International Agency for Research on Cancer consensus group
stated in 1994 that there was sufficient epidemiologic and
histologic evidence to classify H. pylori as a definite
carcinogen, and most but not all recent studies have found H.
pylori to be associated with gastric cancer. The rates of
infection in patients with gastric cancer vary greatly among
studies, and these variations may be attributable to differences
in the methods of detecting H. pylori or to differences in the
patient groups. In Japan, where the incidence of gastric cancer
is high, *endoscopy is performed frequently for the early
detection of gastric cancer, even as part of the examination of
patients without symptoms of the disease. The authors conducted a
prospective long-term study of a large group of patients (1526)
with various gastric disorders who were assessed for H. pylori
infection by endoscopy and biopsy, followed by histologic
examination and various tests to determine the relation between
H. pylori infection and the future development (1 to 10.6 years)
of gastric cancer. The authors conclude that gastric cancer
develops in persons infected with H. pylori but not in uninfected
persons. In an editorial comment on this study, J.G. Fox and T.C.
Wang (Massachusetts Institute of Technology, US) state: "In the
future physicians may need to view H. pylori less as a beneficial
commensal organism and more as something akin to tobacco."
-----------
NEJM 2001 345:784,829
-----------
Notes:
... ... *endoscopy: In general, an "endoscope" is any instrument
used for the examination of the interior of a canal or hollow
organ.
-----------
SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
-------------------
Related Background:
BENEFITS AND DANGERS OF HELICOBACTER PYLORI
Gary Hamilton (NS) discusses the bacterium Helicobacter pylori.
It was in 1984 that Barry Marshall carried out the now famous
experiment on his own stomach and demonstrated that Helicobacter
pylori can cause gastritis, the first step towards a potentially
life-threatening ulcer or stomach cancer. Marshall has since
received a number of accolades, including a Lasker Award,
frequently a harbinger of a Nobel Prize. His dangerous experiment
was a turning point in his dogged campaign to convince the
medical establishment that gastric ulcers are caused not by
stress or spicy foods but by a bacterium. Because of that
campaign, stomach ulcer patients can now be cured with
antibiotics, and a race by laboratories around the world to
develop a vaccine against Helicobacter pylori promises to
obliterate this pathogen once and for all and prevent not only
ulcers, but also stomach cancer, which claims more lives than any
other cancer except lung cancer. However, there are dissenters,
with some researchers believing that H. pylori may not be the
root cause of ulcers, and other researchers proposing that H.
pylori, like many other bacteria, has lived so long with humans
that it may now contribute to human health. There is indeed
emerging evidence that H. pylori may protect humans from
diarrhea, and even protect humans from the relatively rare
cancer, esophageal adenocarcinoma. Nevertheless, H. pylori is
designated by the World Health Organization as a "category 1
carcinogen", a label reserved for the most dangerous cancer-
causing agents, with the bacterium believed to be responsible for
up to 60 percent of gastric carcinomas, which are particularly
prevalent in Asia and in many developing countries, and which
kill hundreds of thousands of people each year. The apparent
consensus is to eradicate this pathogen.
-----------
NS 2001 11 Aug
-----------
PRAXIS 15 Oct 2001 http://scienceweek.com/praxis
-------------------
Related Background:
SEQUENCING OF COMPLETE GENOME OF ULCER-CAUSING BACTERIUM
Gastritis, stomach ulcers, and stomach cancers are a complex of
pathologies that are difficult to categorize and often difficult
to diagnose. Until recently, the consensus was that most cases of
gastritis and stomach ulcers involved stress-induced acid
secretion into the stomach, which in the absence of food would
result in a corrosive action of the acid on the wall of stomach.
Then, in 1983, came the startling self-experiments and
conclusions of two Australian physicians, Barry Marshall and
Robert Warren, who proposed that the bacterium Helicobacter
pylori, a spiral organism present in half the world's population,
was the cause of most gastritis and stomach ulcers and perhaps
stomach cancers as well. The initial report was, to put it
bluntly, derided by most gastroenterologists, but during the past
decade most gastroenterologists have in fact come around to agree
with Marshall and Warren. This week, at the non-profit Institute
for Genomic Research (Rockville MD US) headed by J. Craig Venter,
Jean-Francois Tomb et al report the first complete sequencing of
the genome of Helicobacter pylori. Venter's group is publishing
the entire genome, giving it away free, partly in protest against
a corporate entity (Genome Therapeutics, Waltham MA US) that
claimed two years ago that it had sequenced the genome but would
not publish it because of its proprietary value and a contract
with Astra AB of Sweden. Now the complete genome of H. pylori is
available to everyone. The cost to the Venter and Tomb group was
apparently $1 to $2 million out of its own funds. The bacterium
has 1,667,867 base pairs of DNA in a single circular chromosome
organized into coding sequences for 1,590 genes. The Venter and
Tomb group have already started comparing various sequences of
the H. pylori genome with known sequences of other organisms to
identify what proteins are expressed and for which functions.
This is only the 5th bacterial genome to be published, and it is
an important step in molecular medicine.
-----------
NAT 1997 7 Aug
-----------
PRAXIS 15 Oct 2001 http://scienceweek.com/praxis
-------------------
Related Background:
EVIDENCE THAT COMMON HOUSE FLY MAY BE ULCER PATHOGEN CARRIER
Medicine has its own canons, much less stable than the canons of
physics, but canons nevertheless. Until a decade ago, one of the
medical canons was that most stomach ulcers are caused by excess
acid secretion produced by a hyperactive gastric system. That
canon has been demolished, and it is now generally agreed that
most stomach ulcers are caused by a pathogen, the bacterium H.
pylori, which may also cause some stomach cancers. Now Peter
Grubel and colleagues (St. Elizabeth's Medical Center, Boston MA
US) report that H. pylori can survive as long as 12 hours on the
external parts of the housefly, and that flies still excrete the
bacterium up to 30 hours after crawling over a contaminated
source. It is thus possible that flies pick up H. pylori from
human excrement, and then spread the contamination to food.
Houseflies are known to travel as much as 30 miles from a
contaminating source. Studies are under way to determine if flies
are actually linked to ulcers outside the laboratory, and if the
answer is yes, the common housefly will be implicated as a vector
in one more disease.
-----------
J. Clin. Microbiol. 1997 29 May
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PRAXIS 15 Oct 2001 http://scienceweek.com/praxis
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12. CHEMOKINES AND THE MOLECULAR BASIS OF CANCER METASTASIS
Philip M. Murphy (National Institutes of Health, US) discusses
the role of *chemokines in the metastasis of cancers. Metastasis
is the chief cause of morbidity and mortality in most cancers.
Appropriately, the word "cancer" (from the Latin for "crab") and
"metastasis" (from the Greek for "change in position") both refer
to cell movement: the crab-like invasion of cancer into healthy
tissue and the migration of cancer cells to sites distant from
the primary tumor. Since the work of James Paget (1814-1899) more
than 100 years ago, pathologists have recognized that the
movement of cancer cells is not random and that different types
of cancer have different destinations. To explain this
specificity, the "soil and seed theory" states that different
organs provide growth conditions optimized for specific cancers,
whereas the "homing theory" holds that different organs have
special abilities to arrest or attract through chemotactic
factors specific types of cancer cells. There is some evidence to
support each theory, but the underlying molecular mechanisms have
largely remained a mystery. Both metastasis and normal migration
of white blood cells (leukocytes) involve passage through the
blood and *lymphatic circulation and movement across vascular
barriers, and this has suggested that metastatic cancer cells
simply co-opt signals that normally control leukocyte transport.
Chemokines are a large and well-defined family of proteins that
are critical in the development and deployment of leukocytes, and
chemokines can be secreted by virtually all types of cells, but
in different amounts and combinations. There is now new direct
experimental evidence by A. Mueller et al (2001) that chemokines
may indeed be involved in an important way in the metastasis of
human breast cancer (see background material below).
-----------
NEJM 2001 345:833
NAT 2001 410:50
-----------
Notes:
... ... *chemokines: Chemokines are immune system signaling
molecules; the receptors for chemokines are proteins on the
surfaces of cells.
... ... *lymphatic circulation: The lymphatic system is a complex
network for the distribution of lymph fluid (which is similar to
blood plasma -- blood without red cells). Lymph is collected by
drainage from the tissues throughout the body, flows in the
lymphatic vessels through the lymph nodes, and is eventually
added to the venous blood circulation.
-----------
SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
-------------------
Related Background:
MEDICAL BIOLOGY: ON CHEMOKINES AND BREAST CANCER METASTASIS
Although the term "metastasis" is usually used in connection
with the spread of cancer, the term actually refers to the spread
of any disease from an original location to other tissues.
Concerning cancer, what is important is that malignant cells have
the ability to invade healthy body tissues with very little of
the restrictions that normally constrain non-malignant cells.
Following a nearby invasion, a number of malignant cells may
detach from the initial ("primary") tumor and invade a body
cavity (e.g., abdominal or thoracic) or enter the blood or lymph
system. This latter condition can lead to widespread metastasis,
with those malignant cells that survive in the blood or lymph
invading adjacent body tissues and establishing secondary tumors.
In the final stage of metastasis, the secondary tumors become
vascularized, with new networks of blood vessels providing
nutrients for the further growth of these secondary tumors.
(Any macroscopic new tissue, whether resulting from repairing a
wound, normal growth, or tumors, requires a blood supply, with
blood vessel growth in tumors initiated by chemical triggers
called "tumor angiogenesis factors".) In all stages of
metastasis, malignant cells must resist the antitumor defenses of
the body in order to survive.
A common popular misconception is that migration of tumor
cells is a late development in the history of a tumor. On the
contrary, almost from inception, a tumor may shed cells into the
circulation. From animal models, it is estimated that a 1-
centimeter tumor sheds more than 1 million cells a day into the
venous circulation. In animals, circulating tumor cells usually
die as a result of intravascular trauma: the longer a tumor cell
spends in the circulation, the greater the chance of its death.
The probability that a circulating tumor cell will become a
metastatic tumor is estimated to be less than 10^(-6).
Experiments suggest that metastasis is not a random event,
and that the primary tumor may regulate the growth of metastatic
tumors, and removal of the primary tumor often results in
accelerated growth of the metastases.
The term "cytokine" refers to any substance that promotes
cell growth and cell division. As a promoter of cell growth and
division, a cytokine acts as a messenger to cells, and the
transmission of the message requires a binding of the cytokine
molecule to a cytokine-specific receptor on the cell surface.
"Chemokines" are a type of cytokine, the chemokines in general
comprising several groups of polypeptides with molecular weights
in the range 8 to 10 kilodaltons. The chemokines are chemokinetic
and chemotactic, normally stimulating white blood cell
(leukocyte) attraction and movement.
The G-proteins are a family of signal-coupling proteins that
act as intermediaries between activated cell receptors and
effectors, for example, the transduction of hormonal signals from
the cell surface to the cell interior. The G-protein is
apparently embedded in the cell membrane with parts exposed on
the outside surface and inside surface. The outside moiety is
activated by the first messenger, and the inside moiety activates
the second messenger, the G-protein thus acting as a
trans-membrane signal transducer.
Endothelial cells are flat cells forming a layer lining
blood vessels, lymphatic vessels, the heart, etc., and when
migrating tumor cells circulate in blood or lymph, they must
adhere to endothelial cells before they can pass through the
walls of blood or lymph vessels and into the adjacent tissues.
The interaction of migrating tumor cells and endothelial cells is
thus of considerable importance in metastasis. So-called
"adhesion molecules" are molecules expressed on the surface of a
cell that mediate the adhesion of the cell to other cells or
to the extracellular matrix. Adhesion molecules bind to receptors
that are classed collectively as "integrins".
In general, "hematopoietic cells" (hemopoietic cells) are
any cells involved in the formation of blood cells.
In this context, a "growth factor" is any specific substance
that must be present in a culture medium for multiplication of
the cultured cells to occur. Certain growth factors have been
identified as cytokine proteins (peptide hormones) that stimulate
the growth and division of target cells by binding to cell
membrane receptors. "Transforming growth factors" are cytokine
growth factors that produce in target cells some of the growth
characteristics of cancer cells.
The term "actin" refers to a family of ubiquitous structural
proteins present in all cells with a nucleus (eukaryote cells),
and the term "cytoskeleton" refers to the quasi-rigid matrix that
among other things determines cell shape. Both actin and the
cytoskeleton are of considerable importance in the production and
movements of pseudopods, the protoplasmic extensions involved in
the amoeba-like movements of migrating cells.
... ... A. Mueller et al (14 authors at 6 installations, US, DE,
MX) report new evidence of the involvement of chemokine receptors
in breast cancer metastasis, the authors making the following
points:
1) The authors point out that metastasis is the result of
several sequential steps and represents a highly organized, non-
random, and organ-selective process. Although various molecules
have been implicated in the metastasis of breast cancer, the
precise mechanisms determining the directional migration and
invasion of tumor cells into specific organs remains to be
established.
2) The authors point out that through their interaction with
G-protein coupled receptors, the secreted proteins called
chemokines induce cytoskeletal rearrangement, firm adhesion to
endothelial cells, and directional migration. Chemokines act in a
coordinated fashion with cell surface proteins, including
integrins, to direct the specific homing of various subsets of
hematopoietic cells to specific anatomical sites. Breast cancer
is characterized by a distinct metastatic pattern involving the
regional lymph nodes, bone marrow, lung, and liver, with tumor
cell migration and metastasis sharing many similarities with
leukocyte trafficking, which is critically regulated by
chemokines and their receptors.
3) The authors report that the chemokine receptors CXCR4 and
CCR7 are highly expressed in human breast cancer cells, malignant
breast tumors, and metastases, and that the known ligands of
these receptors exhibit peak levels of expression in organs
representing the first destination of breast cancer metastasis.
In breast cancer cells, signaling via CXCR4 and CCR7 mediates
actin polymerization and pseudopodia formation, and subsequently
induces chemotactic and invasive responses. In vivo, neutralizing
the interactions of the ligands of these chemokine receptors
significantly impairs metastasis of breast cancer cells to
regional lymph nodes and lung. Malignant melanoma, which has a
similar metastatic pattern as breast cancer but also a high
incidence of skin metastasis, shows high expression levels of the
chemokine receptor CCR10 in addition to CXCR4 and CCR7. The
authors suggest their findings indicate that chemokines and their
receptors have a critical role in determining the metastatic
destination of tumor cells.
4) The authors conclude: "Our findings are probably not
unique to breast cancer. Other tumor entities of hematopoietic
and non-hematopoietic origin... express functionally active
chemokine receptors that mediate tumor cell migration in vitro.
Our results in breast cancer and malignant melanoma suggest that
malignant cells, in general, express distinct and non-random
patterns of chemokine receptors... Currently, intense efforts are
underway to identify small-molecule antagonists for many
chemokine receptors. We propose that small molecule antagonists
of chemokine receptors... may be useful to interfere with tumor
progression and metastasis in tumor patients."
... ... In a commentary on the above report, Lance A. Liotta
(National Institutes of Health, US) makes the following points:
1) It was already recognized in the 19th century that
secondary tumors are seeded by cells released from the original
tumor, the seeding cells ferried around the body in the lymphatic
and blood circulations. Autopsies have revealed that some organs
have large numbers of secondary tumors, while other organs have
relatively few. The basis for this bias has been a puzzle,
because the pattern cannot be explained simply by differences in
blood and lymph flow in different organs.
2) The author points out that three major theories have been
proposed to explain the bias of metastases towards certain
organs, but animals studies have failed to reveal which theory is
correct. The three theories are as follows:
... ... a) Tumor cells leave the blood and lymphatic systems to
the same extent at all organs, but multiply only in those organs
that have the appropriate growth factors.
... ... b) The endothelial cells that line blood vessels in
target organs express adhesion molecules that cause circulating
tumor cells to stop circulating and invade those organs.
... ... c) The chemokine theory holds that organ-specific
attractant molecules enter the circulation, stimulating the
migrating tumor cells to invade the walls of blood vessels and
enter the organs.
The author (Liotta) points out that there is evidence to
support all three theories, but the mediator molecules -- whether
they be growth factors, adhesion molecules, or chemoattractants
-- have remained unknown. "Now, however, Mueller et al have
identified chemoattractants in target organs and chemoattractant
receptors on tumor cells, providing molecular support for the
chemoattraction theory. Moreover, [they] have shown that they can
block metastasis in animals by blocking the receptors."
-----------
NAT 2001 410:24,50
SW 2001 6 Apr 01
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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13. IN FOCUS: ON THE DEVELOPMENT OF SCIENCE
"Tbere is only one way of seeing one's own spectacles clearly:
that is, to take them off. It is impossible to focus both on them
and through them at the same time. A similar difficulty attaches
to the fundamental concepts of science. We see the world through
them to such an extent that we forget what it would be like
without them: our very commitment to them tends to blind us to
other possibilities. Yet a proper sense of the growth and
development of our ideas will come only if we are prepared to
unthink them. We are justified in placing the trust in them that
we do only because -- and to the extant that -- they have proved
their worth in competition with alternatives: if earlier men had
never thought in other terms than we do, than we ourselves would
simply be carrying on a traditional habit. We shall understand
the merits of our own ideas, instead of taking them for granted,
only if we are prepared to look at these alternatives on their
own terms and recognize why they failed... The invisibility of
our intellectual spectacles may have a further effect. It may
lead us to misunderstand, not only the specific ideas and
doctrines put forward by earlier thinkers, but also the general
character of their enquiries. This effect... is easily
illustrated: for example, from the scornful third-hand accounts
one reads of intellectual history in the days before the
Scientific Revolution. [Questions are asked:] 'What were thinking
men up to in all those years? And why did they shut their eyes to
the merits of the experimental method? How could they have hoped
to get genuine scientific results from mere argument, without
leaving their studies, unless it was through a mixture of
prejudice, muddle-headedness, and metaphysics?' Such questions as
these often criticize the 'pre-scientists' for _failing_ to do
things that it was not their business to attempt. For those
earlier enquiries in natural philosophy that are swept aside as
'pre-scientific' were in fact indispensable. Their effect was to
clear the ground and collect many of the girders and timbers out
of which the structure of science as we know it was in due course
constructed."
-----------
Stephen Toulmin: _Foresight and Understanding: An Enquiry into
the Aims of Science_
(Harper and Row, New York 1963, p.101)
http://www.amazon.com/exec/obidos/ASIN/0061305642/scienceweek
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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14. SW ARCHIVE:
ON THE DISCOVERY OF THE STRATOSPHERE
The atmosphere of Earth is divisible into several layers,
each layer having a characteristic temperature range, pressure
range, and composition. The layers, from the surface of Earth,
are (with thicknesses varying at different latitudes):
troposphere (0 to approximately 10 kilometers), stratosphere
(from approximately 10 to 50 kilometers), mesosphere
(approximately 50 to 80 kilometers), thermosphere (approximately
80 to 500 kilometers), and exosphere (above approximately 500
kilometers. Other layers, essentially meta-layers, are also
recognized: a) the "chemosphere" is the region between
approximately 32 and 92 kilometers where many important chemical
reactions occur; b) the "ionosphere", above approximately 80
kilometers, is a shell of high electron concentration resulting
from very short wavelength sunlight stripping electrons from
atoms and molecules (mainly oxygen and nitrogen) to create an
ionized layer; c) the magnetosphere is the constantly changing
magnetic field generated by the Earth's dynamo, this magnetic
field influencing the behavior of electrically charged particles,
and the field extending approximately 10 Earth radii (64,000)
kilometers into space on the sunward side.
The boundary between troposphere and stratosphere is called
the "tropopause"; that between stratosphere and mesosphere is
called the "stratopause"; and that between mesosphere and
thermosphere is called the "mesopause", in each case the root
"pause" used because of an inflection in the temperature-altitude
curve.
The temperature of the atmosphere undergoes marked but
systematic variation with altitude. In the troposphere, the layer
closest to the surface, the temperature decreases by
approximately 6.5 degrees centigrade per kilometer of altitude,
until at the tropopause (10 to 11 kilometers) the temperature
stabilizes at approximately -53 degrees centigrade. The
temperature remains stable in the stratosphere, and even
increases with altitude to approximately 0 degrees centigrade at
the stratopause. Then in the mesosphere there occurs again a
decline in temperature with altitude, now down to -100 degrees
centigrade, and then after the mesopause and into the upper
atmosphere (thermosphere and exosphere), the temperature rises
markedly in these regions of extremely low air density, so that
at 200 kilometers the temperature range is 300 to 900 degrees
centigrade, depending on solar radiance.
The first hint that Earth's atmosphere is a series of
concentric shells was provided by the meteorologist Leon
Teisserenc de Bort (1855-1913) [the surname is Teisserenc de
Bort]. From 1892 to 1896, Teisserenc de Bort served as chief
meteorologist at the Central Meteorological Bureau in Paris, but
in 1896 he resigned and carried out his meteorological balloon
investigations himself at his estate near Versailles. He
conducted experiments with high-flying instrumented balloons, and
he was one of the pioneers in the use of such devices. He
discovered that above approximately 11 kilometers the
temperature, which drops steadily from sea-level to that
altitude, remained constant up to the highest points he could
reach. Surprised by this result, he accumulated data from 236
balloon ascents before he suggested, in 1902, that the atmosphere
was divided into 2 layers. During the next few years, he termed
the lower layer, the layer involving air movements, the
"troposphere" ("sphere of change"), and the layer above that, a
layer he mistakenly thought consisted of internal further layers,
the "stratosphere" ("sphere of layers"). Thus, to Teisserenc de
Bort we owe both the discovery and the name of the stratosphere.
... ... Mott T. Greene (University of Puget Sound Tacoma, US)
presents an essay on Teisserenc de Bort, the author making the
following points:
1) The author suggests that ripples from Teisserenc de
Bort's discovery of the stratosphere spread far beyond
meteorology. Between 1902 and 1904, the oceanographer Vagn Ekman
(1874-1954) discovered similar layering of the ocean, and in
1909, the meteorologist Andrija Mohorovicic (1857-1936) used
seismology to establish the existence of a similar discontinuity
in the solid Earth, the discontinuity now known as the "Moho".
2) The author (Greene) also suggests that the discovery that
the Earth-ocean-atmosphere system is composed of concentric
shells of different density, from the core of the Earth to the
top of the atmosphere, is the founding insight of modern
geophysics, and that the discovery also profoundly influenced the
thinking of the young meteorologist Alfred Wegener (1880-1930),
leading Wegener in 1912 to propose the theory of continental
drift, with the continents representing the remains of a formerly
continuous Earth shell above the ocean floors. Greene concludes:
"Just as air masses and ocean water masses moved under the
influence of the Earth's rotation, sliding along surfaces of
discontinuity, so, he [Wegener] reasoned, did the continents on a
longer time-scale -- making Teisserenc de Bort not only the
discoverer of the stratosphere but an honorary grandfather of
continental drift."
-----------
NAT 2000 407:947
SW 2000 17 Nov
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SCIENCE-WEEK 26 Oct 2001 http://scienceweek.com
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15. SOURCES:
------------
AS: American Scientist; CEN: Chemical & Engineering News;
CR: Chemical Reviews; GD: Genes & Development;
GR: Genome Research; JACS: J. Amer. Chemical Society;
JAMA: J. Amer. Medical Association; JCE: J. Chem. Education;
MMWR: CDC Morbidity and Mortality Weekly Report; NAT: Nature;
NATM: Nature Medicine; NEJM: New England J. Medicine;
NS: New Scientist; NYT: New York Times; NYR: New York Review;
PNAS: Proceedings of the National Academy of Sciences;
PRL: Physical Review Letters; PT: Physics Today; PRAX: PRAXIS;
SA: Scientific American; SCI: Science; SW: ScienceWeek;
TS: The Scientist.
In the text, the affiliation following the names of authors is
the affiliation of the lead author.
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