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.

March 9, 2001 -- Vol. 5 Number 10

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I should like to find a way of discouraging
unnecessary publications, but I have not found
a solution, save the radical one... that all
scientific papers be published anonymously.
-- Erwin Chargaff

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=-=-=-=-=-=-=-=-=
Section 1
=-=-=-=-=-=-=-=-=

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

1. DEVELOPMENTAL BIOLOGY:
ROLE OF SPERM IN MAMMALIAN EMBRYONIC PATTERNING.
Evidence suggests that the cleavage pattern of normal development
may be involved in specifying the future embryonic axis, but how
and when this pattern becomes established is unclear. In many
animal eggs, the sperm entry position provides a cue for
embryonic patterning, but until now no such role has been found
in mammals. Researchers now report that in the mouse sperm entry
position predicts the plane of initial cleavage of the mouse egg
cell and can define embryonic and abembryonic halves of the
future blastocyst. In addition, the embryonic cell inheriting the
sperm entry position acquires a division advantage and tends to
cleave ahead of its sister. Since cell identity reflects the
timing of the early cleavages, these events together shape the
blastocyst, the organization of which will become translated into
axial patterning after uterine implantation.
(K. Piotrowska and M. Zernicka-Goetz: Nature 25 Jan 01 409:517)

2. DEVELOPMENTAL BIOLOGY: ON DEVELOPMENT OF THE PANCREAS
Classic studies of dissected and recombined embryonic pancreas
tissues published 40 years ago suggested that epithelial-
mesenchymal cell interactions regulate growth and cell
differentiation in the embryonic pancreas. Modern studies have
revealed additional cell interactions involving pancreatic
epithelium and mesoderm-derived tissues essential for normal
pancreatic development. Recently, many of the signaling pathways
likely to govern cell interactions in the developing pancreas
have been identified, allowing detailed studies of the genetic,
molecular, and cellular basis of intercellular signaling that
establishes proper pancreas development and function. The
evolutionary conservation of insulin-signaling and the profound
impact of pancreatic diseases on human health have encouraged
continuing research concerning the signals governing pancreas
development and function.
(S.K. Kim and M Hebrok: (Genes & Development 15 Jan 01 15:111)

3. MEDICAL BIOLOGY: ON BOTULINUM TOXIN AS A BIOLOGICAL WEAPON
Botulinum toxin is the most poisonous substance known. A single
gram of crystalline toxin, evenly dispersed and inhaled, would
kill more than 1 million people, although technical factors would
make such dissemination difficult. The basis of the phenomenal
potency of botulinum toxin is enzymatic: the toxin is a zinc
proteinase that cleaves one or more of the [membrane-] fusion
proteins involved in the release of the neurotransmitter
substance acetylcholine by neuronal synaptic vesicles into the
neuromuscular junction. In this system, acetylcholine activates
muscle fibers, and if acetylcholine release is prevented, nerve
fibers have no means of activating these muscle fibers, and the
muscle is effectively paralyzed. Four of the countries currently
listed by the US as "state sponsors of terrorism" (Iran, Iraq,
North Korea, Syria) have developed, or are believed to be
developing, botulinum toxin as a weapon.
(S.S. Arnon et al: J. Amer. Med. Assoc. 28 Feb 01 285:1059)

4. EARTH SCIENCES:
EVIDENCE FOR AN IMPACT EVENT RELATED TO THE PERMIAN EXTINCTION
The extinction event that marks the Permian/Triassic boundary was
the most severe in the past 540 million years, killing off over
90 percent of all marine species, approximately 70 percent of
terrestrial vertebrate genera, and most land plants. Proposed
catastrophic hypotheses for the Permian/Triassic boundary
extinction include an exploding meteor (bolide) (asteroidal or
cometary) and or massive volcanic lava flows (flood basalt
volcanism). Other extinctions mechanisms involving ocean anoxia,
as well as changes in sea level and climate, have also been
proposed. It is now reported that fullerenes [C(sub60) to
C(sub200)] from sediments of the Permian/Triassic boundary
contain trapped helium and argon with isotope ratios similar to
the apparent planetary component of carbonaceous chondrite
meteorites, and it is suggested that these data imply that an
impact event (asteroidal or cometary) accompanied the Permian
extinction. (L. Becker et al: Science 23 Feb 01 291:1530)

5. ASTROBIOLOGY: ON THE SEARCH FOR LIFE ON EUROPA
No broadly accepted definition of life exists, and most proposed
definitions face severe objections. Nevertheless, one working
definition of life has become influential in the "origin-of-life"
community, the definition that life is a self-sustained chemical
system capable of undergoing Darwinian evolution. The idea that
the origin of life is the same as the origin of evolution is a
popular corollary. However, such a definition is unlikely to
prove useful to a remote _in situ_ search for life in which
experiments to detect evolution are not possible. In a search for
extraterrestrial life in our Solar System, we instead fall back
on a less ambitious notion -- "life as we know it", meaning life
based on a liquid water solvent, a suite of biogenic elements
(most particularly carbon), and a source of free energy. With the
benefit of 25 years hindsight, there are a number of lessons to
be learned from the Viking (1977) Mars exploration experience in
the search for life on Europa.
(C.F. Chyba and C.B. Phillips:
Proc. Natl. Acad. Sci. US 30 Jan 01 98:801)

6. ASTROBIOLOGY: ON INTELLIGENT LIFE IN THE UNIVERSE
It is possible to imagine the existence of forms of life very
different from those found on Earth, occupying habitats that are
unsuitable for our kind of life. Some of those aliens might be
technological, because technology is an autocatalytic process,
and it follows that some aliens might possess technology well in
advance of our own, including interstellar transportation. So
much is clear, but this train of logic begs the obvious question
of where these intelligent non-humanoid aliens might be. The
history of science indicates that any discussion of alien life
will be misleading if it is based on the presumption that
contemporary science is the ultimate in human understanding. If
the past is any guide, then almost everything we now think we
know will be substantially qualified or proven wrong within the
next 25 years, let alone another century. Biology, in particular,
will not persist in its current primitive form. At present,
biology is at a stage roughly analogous to physics when Newton
(1642-1727) discovered his law of gravity.
(J. Cohen and I. Stewart: Nature 22 Feb 01 409:1119)

7. IN FOCUS: ON INVERTEBRATES VS. VERTEBRATES

8. FROM THE SCIENCEWEEK ARCHIVE:
ON AESTHETICS AS A GUIDE TO THEORY

=-=-=-=-=-=-=-=-=
Section 2
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1. DEVELOPMENTAL BIOLOGY:
ROLE OF SPERM IN MAMMALIAN EMBRYONIC PATTERNING.
     In many animal species, the point of entry of a sperm into
an egg cell during fertilization apparently determines the
orientation of the division of the one-celled embryo into two
cells. The entry point also apparently determines the orientation
of one of the 3 embryonic "axes", which form later in development
and which mark the 3 dimensions of the embryo: front to back,
head to tail, and left to right. Until now, this determining
effect of the sperm entry point was believed not to be true for
mammals.
... ... K. Piotrowska and M. Zernicka-Goetz (University of
Cambridge, UK) present a study of the role of sperm in spatial
patterning of the early mouse embryo, the authors making the
following points:
     1) The authors point out that despite a lack of known
determinants of cell fate in the mouse embryo, the spatial
patterning of the embryo is evident early in development. The
vertical axis of the embryo after transplantation in the uterus
can be traced back to organization of the pre-implantation
*blastocyst, and this in turn reflects the organization of the
*cleavage stage embryo and the *animal-vegetal axis of the
*zygote. These findings suggest that the cleavage pattern of
normal development may be involved in specifying the future
embryonic axis, but how and when this pattern becomes established
is unclear. In many animal eggs, the sperm entry position
provides a cue for embryonic patterning, but until now no such
role has been found in mammals.
     2) The authors report that in the mouse sperm entry position
predicts the plane of initial cleavage of the mouse egg cell and
can define embryonic and *abembryonic halves of the future
blastocyst. In addition, the embryonic cell inheriting the sperm
entry position acquires a division advantage and tends to cleave
ahead of its sister. Since cell identity reflects the timing of
the early cleavages, these events together shape the blastocyst,
the organization of which will become translated into axial
patterning after uterine implantation.
     3) The authors conclude that 2 axes of the blastocyst become
specified in the single-cell embryo. One of these axes is defined
by the animal pole, and the second axis, the embryonic-
abembryonic axis, relates to the sperm entry point. These axes
are initially not fixed and can be re-established if development
is perturbed. The authors suggest that the direction of
blastocyst organization by the plane of cleavage and cellular
identity by the order of cleavage may offer an interpretation of
the regulative events that occur following perturbation of
development. In normal development, the orientation and timing of
cleavage are mechanisms that progress together, but one might act
as a failsafe mechanism for the other when development is
perturbed. The authors conclude: "It will be a future challenge
to determine how these axes are initiated by the earliest events
of embryogenesis and how they become transformed into the final
body pattern.
... ... In a commentary on this work, Roger A. Pedersen
(University of California San Francisco, US) states: "This
finding represents a leap forward in our knowledge of how the
mammalian embryo acquires its body pattern. It also suggests that
mammals might share other features of axis formation with species
such as frogs, for which we have a better understanding of the
effects of fertilization on embryonic organization."
-----------
K. Piotrowska and M. Zernicka-Goetz: Role for sperm in spatial
patterning of the early mouse embryo.
(Nature 25 Jan 01 409:517)
QY: Magdalena Zernicka-Goetz: mzg@mole.bio.cam.ac.uk
-----------
Roger A. Pedersen: Sperm and mammalian polarity.
(Nature 25 Jan 01 409:473)
QY: Roger A. Pedersen: pedersen@cgl.ucsf.edu
-----------
Text Notes:
... ... *cleavage: The term "cleavage" refers to a series of
consecutive cell divisions, with the cells produced during
cleavage called "blastomeres". During cleavage, almost no growth
occurs between consecutive divisions, and the total volume of the
embryo does not substantially change: the size of the cells is
reduced by almost half at each division. At the beginning of
cleavage, cell divisions tend to occur synchronously in all
blastomeres, and the number of cells is doubled at each division.
As cleavage progresses, this synchrony of division is lost. In
most animals, cleavage follows an orderly pattern, with the first
division in the plane of the main axis of the egg cell. This
cleavage plane is arbitrarily called "vertical". The second
cleavage plane is again vertical but at right angles to the
first, producing 4 equal cells arranged around the main axis of
the egg. The third cleavage plane is at right angles to both the
first and second cleavage planes and is horizontal (or
equatorial). Subsequent divisions may alternate between vertical
and horizontal cleavage planes, but ultimately cleavage divisions
become randomly oriented. The above cleavage pattern is typical
of many animal groups, but variations are common in many species.
Although the shape and volume of the embryo do not change during
cleavage, an important change in gross organization occurs: as
the blastomeres are produced, they move outward, leaving a
centrally-located fluid-filled cavity, and the embryo at this
stage approximates a hollow ball and is known as a "blastula".
The formation of the blastula marks the end of the cleavage stage
of embryonic development.
... ... *blastocyst: The "morula" is an early embryonic *cleavage
stage consisting of a solid mass of cells (blastomeres). The next
stage is the "blastula" stage, in which the cells form a hollow
sphere. The "blastocyst" is an early form of the blastula stage,
an egg in the later stages of cleavage. The blastocyst consists
of a hollow fluid-filled ball of cells and an inner cell mass
(embryonic stem cells) from which the embryo develops.
... ... *animal-vegetal axis: After fertilization, the egg cell
acquires polarity, two poles of the egg becoming distinct from
each other. At one pole, known as the "animal pole", the
cytoplasm is apparently more active and contains the nucleus. At
the other pole, called the "vegetal pole", the cytoplasm is
apparently less active and contains most of the nutritive
material ("yolk") of the egg. The general organization of the
future animal is apparently closely related to the polarity of
the egg. (The archaic and confusing terminology "animal" vs.
"vegetal" in this context derives from 19th century microscopy,
but is still in use.)
... ... *zygote: In general, the term "zygote" refers to the cell
formed by the union of male and female gametes (sperm and egg
cells).
... ... *abembryonic [region]: In general, the area of the
mammalian blastocyst opposite the region where the embryo is
formed (opposite the stem cell region).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE MOLECULAR BIOLOGY OF SPERM-EGG FERTILIZATION
Sexual reproduction, defined in this context as the fusion of two
haploid cells (gametes) during fertilization to form a diploid
zygote, occurs in almost all eukaryotes. Fertilization is in
effect the cellular bridge between generations, but although it
has been studied for more than a century, it remains one of the
least understood fundamental biological processes [*Note #1]. In
general, most animal sperm have an *acrosomal vesicle anterior to
the nucleus. The opening (*exocytosis) of the vesicle is required
for fertilization. Animal eggs are contained within an egg
envelope, with a gelatinous matrix external to the envelope.
... ... V.D. Vacquier (University of California San Diego, US)
presents a review describing the molecular diversity in the steps
of sperm-egg interaction, with examples of the apparent rapid
evolution of fertilization molecules. The author makes the
following points:
     1) A generalized scheme of animal fertilization involves 5
steps in sperm-egg interaction. Spermatozoa may be
*chemotactically attracted to swim toward the egg by egg-released
molecules (step A). Depending on the species, sperm bind to the
egg envelope either before or after the opening of the sperm
acrosomal vesicle (steps B and C). Soon after exocytosis of the
acrosomal vesicle occurs (called the "acrosomal reaction"), a
hole is created in the egg envelope through which the sperm
passes (step D). Once the sperm is through the envelope, the two
cells fuse (step E) and the sperm nucleus is incorporated into
the egg cytoplasm.
     2) One or more of the 5 steps can exhibit species
specificity, meaning that if spermatozoa and eggs are from the
same species, their interaction leading to fusion is more
efficient than if two gametes are from different species.
     3) The author presents details of current knowledge of the
molecular events involved in each of the 5 steps. Concerning
chemoattraction, the author notes that although sperm
chemoattraction to egg-derived factors has been
phenomenologically demonstrated in most invertebrate and some
vertebrate groups, the chemical nature of the attractants is
known in but a few species. The structures of known sperm
chemoattractants are chemically unrelated, indicating that they
evolved independently in different phyla. For example, in *brown
algae, female gametes release species-specific 11-carbon cyclo-
olefinic hydrocarbons, which in picomolar concentrations attract
male gametes. In the *ciliate protozoan Euplotes, small mating-
type-specific proteins control the cellular activities of
chemotaxis, *conjugation, and growth. In marine invertebrates,
sperm swim up gradients of an egg-derived peptide... Although
human sperm are attracted to the *follicular fluid surrounding
the human egg cell, the chemoattractant remains unknown.
-----------
V.D. Vacquier (University of California San Diego)
Evolution of gamete recognition proteins.
(Science 25 Sep 98 281:1995)
QY: Victor D. Vacquier, Univ. of Calif. San Diego 619-534-2230
-----------
Text Notes:
... ... *Note #1: When sperm were first discovered by Leeuwenhoek
and others in 1678, sperm were thought to be parasites living in
semen (the term "spermatozoa" literally means "sperm animals").
Although Leeuwenhoek soon came to believe that each sperm
contained a preformed embryo, most investigators did not accept
this idea and continued to regard sperm as unimportant in
reproduction. The first experimental evidence suggesting the
importance of sperm in reproduction came from experiments by
Lazzaro Spallanzani in the 18th century. Spallanzani demonstrated
that filtered toad semen devoid of sperm would not fertilize
eggs. But he concluded that the viscous fluid retained by the
filter paper -- and not the sperm -- was the agent of
fertilization. Like those who preceded him, Spallanzani believed
the sperm cells were parasites. His experiment stands, but not
his conclusion. It was not until 1824 that sperm cells were
proposed as the active agents in fertilization by J.L. Prevost
and J.B. Dumas, but several decades would pass before the idea
was generally accepted by most biologists. The involvement of
sperm cells in the fertilization of egg cells is therefore a
relatively modern idea.
... ... *acrosomal vesicle: (acrosome) The acrosome is a
specialized penetrating vesicular organelle at the tip of a
spermatozoan. It contains several enzymes (e.g., hyaluronidase)
that are released when the sperm contacts the egg cell, and which
effectively puncture the egg cell envelope and/or egg cell
membrane.
... ... *exocytosis: In general, any process in which an
intracellular vesicle fuses with the plasma membrane of the cell
with a resultant release of the contents of the vesicle into the
extracellular phase.
... ... *chemotactically attracted: In general, the term
"chemotaxis" refers to any movement of an organism in response to
chemical concentration gradients. 
... ... *brown algae: (Phaeophyta) Mostly brown seaweeds of
coastal and marine environments. Brown algae produce *flagellated
motile sperm cells that fertilize egg cells.
... ... *flagellated: A flagellum is a long threadlike extension
providing locomotion for a cell.
... ... *ciliate protozoan: (Ciliophora) Cilia are short
threadlike extensions, hundreds usually present on an individual
ciliated cell, the cilia undergoing synchronized movements to
produce locomotion of the protozoan (e.g., the common
Paramecium).
... ... *conjugation: This is the usual reproductive process in
ciliated protozoa, but it also occurs in some species of
bacteria. The essential process is physical contact of two cells
with transfer of genetic material between them.
... ... *follicular fluid: In this context, the liquid
environment of the mammalian egg-cell containing follicle.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Oct98
For more information: http://scienceweek.com/swfr.htm

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2. DEVELOPMENTAL BIOLOGY: ON DEVELOPMENT OF THE PANCREAS
     The pancreas is a compound gland found in all vertebrates,
functioning both as a duct-gland (exocrine gland) discharging
digestive enzymes into the upper portion of the small intestine
(into the duodenum), and as a ductless-gland (endocrine gland)
secreting hormones that include insulin and glucagon into the
bloodstream. Many invertebrates have a gland involved in the
secretion of digestive enzymes, and sometimes such a gland is
also called "pancreas".
     As an organ, the human pancreas is oblong, approximately
12.5 centimeters long and 2.5 centimeters thick. It lies
posterior to the great curvature of the stomach and is connected,
usually by two ducts, to the duodenum. These two ducts carry the
exocrine secretions, called "pancreatic juice", directly into the
intestine. Each day, the pancreas produces 1.2 to 1.5 liters of
pancreatic juice, which is a clear and colorless liquid
consisting mostly of water, some salts, sodium bicarbonate (which
neutralizes the acid draining from the stomach), and several
digestive enzymes.
     The cells in the pancreas that produce the digestive enzymes
are called "acinar cells" [from Latin _acinus_, grape], so named
because they are found in grape-like clusters. Between these
clusters are islands of another type of secreting tissue,
collectively known as the islets (or islands) of Langerhans.
There are approximately 1 to 2 million islets of Langerhans in
the human pancreas, and it is these groups of cells that are
responsible for secreting hormones that include insulin and
glucagon.
     Four types of hormone-secreting cells compose the pancreatic
islets: a) alpha cells secrete glucagon; b) beta cells secrete
insulin; c) delta cells secrete the growth-hormone-inhibiting
hormone called "somatostatin", which acts locally to inhibit the
secretion of insulin and glucagon; d) F-cells secrete a
pancreatic polypeptide which regulates release of pancreatic
digestive enzymes. The islets are infiltrated by blood
capillaries into which their secretions diffuse.
     The principal physiological activity of glucagon is to
increase blood glucose level when it falls below normal; the
chief physiological action of insulin is opposite to that of
glucagon: insulin helps adjust blood glucose level by decreasing
the blood glucose level if necessary.
     The main target tissue of glucagon is the liver, and the
hormone has a number of actions: a) glucagon accelerates the
conversion of glycogen into glucose (glycogenolysis); b) glucagon
promotes formation of glucose from lactic acid and certain amino
acids (gluconeogenesis; c) as a result, glucagon enhances release
of glucose into the blood and blood glucose level rises.
     In general, insulin also has a number of important actions:
a) insulin accelerates the transport of glucose from the blood
into cells, especially into the muscle fibers involved in
voluntary movements (skeletal muscle fibers); b) insulin
accelerates the conversion of glucose into glycogen
(glycogenesis); c) insulin accelerates the entry of amino acids
into cells, and accelerates the synthesis of proteins; d) insulin
accelerates the conversion of glucose or other nutrients into
fatty acids (lipogenesis); e) insulin decreases the breakdown of
glycogen (decreases glycogenolysis); f) insulin decreases the
metabolic formation of carbohydrates from non-carbohydrates
(decreases gluconeogenesis).
     A deficiency of insulin or defects of insulin receptors on
target cells produces the disease known as "diabetes mellitus".
Hypersecretion of insulin results in the disease called
"hyperinsulinism". (See related background material below for
history of the discovery of insulin.)
     Concerning the digestive enzymes secreted by the acinar
cells of the pancreas, these include a) the carbohydrate-
digesting enzyme pancreatic amylase; b) several protein-digesting
enzymes (trypsin, chymotrypsin, carboxypeptidase); c) pancreatic
lipase, the principal triglyceride-digesting enzyme in the adult
body; d) the nucleic-acid-digesting enzymes ribonuclease and
deoxyribonuclease. In order to prevent digestion of the cells of
the pancreas, the digestive enzymes are secreted in an inactive
form which become activated in the duodenum. In general, the
pancreatic secretions are regulated by both neural and hormonal
mechanisms.
     The islets of Langerhans are named after Paul Langerhans
(1847-1888), who discovered and named the islets in the course of
research for his doctoral dissertation, a project during which he
prepared the first careful description of the microscopic
structure of the pancreas. The research career of Langerhans was
soon after terminated when he contracted tuberculosis, and at the
age of 27 he abandoned research and moved to the island of
Madeira in an attempt to find a cure for his pulmonary disease.
He practiced medicine on Madeira and died there at the age of 41.
     The pancreas is an unusual gland in that it is bifunctional,
containing two types of secreting cells arranged in two different
morphologies. The gland is also the site of a number of serious
diseases involving acute or chronic infections, tumors, and
cysts. The entire gland can be removed surgically and life
sustained by the administration of insulin and potent pancreatic
extracts. Approximately 80 to 90 percent of the pancreas can be
removed surgically without producing an insufficiency of either
endocrines (insulin or glucagon) or exocrines (water,
bicarbonate, and enzymes).
     In this context, the term "anlage" (pl. anlagen or anlages)
refers in general to any group of embryonic cells identified as a
future organ or body part. Also, in this context, the term
"signal" refers to a specific chemical signal or "messenger"
which provokes a specific response in cells, usually by
interacting with a cell-surface receptor, and the term "signal
pathway", in its broadest sense, refers to the biochemical
pathway involved in the production of the signal, the interaction
of the signal with the cell, and the response of the cell to the
signal.
     In animals, "epithelial cells" compose the cell layers that
form the interface between a tissue and the external environment,
for example, the cells of the skin, the lining of the intestinal
tract, the lung airway passages, and the lining of various ducts.
The term "mesenchyme" refers to an embryonic connective tissue
from which all other connective tissue derives. In general, all
embryonic organs consist of an epithelium and an associated
mesenchyme, and epithelial-mesenchyme interactions are apparently
of great importance in development.
     In the embryos of higher animals, there occurs the
transformation of a spherical single-layer "blastula" into a
3-layered "gastrula" consisting of ectoderm (outermost layer),
mesoderm (middle layer), and endoderm (innermost layer)
surrounding a cavity with one opening. The 3 layers are called
the "germ layer", and these layers, via further cell
differentiation and proliferation, determine the development of
all the major body systems and organs.
... ... S.K. Kim and M. Hebrok (2 installations, US) present a
review of current research on intercellular signals regulating
pancreas development and function, the authors making the
following points:
     1) The authors point out that classic studies of dissected
and recombined embryonic pancreas tissues published 40 years ago
suggested that epithelial-mesenchymal cell interactions regulate
growth and cell differentiation in the embryonic pancreas. Modern
studies have revealed additional cell interactions involving
pancreatic epithelium and mesoderm-derived tissues essential for
normal pancreatic development. Recently, many of the signaling
pathways likely to govern cell interactions in the developing
pancreas have been identified, allowing detailed studies of the
genetic, molecular, and cellular basis of intercellular signaling
that establishes proper pancreas development and function.
     2) During embryogenesis, organs develop in a stereotyped
sequence along the respiratory and gastrointestinal tracts. This
organization is accomplished through the temporal and spatial
regulation of signaling pathways that specify and thereby
separate the distinct organ anlagen. Many endoderm-derived
organs, including lungs, trachea, thyroid, liver, and
gallbladder, develop from a ventral portion of the embryonic gut.
In contrast, the first sign of pancreas morphogenesis in birds
and mammals is a dorsal evagination of the foregut caudal to the
stomach anlage. Subsequently, a ventral bud develops adjacent to
a portion of the liver (liver diverticulum). This ventral bud
translocates to the dorsal side during gut rotation to form the
mature pancreas. In some species, including humans, this is
accomplished by fusion of the dorsal and ventral lobes. Pancreas
development, therefore, requires specification of the pancreas
anlage along both the anterior-posterior and dorsal-ventral axes
of the embryo.
     3) The authors point out that genetic and biochemical
relationships of the signaling pathways involved in pancreas
development have been revealed largely from in vitro studies and
examination of embryonic axial patterning, organogenesis, and
cell fate determination in the fruit fly Drosophila melanogaster,
the nematode worm Caenorhabditis elegans, and the toad Xenopus
laevis. Given apparent evolutionary conservation of insulin
activities, insulin regulation, or insulin signal transduction
machinery, studies in these organisms are likely to remain
important for understanding the developmental genetics and
function of insulin-producing cells. The authors conclude: "The
evolutionary conservation of insulin-signaling and the profound
impact of pancreatic diseases on human health insure continuing
interest and relentless growth in our understanding of the
signals governing pancreas development and function."
-----------
S.K. Kim and M Hebrok: Intercellular signals regulating pancreas
development and function.
(Genes & Development 15 Jan 01 15:111)
QY: Seung K. Kim: seungkim@cmgm.stanford.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PHYSIOLOGY: ON INSULIN
     The term "hormone" was first used in 1902 by William Bayliss
(1860-1924) and Ernest Starling (1866-1927) to describe the
action of secretin, a hormone produced by the mammalian *duodenum
and which stimulates the secretion of pancreatic juice. Based
more on physiological effects than on chemical structure,
subsequent use of the term "hormone" led to the definition of
hormones as signal molecules, products of glandular cells, with
the signal molecules secreted into the internal milieu, most
frequently into the blood. Acting on target cells, these chemical
messengers coordinate activities of different parts of the body.
Target cells, in turn, respond according to their degree of
differentiation, age, and functional and nutritional status, the
target cells integrating many hormonal and neuronal regulatory
stimuli. The target cell "receptor" is a specific chemical
structure required for target cells to receive and recognize a
hormone messenger. In general, hormone receptors transduce the
external chemical signals provided by hormones and are
responsible for the initiation of the first cellular responses to
hormones, this first response usually involving a cascade of
specific biochemical reactions inside the cell.
     The disease *diabetes mellitus has a long history, but it
was only in 1869 that Paul Langerhans (1847-1888) identified a
new type of cell in the pancreas, cells apparently glandular in
character, and histological groups of these cells came to be
called "islets of Langerhans". In 1889, von Mering and Minkowski
demonstrated that diabetes mellitus, characterized in its most
evident form by permanent high blood sugar (hyperglycemia) and
glucose in the urine (glycosuria; glucosuria) could be induced
experimentally in the dog by total removal of the pancreas. This
demonstrated the essential role of the pancreas in the regulation
of glucose balance (glucose homeostasis). The hormone responsible
for this action was called "insulin", and was finally isolated
from the pancreas in 1922 by Frederick Banting (1891-1941) and
Charles Best (1899-1978) [*Note #1]. This discovery had an
enormous impact in physiology, biochemistry, and medicine. The
discovery had an extremely beneficial effect on the prognosis and
therapy of insulin-dependent diabetes, allowing a specific
replacement treatment for endogenous insulin deficiency, which if
untreated is potentially fatal. The arrival of the insulin era
was also of major importance in protein chemistry. Insulin was
one of the first proteins to be crystallized (Abel 1926), and its
primary structure was the first to be elucidated (Sanger 1953).
Partial synthesis was accomplished between 1964 and 1966, and
total synthesis was completed in 1974. Human insulin, available
commercially, is currently prepared by a modification of pork
insulin or by a biosynthetic process involving genetic
engineering.   
     The insulin molecule consists of two polypeptide chains
connected by two disulfide bridges, with a third disulfide bridge
linking parts of one chain. This two-chain structure has
evidently been present throughout evolution, but major variations
in the amino acid sequences are observed between species. Various
mammalian insulins usually have similar potencies in all mammals,
including humans; fish insulin has considerable potency in
mammals. It is evidently the 3-dimensional structure of insulin,
and not the primary sequence of amino acid residues, which is
responsible for its potency across different species: variations
in amino acid sequence are still potent, provided the specific 3-
dimensional structure is maintained.
     Insulin apparently exerts its glucose-lowering effects by
stimulating glucose uptake in tissues such as skeletal muscle,
suppressing fatty acid release from fat (adipose) tissue, and
inhibiting production of glucose by the liver. Muscle, liver, and
fat, therefore, are widely viewed as the principal insulin-
sensitive tissues in the body. The brain, in contrast, has
historically been considered insulin-insensitive because its
ability to use glucose does not require insulin. Because of this,
the idea that insulin participates in the central nervous system
control of food intake and body weight was received with
skepticism when it was first proposed more than 20 years ago.
Since then, however, support for this hypothesis has steadily
accumulated, including the demonstration that insulin is
transported across the *blood-brain barrier, that it is effective
in suppressing food intake when given directly into the brain,
and that insulin receptors are concentrated in brain areas
involved in energy homeostasis.
... ... J.C. Bruening et al (10 authors at 3 installations, DE
US) present new evidence concerning insulin signaling in the
brain, the authors making the following points:
     1) The authors point out that insulin receptors and insulin
signaling proteins are widely distributed throughout the central
nervous system. To study the physiological role of insulin
signaling in the brain, the authors created mice with a neuron-
specific disruption of the insulin receptor gene (no-insulin-
receptor knockout mice = NIRKO mice). Inactivation of the insulin
receptor had no effect on brain development or neuronal survival.
     2) The authors report that female NIRKO mice showed
increased food intake, and both male and female mice developed
diet-sensitive obesity with increases in body fat and plasma
*leptin levels, mild insulin resistance, elevated plasma insulin
levels, and *hypertriglyceridemia. NIRKO mice also exhibited
impaired *spermatogenesis and *ovarian follicle maturation
because of *hypothalamic dysregulation of *luteinizing hormone.
The authors conclude: "Thus, insulin receptor signaling in the
central nervous system plays an important role in regulation of
energy disposal, fuel metabolism, and reproduction."
... ... In a commentary on the this work, Michael W. Schwartz
(University of Washington Seattle, US) states: "Bruening and
colleagues provide important evidence to support [the hypothesis
that insulin participates in the central nervous system control
of food intake and body weight]... The stage is now set for
studies to determine if impaired central nervous system signaling
by insulin and leptin contribute to the pathogenesis of two
common metabolic diseases, obesity and *type 2 diabetes."
-----------
J.C. Bruening et al: Role of brain insulin receptor in control of
body weight and reproduction.
(Science 22 Sep 00 289:2122)
QY: Jens C. Bruening: jens.bruening@uni-koeln.de
-----------
Michael W. Schwartz: Staying slim with insulin in mind.
(Science 22 Sep 00 289:2066)
QY: Michael W. Schwartz: mschwart@u.washington.edu
-----------
Text Notes:
... ... *duodenum: The duodenum is the first segment of the
intestine attached to the stomach.
... ... *diabetes mellitus: A metabolic disease in which
carbohydrate utilization is reduced and that of lipid and protein
enhanced, the disease caused by an absolute or relative
deficiency of the hormone insulin. 
... ... *Note #1: For their isolation of insulin, Banting and
MacLeod received the Nobel Prize for Physiology and Medicine in
1923. The human story has been told many times in books and in
film. Briefly, Banting was a scientifically-minded physician
without a laboratory who came to the established physiologist
MacLeod in the early part of 1921 to beg laboratory space to work
on the isolation of insulin. MacLeod tried to discourage Banting,
saying he would not succeed, but finally MacLeod agreed to give
Banting some laboratory space and an assistant, Charles Best.
MacLeod then went off to Europe and did not return until
September 1921, only to find Banting and Best had indeed isolated
insulin. Banting and Best wanted to present their work to the
December 1921 meeting of the American Physiological Society, but
they were unable to do this because neither of them were members
of the Society. MacLeod, who was a member, attached his name to
the paper, and the paper was published under the three names in
1922. When the Nobel Prize was awarded to Banting and MacLeod,
Banting was furious and at first refused to accept the prize. He
finally did accept, but he immediately transferred half the prize
money to Charles Best. MacLeod then gave half _his_ prize money
to James Collip, an assistant who had helped in the later
purification of insulin.
... ... *blood-brain barrier: A selective mechanism opposing the
passage of most ions and large molecular-weight compounds from
the blood to brain tissue, the mechanism operating in a
continuous layer of endothelial cells connected by tight
junctions between cells. (Endothelial cells are flat cells
forming a layer lining blood vessels, lymphatic vessels, the
heart, etc.)
... ... *leptin: First isolated in 1994 by Y. Zhang et al, leptin
is a hormone secreted by fat cells (adipocytes), the hormone
circulating in blood at levels proportionate to fat stores and
acting in the brain to reduce food intake and body weight.
Insulin deficiency in type 1 diabetes does not cause weight gain,
but rather is associated with severe and progressive weight loss.
In contrast, leptin deficiency is associated with severe obesity
syndrome.
... ... *hypertriglyceridemia: Abnormally high concentrations of
triglycerides in blood.
... ... *spermatogenesis: In general, the entire process that
results in the production of sperm cells.
... ... *ovarian follicle: One of the spherical cell aggregations
in the ovary, the aggregation containing an egg cell (ovum).
... ... *hypothalamic: The hypothalamus is a deep brain
structure with various clusters of nerve cells controlling
several important homeostatic functions such as temperature
regulation and food intake, and in addition the sex drive,
aggressive emotions, psychosomatic effects, etc. The
hypothalamus essentially integrates the activity of the
autonomic nervous system, and it acts as an intermediary between
the endocrine (hormone) system and the nervous system, with
various hypothalamic neuron types secreting hormones themselves.
... ... *luteinizing hormone: A hormone produced by the pituitary
gland. Luteinizing hormone has a complex interaction spectrum,
but in general, this hormone stimulates secretion of testosterone
in males, and stimulates secretion of estrogen in females. The
hormone is important both in the production of sperm and in the
production of egg cells.
... ... *type 2 diabetes: Adult-onset diabetes mellitus.
Juvenile-onset diabetes mellitus is type 1 diabetes mellitus.
There are two major forms of diabetes: diabetes mellitus and
diabetes insipidus. When the term "diabetes" is used alone, the
usual referent is diabetes mellitus, which in turn has two types:
juvenile-onset (type 1) and adult-onset (type 2). The various
forms and types of diabetes differ in important ways in both the
physiology and biochemistry of the disease processes. 
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 8Dec00
For more information: http://scienceweek.com/swfr.htm

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3. MEDICAL BIOLOGY: ON BOTULINUM TOXIN AS A BIOLOGICAL WEAPON
     As the story goes, in the year 1793 in Wildbad, Germany, 13
people sharing an uncooked smoked sausage that had been sitting
for hours became ill and 6 died. To describe their illness, the
word "botulism", from the Latin _botulus_, meaning "sausage",
came into use. It took more than a century before it was
understood that the skin of the sausage had made the meat below
it low in oxygen content, so that with enough time the common
bacterial organism Clostridium botulinum could multiply and
produce enough toxin to make the sausage extremely lethal.
     The organism C. botulinum, an *anaerobic spore-forming
bacillus, is distributed throughout the environment, particularly
in ordinary soil. When the spores contaminate preserved or canned
foods with low oxygen levels and nutrients that support growth,
the spores germinate and the clostridium bacteria soon begin
producing toxins. Botulinus (or botulinum) neurotoxins (there are
a number of types) are the most potent toxins known. The toxins
are heat-labile, so properly heated foods do not produce the
toxic condition called "botulism". In general, the toxin acts on
the peripheral nervous system by inhibiting the release of
acetylcholine at cholinergic synapses such as the neuromuscular
junction. This causes a muscular paralysis that is usually fatal.
     This report concerns botulinum toxin as a biological weapon
of warfare or terrorism. Although biological warfare is usually
treated as a modern phenomenon, its history is more than 600
years old. In 1347, in the siege of Caffa in the Crimea, the
Mongols apparently hurled bodies of plague victims over the walls
protecting the Genoese defenders. Subsequently, Genoese ships
carried the plague bacillus (Yersinia pestis) to Europe with the
resultant extensive epidemic known as the "Black Death". In World
War I (1914-1918), the Germans apparently infected Romanian
cavalry horses, and US livestock destined for shipment to the
Allies, with glanders, a lesion-producing bacterial disease.
Charges of germ warfare were made by the Chinese against United
Nations forces in the Korean War of 1950-1953, but there was no
substantiation of these charges. More recently (see below) there
has been evidence of both intended and actual use of botulinum
toxin for either military or terrorist purposes.
... ... S.S. Arnon et al (18 authors at 10 installations, US)
present a report on botulinum toxin as a biological weapon, the
authors making the following points:
     1) The authors point out that botulinum toxin is the most
poisonous substance known. A single gram of crystalline toxin,
evenly dispersed and inhaled, would kill more than 1 million
people, although technical factors would make such dissemination
difficult. The basis of the phenomenal potency of botulinum toxin
is enzymatic: the toxin is a zinc proteinase that cleaves one or
more of the [membrane-] fusion proteins involved in the release
of the neurotransmitter substance acetylcholine by neuronal
synaptic vesicles into the neuromuscular junction. In this
system, acetylcholine activates muscle fibers, and if
acetylcholine release is prevented, nerve fibers have no means of
activating these muscle fibers, and the muscle is effectively
paralyzed.
     2) In the US, botulinum toxin is licensed for treatment of
various diseases and conditions, including migraine headache,
lower back pain, stroke, etc. Therapeutic botulinum toxin,
however, is an impractical bioterrorist weapon: a vial of a
typical therapeutic preparation of botulinum toxin contains only
approximately 0.3 percent of the estimated lethal human
inhalation dose, and 0.005 percent of the estimated lethal oral
dose.
     3) Terrorists have already attempted to use botulinum toxin
as a bioweapon. Between 1990 and 1995, aerosols containing
botulinum toxin were dispersed at multiple sites in downtown
Tokyo (JP) and at US military installations in Japan on at least
3 occasions, the dispersals by the Japanese cult Aum Shinrikyo.
These attacks apparently failed because of faulty microbiological
technique, deficient aerosol-generating equipment, or internal
sabotage. The perpetrators evidently obtained C. botulinum from
soil they had collected in northern Japan.
     4) Military development and use of botulinum toxin evidently
began in the 1930s. The head of the Japanese biological warfare
group known as "731" admitted to feeding cultures of C.
botulinum, with lethal effect, to prisoners during the Japanese
occupation of Manchuria in the 1930s. The US biological weapons
program first produced botulinum toxin during World War II (1939-
1945). Evidently, because of concerns that Nazi Germany had
weaponized botulinum toxin, more than 1 million doses of
botulinum toxoid vaccine were made for Allied troops preparing to
invade Normandy on D-Day.
     5) Four of the countries currently listed by the US as
"state sponsors of terrorism" (Iran, Iraq, North Korea, Syria)
have developed, or are believed to be developing, botulinum toxin
as a weapon. After the 1991 Persian Gulf War, Iraq admitted to
the United Nations inspection team to having produced 19,000
liters of concentrated botulinum toxin, of which approximately
10,000 liters were loaded into military weapons. These 19,000
liters of concentrated toxin are currently not fully accounted
for, and they constitute approximately 3 times the amount needed
to kill by inhalation the entire present human population of the
world. In 1990, Iraq deployed specially designed missiles with a
600-kilometer range, and 13 of these missiles were filled with
botulinum toxin, 10 with *aflatoxin, and 2 missiles with *anthrax
spores. Iraq also deployed special 180-kilogram bombs for
immediate use, with 100 bombs containing botulinum toxin, 50
bombs containing anthrax spores, and 7 bombs containing
aflatoxin. As far as is known, Iraq weaponized more botulinum
toxin than any other of Iraq's known biological agents.
     6) Concerning the chemistry of botulinum toxin, the authors
point out that botulinum toxin is a simple di-chain polypeptide
that consists of a 100-kilodalton heavy chain joined by a single
disulfide bond to a 50-kilodalton light chain. The 3-dimensional
structure of botulinum toxin was recently resolved to 3.3
angstroms. The toxin's light chain is a Zn(++)-containing
*endopeptidase that blocks acetylcholine containing vesicles from
fusing with the terminal membrane of the motorneuron axon, this
resulting in flaccid muscle paralysis. The estimated human lethal
dose (70 kilogram individual) of botulinum toxin is approximately
0.09 to 0.15 micrograms intravenously or intramuscularly, 0.70 to
0.90 micrograms by inhalation, and 70 micrograms orally.
-----------
S.S. Arnon et al: Botulinum toxin as a biological weapon.
(J. Amer. Med. Assoc. 28 Feb 01 285:1059)
QY: Stephen S. Arnon: sarnon@dhs.ca.gov
-------------------
-----------
Text Notes:
... ... *anaerobic: In general, requiring an absence of oxygen.
... ... *aflatoxin: A potent toxin produced by a species of mold
(Aspergillus flavus), the organism a frequent contaminant of
peanuts, corn, grains, and other foods.
... ... *anthrax: See related background material below.
... ... *endopeptidase: (endoproteinase; proteinase) In general,
any enzyme that hydrolyzes non-terminal peptide bonds in
oligopeptides.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BIOLOGICAL TERRORISM
Although political events during the past decade have reduced
fears of biological warfare among the major powers, the pathogens
likely to be used in such warfare have not vanished, and there is
now an apparent growing concern about the use of such pathogens
in acts of biological terrorism ("bioterrorism").
... ... Donald A. Henderson (Johns Hopkins University, US)
presents a review of current views, expectations, and contingency
plans, the author making the following points concerning the
scientific fundamentals: 1) The expected scenario following
release of an aerosol cloud of a biological agent is entirely
different from that following an attack of nuclear or chemical
terrorism. A biological agent aerosol release could be silent and
would almost certainly be undetected. The cloud would be
invisible, odorless, and tasteless. It would behave much like a
gas in penetrating interior areas, and the release would not be
suspected for days or weeks later. 2) The implicit assumption has
frequently been that chemical and biological threats and the
responses to them are so generically similar that they can be
readily handled by a single "chembio" expert, usually a chemist.
This is a serious misapprehension. 3) Any of thousands of
biological agents that are capable of causing human infection
could be considered a potential biological weapon, but
realistically only a few pose serious problems. Only a very small
number of species of these pathogens can be cultivated and
dispersed effectively so as to cause cases and deaths in numbers
that would threaten the functioning of a large community. The
current consensus is that there are 11 pathogens "very likely to
be used." *Smallpox, *plague, *anthrax, and *botulism are
considered the top four candidates. The others are *tularemia,
*glanders, *typhus, *Q fever, *Venezuelan equine encephalitis,
*Marburg virus, and *influenza virus. 4) Any group with
sufficient resources could purchase prepared supplies of
aerosolizable organisms and could transport them easily, because
only small quantities are needed to inflict casualties over a
wide area. No mechanisms currently exist for screening to
intercept such materials at state or national borders. 5) Of the
potential biological weapons, smallpox and anthrax pose by far
the greatest threats, but these pathogens have different clinical
and epidemiological properties. Smallpox poses an unusually
serious threat, in part because virtually everyone is now
susceptible, vaccination having stopped worldwide 20 or more
years ago as a result of the eradication of the disease. It is
probable that no more than 20 percent of the world population is
protected; for the unprotected, fatality rates after infection
are 30 percent. Another problem is that there are no longer any
manufacturers of smallpox vaccine, which means large-scale
vaccination immediately after an outbreak is currently not
possible. 6) Concerning an inhalation anthrax epidemic, the
scenario is as dangerous as that for smallpox. After 2 to 3 days
anthrax-infected individuals would appear in emergency rooms and
doctors' offices with a variety of nonspecific symptoms such as
fever, cough, and headache. Within a day or two, patients would
become critically ill and then die within 24 to 72 hours. The
fatality rate for anthrax is 80 percent or greater. 7) The author
concludes: "Once the medical community rallied... in educating
peoples and policy-makers everywhere about the dread realities of
a nuclear winter. Perhaps the same should now be done with
respect to the realities of biological weapons, which are now
considered to be a more serious threat than the nuclear ones."
-----------
Donald A. Henderson: The looming threat of bioterrorism.
(Science 26 Feb 99 283:1279)
QY: Donald A. Henderson, Johns Hopkins Center for Civilian
Biodefense Studies, Johns Hopkins Univ., Baltimore, MD 21202 US.
-----------
Text Notes:
... ... *Smallpox: This is an acute eruptive contagious disease
caused by a poxvirus (Orthopoxvirus, a member of the family
Poxviridae). The average incubation period is 8 to 14 days.
Following the incubation period, the onset symptoms are
constitutional: chills, high fever, backache, headache. In from 2
to 5 days, these symptoms subside and the skin eruptions appear.
Considering the temporal course of the disease, a smallpox
epidemic would probably not become evident until 2 to 3 weeks
after release of an aerosol.
... ... *plague: In this context, this term refers to the acute
infectious disease caused by the bacterium Yersinia pestis, the
disease marked by high fever, toxemia, and prostration. The
pathogen is usually transmitted to man by fleas that have bitten
infected rodents, and there are various forms of the disease. The
incubation period is 2 to 7 days. The fatality rate is near 50
percent, with usually 100 percent fatality for the pneumonic form
of the disease.
... ... *anthrax: This disease is caused by the bacterium
Bacillus anthracis, and is usually transmitted by infected
animals through traumatized human skin. The disease is marked by
hemorrhage and blood effusions in various organs and body
cavities, and by symptoms of extreme prostration. In the context
of this report, the disease entity of concern is "inhalation
anthrax", which is a more serious human disease than anthrax
contracted from an animal through the skin. Inhalation anthrax
produces hemorrhagic pneumonia with shock and is usually fatal
(fatality above 80 percent).
... ... *botulism: This disease is caused by toxins of the
bacterium Clostridium botulinum, an organism common in soil and
sometimes in animal feces. Symptoms appear 18 to 24 hours after
entry of the toxins, and the most severe symptoms are the result
of effects on the neuromuscular system. Death occurs from
respiratory paralysis or cardiac arrest. The fatality rate is
high. Ordinarily, botulism is not an actual human infection,
since the human disease is almost always caused by ingestion of
food contaminated with toxins produced by C. botulinum, which is
anaerobic and grows only under conditions of low or absent oxygen
(e.g., in canned foods). The botulinum toxins are among the most
highly toxic substances known: the lethal dose for a human is
estimated to be in the range 1 to 2 micrograms.
... ... *tularemia: This disease is caused by the bacterium
Francisella tularensis, a pathogen usually transmitted to humans
by biting arthropods (e.g., insects), direct contact with
infected animal tissue, ingestion of contaminated food or water,
and inhalation of aerosols. Apparently, inhalation of only 50
individual F. tularensis bacteria can result in infection.
Symptoms appear within a week. The disease can usually be
controlled with antibiotics.
... ... *glanders: A common disease of horses, mules, and
donkeys, caused by the bacterium Burkholderia mallei. The
inhalation form of the disease may lead to primary pneumonia.
The disease can usually be controlled with antibiotics.
... ... *typhus: A group of acute infectious and contagious
diseases caused by the bacterial group Rickettsaie. These
diseases are characterized by fever, headache, malaise, and
prostration.
... ... *Q fever: Also caused by a Rickettsaie bacterium, but the
symptoms resemble influenza, nonbacterial pneumonia, hepatitis,
or encephalopathy.
... ... *Venezuelan equine encephalitis: This is a viral disease
usually transmitted by mosquitoes from horses to humans. It is
caused by a togavirus, subgroup alphavirus. In humans, the
symptoms are similar to those of influenza.
... ... *Marburg virus: One of the two notorious African
Hemorrhagic Fevers (the other is Ebola virus), highly virulent,
with infections usually ending in death. These viruses have the
highest mortality rate (as much as 90 percent) of all the viral
hemorrhagic fevers. The disease was first recognized in 1967.
... ... *influenza virus: Any of a group of influenza viruses,
all of the family Orthomyxoviridae. The influenza diseases
usually have a sudden onset, are highly contagious, and easily
produce large-scale epidemics. Apparently, if only a few cells of
the respiratory *epithelium are infected by deposited virus
particles, the infection can proceed. The severity of symptoms
and the outcome depends on which strain of the virus is the
pathogen.
... ... *epithelium: In animals and humans, epithelial cells
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 21May99
For more information: http://scienceweek.com/swfr.htm

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4. EARTH SCIENCES:
EVIDENCE FOR AN IMPACT EVENT RELATED TO THE PERMIAN 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.
     The geological period ranging approximately from 146 million
years ago to 65 million years ago is called the "Cretaceous
period", and the geological period comprising the approximate 
time-frame 65 million to 3 million years ago is called the
"Tertiary period". The largest mass extinction of the past 200
million years apparently occurred 65 million years ago at the end
of the Cretaceous period, when approximately half of the genera
of multicellular organisms on Earth, including all of the
dinosaurs, suddenly died off. The geological record indicates
that a layer of impact-produced minerals and the element iridium
(an element rare in the crust of the Earth but more abundant in
primitive meteorites) was deposited at the time the dinosaurs
vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In
addition to this, the largest known crater on Earth (Chicxulub,
Yucatan, MX) to be dated at less than 1 billion years old was
apparently formed at this time. Taken together, these data imply
that the K/T mass extinction was caused by the impact into the
Yucatan peninsula of an asteroid or comet of approximately 10
kilometers in radius. 
     The "Permian period" comprises the approximate time-frame
286 to 248 million years ago, and the "Triassic period" comprises
the approximate time-frame 248 to 213 million years ago. At the
end of the Permian period, many groups of animals and plants
apparently vanished in the greatest known crisis in the history
of life on Earth. This Permian/Triassic mass extinction has
traditionally been considered to have had a slow time course, but
recent evidence has suggested this event was more abrupt than
previously recognized.
     "Stony" meteorites (aerolites) are meteorites formed solely
of rock-forming silicates, and chondrites are a type of stony
meteorite consisting of an agglomeration of millimeter-sized
globules (chondrules) that are thought to be unchanged since the
original condensation out of the nebula from which the Sun and
Solar System formed. A "carbonaceous chondrite" is a chondritic
meteorite that contains a relatively large amount of carbon, with
a resultant dark appearance. The "Murchison meteorite" is a
carbonaceous chondrite that fell in 1969 near Murchison,
Australia.
     "Fullerenes" are large molecules composed entirely of
carbon, with the chemical formula C(n), where n is any even
number from 20 to over 100. They apparently have the structure of
a hollow spheroidal cage with a surface network of carbon atoms
connected in hexagonal and pentagonal rings, and the cage large
enough to trap atoms and small molecules. Fullerenes have been
identified in the Murchison meteorite.
     In this context, the term "planetary" refers to the
primordial aggregates (primitive planets) surrounding the Sun
from which the current planets were formed.
... ... L. Becker et al (5 authors at 4 installations, US)
present new evidence concerning an impact event at the Permian-
Triassic boundary, the authors making the following points:
     1) The authors point out that the extinction event that
marks the Permian/Triassic boundary (251.4 +- 0.3 million years
ago) was the most severe in the past 540 million years, killing
off over 90 percent of all marine species, approximately 70
percent of terrestrial vertebrate genera, and most land plants.
Several new studies have demonstrated that these extinctions were
much more abrupt than previously thought, with estimates of the
extinction interval ranging from less than 500,000 to
approximately 8000 years. Proposed catastrophic hypotheses for
the Permian/Triassic boundary extinction include an exploding
meteor (bolide) (asteroidal or cometary) and or massive volcanic
lava flows (flood basalt volcanism). Other extinctions mechanisms
involving ocean anoxia, as well as changes in sea level and
climate, have also been proposed.
     2) The authors report that fullerenes [C(sub60) to
C(sub200)] from sediments of the Permian/Triassic boundary
contain trapped helium and argon with isotope ratios similar to
the apparent planetary component of carbonaceous chondrites. The
authors suggest these data imply that an impact event (asteroidal
or cometary) accompanied the extinction, as was the case for the
Cretaceous/Tertiary extinction event approximately 65 million
years ago.
     3) The authors conclude: "Based on the measured helium-3
content for the Permian/Triassic boundary and Murchison
fullerenes, the estimated size of the bolide is 9 +- 3 kilometers
or comparable to the K/T Chicxulub impactor. Such an event could
have caused the severe end-Permian mass extinction. Our results
are consistent with recent paleontological studies that now point
to a very rapid extinction event. The unique planetary signature
measured in fullerenes isolated from the Murchison carbonaceous
chondrite and from the Permian/Triassic boundary sediments
demonstrates that this distinctive noble gas carrier can survive
major impact events and contribute to the unique gas signature of
the terrestrial planetary atmospheres."
-----------
L. Becker et al: Impact event at the Permian-Triassic boundary:
Evidence from extraterrestrial noble gases in fullerenes.
(Science 23 Feb 01 291:1530)
QY: Luann Becker: lbecker00@u.washington.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PLANETARY SCIENCE:
FULLERENES AND CAGED NOBLE GASES IN METEORITES
     Fullerenes are large molecules composed entirely of carbon,
with the chemical formula C(n), where n is any even number from
32 to 100+. They apparently have the structure of a hollow
spheroidal cage with a surface network of carbon atoms connected
in hexagonal and pentagonal rings. They were discovered by
Richard E. Smalley, who received the Nobel Prize in Chemistry for
the discovery in 1996.
     Since the synthesis, isolation, and characterization of
fullerenes, there has been considerable interest in determining
whether or not this family of carbon molecules occurs naturally
on the Earth and elsewhere in the universe. But despite an
intensive effort by researchers, terrestrial discoveries of
fullerenes have been limited to the detection of mostly C(sub60)
and C(sub70) fullerenes, while extraterrestrial evidence is based
on trace findings of C(sub60) and a single mass spectrum of a
high mass carbon envelope from the *Allende meteorite.
... ... L. Becker et al (3 authors at 3 installations, US) now
report the discovery of naturally occurring fullerenes [C(sub60)
to C(sub400)] in the Allende and *Murchison meteorites and in
some sediment samples from the 65 million-year-old
*Cretaceous/Tertiary boundary layer. The authors point out that
unlike the other pure forms of carbon (diamond and graphite),
fullerenes are extractable in an organic solvent. By exploiting
this property, and also the unique ability of the fullerene cage
structure to encapsulate and retain noble gas atoms, the authors
report they have determined that both the Allende and Murchison
fullerenes and the Cretaceous/Tertiary boundary fullerenes
contain trapped noble gases with isotope ratios that can only be
described as extraterrestrial in origin. The authors suggest they
have thus identified a carrier phase for noble gases associated
with impact (asteroidal or cometary) and meteorite carbon, and
that the recognition of the fullerene carrier phase with an
extraterrestrial noble gas signature in *carbonaceous chondrites
provides pathways for understanding the origin and evolution of
planetary atmospheres and/or presolar environments. The authors
conclude: "Future searches for fullerene and (sup3)He as a tracer
of the flux of extraterrestrial material in the sediments
throughout the geologic record, and its possible association with
changes observed in climate and the biostratospheric record,
could have broad implications for the [study of the] evolution of
life on the Earth."
-----------
L. Becker et al: Fullerenes: An extraterrestrial carbon carrier
phase for noble gases.
(Proc. Natl. Acad. Sci. US 28 Mar 00 97:2979)
QY: Luann Becker [lbecker@soest.hawaii.edu]
-----------
Text Notes:
... ... *Allende meteorite: One of the more interesting
carbonaceous chondrites (see note below) is the meteorite called
Allende, which fell on 8 February 1969 at Pueblito de Allende in
northern Mexico, and which scattered 5 metric tons of material
over an area 48 kilometers long by 7 kilometers wide.
... ... *Murchison meteorite: This meteorite, which fell near
Murchison AU in 1969, contains 8 amino acids and the nucleotide
bases adenine, guanine, and uracil.
... ... *Cretaceous/Tertiary boundary layer: See main report.
... ... *carbonaceous chondrites: Chondrite meteorites are
characterized by the presence of chondrules, small spherical
inclusions of glassy rock that can be seen easily with the naked
eye if the meteorite is cut and one of the cut faces polished.
Carbonaceous chondrites contain both chondrules and volatiles,
and since the presence of the volatiles is believed to indicate
an origin which did not involve heat, the carbonaceous chondrites
are considered the least altered remains of the solar nebula from
which the planets are believed to have formed.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 26May00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE PERMIAN EXTINCTION AND THE GREENHOUSE EFFECT
In geology and paleontology, a "period" is a major subdivision of
an era of geological time distinguished by a particular system of
rocks and associated fossils. The Permian Period is the most
recent period of the Paleozoic Era, and occurred approximately
225 to 270 million years ago. The Permian was characterized by an
abundance of vertebrate and invertebrate forms, a proliferation
of reptilian forms, and a variety of vegetation forms. The
arrangement of the continents was apparently quite different than
at present, with a closer packing of land masses, and the
southern end of Africa well below the Antarctic circle. One of
the more interesting features of the Permian Period is that it
was apparently terminated by a mass biotic extinction of enormous
proportions. ... ... Peter D. Ward (University of Washington
Seattle, US), reviewing the Permian extinction and a current
model to explain it, makes the following points: 1) The Permian
extinction was the greatest mass extinction in Earth's history,
involving the extinction of 90 percent of the species in the
ocean and 70 percent of the species on land. Despite its scale,
the Permian extinction remains a deep mystery. 2) In the fossils
of the past 530 million years, there is evidence of many mass
extinctions, but evidence only of 5 extinctions that killed more
than half the extant species. The best known extinction is the
Cretaceous-Tertiary (also called K/T) event of 65 million years
ago, apparently caused by the impact of a comet or an asteroid,
the event characterized by the extinction of the dinosaurs. But
the K/T event destroyed only about 50 percent of the species on
Earth, which means it was a much less extensive extinction than
the Permian. 3) Knoll et al, in 1995, proposed that the Permian
extinction in the oceans was essentially caused by the release in
the ocean, due to movements of land masses, of carbon dioxide
trapped in sediments, and that it was ocean carbon dioxide, known
to be highly toxic to marine life, that was responsible for the
ocean mass biotic extinction. 4) In the current article, Ward
suggests that ocean carbon dioxide and volcanic gases, both
emerging into the atmosphere, resulted in a heating of the
atmosphere to critical levels. Ward proposes that the surge in
temperature is reflected in the common redness of rock strata
associated with the end of the Permian, the red color a result of
the rusting of iron compounds, and suggesting a climate change of
massive proportions. 5) The author suggests that the Permian
extinction now appears to be a new type of mass extinction,
unrelated to extraterrestrial causes, and occurring much faster
than typical extinctions triggered by internal changes to the
climate and chemistry of the Earth. The author concludes: "Are we
walking down the same path that killed off so much life 250
million years ago -- not from carbon dioxide liberated from the
oceans but from carbon dioxide liberated from our cars and
industry?" 
QY: Peter D. Ward, Univ. of Washington Seattle 206-543-8992.
(Discover August 1998) (Science-Week 28 Aug 98)
For more information: http://scienceweek.com/swfr.htm

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5. ASTROBIOLOGY: ON THE SEARCH FOR LIFE ON EUROPA
     It is an irony that although the term "living system" is
widely used in science, the term has no consensus definition.
Instead, there are many definitions: physiological, biochemical,
genetic, metabolic, thermodynamic, and so on. One might think the
question of the definition of "life" is only of philosophical
significance, but if you are designing a robot to search for
"life" on another astronomical body, precisely what is the robot
to search for? At the present time, the answer to this question
is not at all clear.
     Concerning definitions, the various definitions of "life"
currently in use can in general be summarized as follows:
     1) The physiological definition defines a living system as a
system that performs various functions such as eating,
metabolizing, excreting, breathing, moving, growing, reproducing,
and being responsive to external stimuli.
     2) The biochemical definition defines a living system as a
system that contains reproducible hereditary information coded in
nucleic acids and that metabolizes by controlling the rate of
chemical reactions with protein catalysts (enzymes).
     3) The genetic definition defines a living system as a
system capable of evolution by natural selection.
     4) The metabolic definition defines a living system as a
system with a definite boundary, the system continually
exchanging some of its materials with its surroundings, but the
exchange not altering the general properties of the system, at
least over some period of time.
     5) The thermodynamic definition defines a living system as
an open system manifesting a local increase in order (decrease in
entropy) at the expense of a larger decrease in order outside the
system.
     Of course, physical scientists can immediately think of many
"non-living" physical systems that can be described by one or
more of the above definitions, but biologists are fully aware of
such counter-examples and admit the ambiguities of all the
definitions. Research on Earth-bound living systems goes on,
ambiguity or no ambiguity, but when the focus is a search for
living systems elsewhere, the ambiguities become critical.
     Jupiter's satellite system consists of at least 16 moons,
the four largest of which are called the Galilean moons, since
they were discovered by Galileo Galilei (1564-1642). They are Io,
Europa, Ganymede, and Callisto, in order of their orbital
distance from Jupiter. Europa, which is slightly smaller than
Earth's moon, has a thick icy crust, and may also have a liquid
water mantle beneath this crust. Very few craters are present on
Europa, which suggests an active surface that renews itself and
thus erases craters as fast as they form from impacts. The
surface also shows numerous lines about 30 kilometers wide and
1000 kilometers long, and these have been interpreted to be
breaks in the crust where water from below has refrozen. The
possible existence of a liquid water mantle beneath the ice on
Europa is of great interest to planetary scientists, since such a
mantle might contain life forms. [See related background material
below for more information about Europa.]
... ... C.F. Chyba and C.B. Phillips (Stanford University, US)
present a commentary on the search for life on Europa, the
authors making the following points:
     1) The authors point out that no broadly accepted definition
of life exists, and that most proposed definitions face severe
objections. Nevertheless, one working definition of life has
become influential in the "origin-of-life" community, the
definition that life is a self-sustained chemical system capable
of undergoing Darwinian evolution. The idea that the origin of
life is the same as the origin of evolution is a popular
corollary. The authors (Chyba and Phillips) suggest, however,
that such a definition is unlikely to prove useful to a remote
_in situ_ search for life. In a search for extraterrestrial life
in our Solar System, we instead fall back on a less ambitious
notion -- "life as we know it", meaning life based on a liquid
water solvent, a suite of biogenic elements (most particularly
carbon), and a source of free energy. The authors state: "The
availability of these on a given world would suggest life to be
possible, so that further exploration may be warranted."
     2) The authors point out that only once before have we
conducted a robotic search for extraterrestrial life. The Viking
spacecraft carried three experiments to search for life in
Martian soil samples, the experiments as designed implicitly
adopting a metabolic definition by searching for chemical changes
associated with metabolism. But instead of finding unambiguous
evidence of Martian biology, Viking appears to have encountered
unanticipated non-biological oxidant chemistry. The Viking gas
chromatograph mass spectrometer failed to find any organic
molecules in the Martian soil at the parts-per-billion or parts-
per-million level. The instrumentation provided a de facto search
for life that implicitly assumed a biochemical definition: no
(detected) organics, no life. In effect, a metabolic search for
life that yielded some ambiguously positive results was undercut
by the negative results of a search based on biochemistry.
     3) The authors suggest that with the benefit of 25 years
hindsight, there are a number of lessons to be learned from the
Viking experience in the search for life on Europa. a) If payload
limits permit, a remote search for life should use experiments
that assume contrasting definitions of life. b) If only one life-
detection experiment can be flown, the biochemical definition
should probably be primary. c) It is crucial to establish the
geological and chemical context within which biological
experiments will be conducted. Had the presence of the Martian
oxidants already been demonstrated, different biology experiments
would have been flown on Viking. d) Life detection experiments
should provide valuable information even if they fail to find
life. e) Nevertheless, exploration often cannot be hypothesis-
testing: much of what we do in planetary missions is simply
exploration.
-----------
C.F. Chyba and C.B. Phillips: Possible ecosystems and the search
for life on Europa.
(Proc. Natl. Acad. Sci. US 30 Jan 01 98:801)
QY: Christopher F. Chyba: chyba@seti.org
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PLANETARY SCIENCE: THE MISSING ORGANIC MOLECULES OF MARS
One of the puzzles about the planet Mars is the fact that the
1976 Viking Mars Mission found no evidence at all of any organic
material on the Martian surface, including no evidence of organic
material expected to result from meteorite impacts. This result
has suggested that the surface layers of Mars (the Martian
regolith) may contain a powerful oxidant that converts all
organic molecules to carbon dioxide at a rate which is rapid
relative to the rate at which they arrive, and this idea is
currently influencing the planning of future Mars missions.
... ... S.A. Benner et al (4 authors at University of Florida
Gainesville, US) examine the idea of a potent Martian surface
oxidant in terms of what is known about the oxidation of organic
compounds generally and the nature of organics likely to come to
Mars via meteorites. The authors make the following points:
     1) The authors suggest that nonvolatile salts of
benzenecarboxylic acids, and perhaps oxalic acid and acetic acid,
should be metastable intermediates of meteoritic organics under
oxidizing conditions. Salts of these organic acids would have
been largely invisible to the gas chromatography-mass
spectroscopy measurements made by the Viking Mars mission.
     2) Experiments indicate that benzenehexacarbolic acid
(mellitic acid) is generated by oxidation of organic matter known
to come to Mars, is rather stable to further oxidation, and would
not have been easily detected by the Viking experiments.
     3) The authors suggest that approximately 2 kilograms of
meteorite-derived mellitic acid may have been generated per
square meter of Martian surface over 3 billion years. How much
remains depends on decomposition rates under Martian conditions.
     4) The authors suggest that as available data do not require
that the surface of Mars be very strongly oxidizing, some organic
molecules might be found near the surface of Mars, perhaps in
amounts sufficient to be a resource. Missions should seek these
organics and recognize that these complicate the search for
organics from entirely hypothetical Martian life.
     5) The authors state: "As in any organic reaction, the
specific oxidant, specific ambient conditions, and specific
catalysts determine what intermediates will accumulate in the
oxidative degradation of organic compounds on Mars. Only by
missions to Mars can we learn these specifics to decide what has
actually happened to meteoritic organics and, by inference, to
other organics that might have come to the Martian surface."
-----------
S.A. Benner et al: The missing organic molecules of Mars.
(Proc. Natl. Acad. Sci. US 14 Mar 00 97:2425)
QY: Steven A. Benner [benner@chem.ufl.edu]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ASTROBIOLOGY:
EVIDENCE FOR SUFFICIENT ENERGY FOR A BIOSPHERE ON EUROPA
... ... Christopher F. Chyba (Stanford University, US) presents
an analysis of energy on Europa available to fuel a biosphere.
The author proposes that disequilibrium chemistry in the ice
cover of Europa's oceans, driven by charged particles accelerated
in Jupiter's *magnetosphere, should produce enough organic and
oxidant molecules to fuel a substantial Europan biosphere. The
author suggests that microbial life could exist in concentrations
detectable by surface landers able to filter meltwater from
Europa's ice. The author suggests that his calculations indicate
that a particular radiation-driven ecosystem is plausible and
quantifiable using our current knowledge of Europa, but that
other radiation-driven ecosystems are possible. The author
concludes: "Neither photosynthesis nor *hydrothermal vents need
by postulated. But only direct exploration will reveal whether
life on Europa actually exists."
------------
Christopher F. Chyba: Energy for microbial life on Europa.
(Nature 27 Jan 00 403:381)
QY: Christopher F. Chyba [chyba@seti.org]
-----------
Text Notes:
... ... *magnetosphere: In general, a region surrounding a planet
in which charged particles are controlled by the magnetic field
of the planet rather than by the magnetic field of the Sun (or of
the star of which the planet is a satellite). The charged
particles originate in the *solar wind (or stellar wind, if the
planet is extrasolar), and they form a tenuous ionized gas
(*plasma) surrounding the planet.
... ... *plasma: In general, a fully ionized gas consisting of
ions and electrons moving freely. 
... ... *solar wind: The solar wind is the steady flow of charged
particles, consisting primarily of protons and electrons, from
the solar corona into interplanetary space. The solar-wind
particles have energies high enough to enable the particles to
escape the Sun's gravitational field, but the wind is influenced
by the Sun's magnetic field, and the particles can be trapped by
planetary magnetic fields.
... ... *hydrothermal vents: In general, hydrothermal vents are
hot springs occurring in volcanic regions of an ocean floor. On
Earth, it has been discovered that volcanically heated areas of
the ocean floor have a rich variety of biological organisms --
the hyperthermophiles, which thrive above 80 degrees centigrade.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PLANETARY SCIENCE:
MORE EVIDENCE FOR AN OCEAN ON THE JOVIAN MOON EUROPA
... ... David Stevenson (California Institute of Technology, US)
presents a commentary on recent research on Europa's apparent
ocean, the author making the following points:
     1) The author points out that when a conductor is placed in
a time-varying magnetic field, electrical currents are induced,
and these currents in turn create magnetic fields that can be
detected. Thus the magnetometers on the *Galileo Spacecraft have
the ability to detect electrical conductors inside the moons of
Jupiter, and now M.G. Kivelson et al (Science 2000 289:1340) have
presented "overwhelming evidence for a conducting layer near the
surface of... Europa. The most likely explanation is that Europa
has a salty, global water ocean beneath its ice shell."
     2) Stevenson summarizes present and possible future evidence
for Europa's ocean as follows:
... ... a) Theoretical studies of tidal deformation and heating
predict that an ocean on Europa would persist once formed. A
qualification is that the rheology of ice is poorly known,
especially at tidal frequencies, so such predictions are
uncertain.
... ... b) Observations of surface deformation on Europa suggests
the presence of thin ice and highly mobilized ice, both
consistent with an underlying ocean. However, this might be
explained by thin, cold, brittle ice "floating" on thick, warm,
soft, easily deformed ice.
... ... c) Near-infrared spectroscopy suggests salt deposits on
the surface, and the salt could arise from sublimation of salty
water eruptions. However, even if water is implicated, it need
not come from an ocean: there may be melting within the ice.
... ... d) Magnetic field evidence for an induction response
(e.g., the work of Kivelson et al) suggests a near-surface global
conducting layer, and this is most readily explained by a salty
ocean. Are there other possible conducting layers?
... ... e) Altimetry and gravity field experiments with
sufficient resolution to determine tidal variation would provide
a clear determination of whether there is an ocean, and also
provide information about ice thickness. However, such
experiments require a Europa orbiter and have not yet been
accomplished (a Europa orbiter mission is in the planning stage:
see related background material below).
     3) Stevenson concludes: "The [Europa] orbiter results will
likely settle the fascinating question of whether Europa has an
ocean. Defined broadly enough, oceans may not be that rare, but
Europa's case may be special because the tidal heating may allow
liquid water to get closer to the surface, possibly including
occasional eruptions or flows. After Mars, [Europa] remains the
most attractive extraterrestrial environment within our Solar
System in which to seek evidence of past or present life."
-----------
David Stevenson: Europa's ocean -- the case strengthens.
(Science 25 Aug 00 289:1305)
QY: David Stevenson djs@gps.caltech.edu
-----------
Text Notes:
... ... *Galileo Spacecraft: This Jupiter probe was first
launched in October 1989. The current data resulted from a flyby
of Europa on 3 January 2000.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 8Sep00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PLANETARY SCIENCE: THE OCEAN OF THE JOVIAN MOON EUROPA
... ... R.T. Pappalardo et al (3 authors 2 installations, US)
present a review of current research on Europa's apparent
subsurface ocean, the authors making the following points:
     1) In the 1960s, spectroscopic data indicated that the
Jovian satellite Europa, like many others in the outer Solar
System, is covered with ice. With surface temperatures of 110
degrees kelvin near the equator and 50 degrees kelvin near the
poles, the ice on Europa must form a rock-hard skin. Recently,
spectacular images radioed from visiting spacecraft have revealed
a young and tremendously deformed surface. Apparently, somewhere
under the icy shell must be a warm mobile interior. Is it glacial
ice? Or are the innards of Europa warm enough to sustain an ocean
of liquid water?
     2) Theory and observation have combined to provide a strong
self-consistent case for a global ocean within Europa. But the
existence of this ocean is not unequivocally proved. Warm
subsurface ice could mimic many of the effects of an internal
ocean. Although the surface of the satellite is sparsely cratered
and probably geologically young, searches for definitive evidence
of ongoing geological activity have been fruitless. Europa might
have had an ocean in the recent past that is now frozen solid.
The only way to be certain is to return a spacecraft to Europa
and this time go into orbit.
     3) Such a mission is being planned by the US National
Aeronautics and Space Administration (NASA). The Europa Orbiter
Mission could be launched as early as November 2003 and would
enter Jupiter's orbit 3 years later. Approximately 2 years after
that, the spacecraft would go into orbit around Europa at an
average altitude of just 200 kilometers. Precise tracking of the
position and altitude of the spacecraft would map the
gravitational field and shape of Europa in enough detail to track
the ebb and flow of tides as the moon trundles around Jupiter. If
Europa does have a subsurface sea, the surface of the moon should
rise and fall 30 meters every 3.6-day orbit. In this way, the
Europa Orbiter will provide the definitive test for an ocean.
-----------
R.T. Pappalardo et al: The hidden ocean of Europa.
(Scientific American October 1999)
QY: Robert T. Pappalardo, Brown University, US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 3Dec99
For more information: http://scienceweek.com/swfr.htm

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6. ASTROBIOLOGY: ON INTELLIGENT LIFE IN THE UNIVERSE
     A search for extraterrestrial life (see previous report) is
one thing, a search for _intelligent_ extraterrestrial life is
something else. Enrico Fermi (1901-1954) once asked: "If
intelligent aliens exist, why aren't they here?" Some people say
aliens are already here; other people say aliens don't exist;
still other people say they exist but Earth is too boring to
visit. In general, questions concerning the existence or non-
existence of intelligent alien life have been left to science
fiction writers and UFO enthusiasts and the mental health people
who study UFO enthusiasts.
     Recently, however, perhaps provoked by the new discoveries
concerning extrasolar planets, many people in the scientific
community have shown a renewed interest in the question of the
possible existence of intelligent alien life. The field of
astrobiology (xenobiology) has become established, with the
emergence of research institutes and academic departments, and
the general view is that yes, intelligent extraterrestrial life
is possible, but it is most probably extrasolar and it is not
going to be easy to find it.
... ... J. Cohen and I. Stewart (University of Warwick, UK)
present an essay on current ideas concerning intelligent
extraterrestrial life (aliens), the authors making the following
points:
     1) The authors point out that it is possible to imagine the
existence of forms of life very different from those found on
Earth, occupying habitats that are unsuitable for our kind of
life. Some of those aliens might be technological, because
technology is an autocatalytic process, and it follows that some
aliens might possess technology well in advance of our own,
including interstellar transportation. So much is clear, but this
train of logic begs the obvious question of where these
intelligent non-humanoid aliens might be.
     2) The authors point out that the subject area of this
discussion is often called "astrobiology", although in science
fiction circles (where the topic has arguably been thought
through more carefully than it has been in academic circles) the
term "xenobiology" is favored. The authors suggest the difference
is significant: Astrobiology is a mixture of astronomy and
biology, and the tendency is to assume that the field must be
assembled from contemporary astronomy and biology; in contrast,
xenobiology is the biology of the strange, and the name
inevitably involves the idea of extending contemporary biology
into new and alien realms.
     3) The authors ask: Upon what science should xenobiology be
based? The authors suggest that the history of science indicates
that any discussion of alien life will be misleading if it is
based on the presumption that contemporary science is the
ultimate in human understanding. Consider the position of science
a century ago. We believed then that we inhabited a newtonian
clockwork Universe with absolute space and absolute time; that
time was independent of space; that both were of infinite extent;
and that the Universe had always existed, always would exist, and
was essentially static. We knew about the biological cell, but we
had a strong feeling that life possessed properties that could
not be reduced to conventional physics; we had barely begun to
appreciate the role of natural selection in evolution; and we had
no idea about genetics beyond mendelian numerical patterns. Our
technology was equally primitive: cars were inferior to the
horse, and there was no radio, television, computers,
biotechnology or mobile phones. Space travel was the stuff of
fantasy. If the past is any guide, then almost everything we now
think we know will be substantially qualified or proven wrong
within the next 25 years, let alone another century. Biology, in
particular, will not persist in its current primitive form. At
present, biology is at a stage roughly analogous to physics when
Newton (1642-1727) discovered his law of gravity. "There is an
awfully long way to go."
     4) The authors point out that evolution on Earth has been in
progress for at least 3.8 billion years. "This is deep time --
too deep for scenarios expressed in human terms to make much
sense. A hundred years is the blink of an eye compared with the
time that humans have existed on Earth. The lifespan of the human
race is similarly short when compared with the time that life has
existed on Earth. It is ridiculous to imagine that somehow, in a
single century of human development, we have suddenly worked out
the truth about life. After all, we do not really understand how
a light switch works at a fundamental level, let alone a
mitochondrion."
-----------
J. Cohen and I. Stewart: Where are the dolphins?
(Nature 22 Feb 01 409:1119)
QY: Jack Cohen: Mathematics Institute, University of Warwick,
Coventry CV4 7AL, UK.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm

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7. IN FOCUS: ON INVERTEBRATES VS. VERTEBRATES
"There are over a million described species of animals. Of this
number about 5 percent possess a backbone and are known as
vertebrates. All others, comprising approximately 95 percent of
the Animal Kingdom, are invertebrates... Divisions of the Animal
Kingdom into vertebrates and invertebrates is artificial and
reflects human bias in favor of man's own relatives. One
characteristic of a single subphylum of animals is used as the
basis for separation of the entire Animal Kingdom into two
groups. One could just as logically divide animals into mollusks
and non-mollusks or arthropods and non-arthropods. The latter
classification could be supported at least from the standpoint of
numbers, since approximately 75 percent of all animals are
arthropods. The artificiality of the invertebrate concept is
especially apparent when one considers the vast and heterogeneous
assemblage of groups that are lumped together in this category.
There is not a single positive characteristic that invertebrates
hold in common. The range in size, in structural diversity, and
in adaptations to different modes of existence is enormous. Some
invertebrates have common phylogenetic origins; others are only
remotely related. Some are much more closely related to the
vertebrates than to other invertebrate groups. Quite obviously,
invertebrate zoology cannot be considered a special field of
zoology, certainly not in the same sense as protozoology or
entomology. A field that embraces all biological aspects --
morphology, physiology, embryology, and ecology -- of 95 percent
of the Animal Kingdom represents no distinct area of zoology; it
is virtually the subject of zoology itself."
-----------
Robert D. Barnes: _Invertebrate Zoology_
W.B. Saunders, Philadelphia 1963, p.1
-------------------
SCIENCE-WEEK http://scienceweek.com 9Mar01

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8. FROM THE SCIENCEWEEK ARCHIVE:
ON AESTHETICS AS A GUIDE TO THEORY
Every branch of science has its stories of elegant theories
immediately acclaimed by everyone and then subsequently proved to
be completely wrong. Usually, one important driving force behind
the immediate acclamation is the elegance of the theory, its
aesthetic aspects. In general, we have a tendency to look for
symmetries and other elegant relations in nature, and when we
find them we feel a certain exhilaration. But there is no a
priori reason, after all, why the laws that govern natural
processes must follow human notions of aesthetics, and indeed an
argument can be made that the search for and appeal of such
aesthetic laws may be based merely on certain properties of the
human nervous system, properties that make certain symmetries and
simplicities appealing. In many cases where an acclaimed elegant
theory turns out to be completely wrong, there is indeed a
feeling of surprise: How could this possibly be wrong? Usually,
it is a case of an apparently reasonable but false premise
forming the basis of an elegant but false theoretical
construction.
... ... In an essay on elegant scientific theories subsequently
proved to be wrong, John Maynard Smith (University of Sussex, UK)
suggests that the cleverest idea in the history of science that
turned out to be wrong was a proposition published by Francis
Crick, John Griffith, and Leslie Orgel in 1957, a proposition
often called "the comma-free code". The essentials of the story
are as follows:
     1) Upon publication of the Watson and Crick model of DNA in
1953, there was an immediate (and somewhat frenzied) interest
among biologists (and some physicists) in unraveling the
mechanics of the genetic code as apparently expressed in the
sequence of nucleotide bases that constitute DNA. An important
consideration was that whatever the code was, it made possible
the coding of the 20 different amino acids found in proteins.
Since 4 nucleotide bases were involved, it was apparent that the
basis of the code could not be nucleotide doublets, since a
doublet basis would yield 4^(2) or only 16 possible amino acids.
It was thus assumed that the code involved a nucleotide triplet
basis, which would yield 4^(3) or 64 possible coded amino acid
entities. But given a discrete triplet basis for the code, how
does the cell know which triplets to read, since there are no
markers in DNA to separate the triplets? Also, why are there only
20 amino acid possibilities? In their 1957 paper, Crick et al
suggested that it is implausible to suppose that translation of
the code starts at the beginning of a gene and "counts off in
threes". Instead, Crick et al suggested that only some of the
triplets are meaningful, in the sense of specifying an amino
acid, that no matter in what order these meaningful triplets are
arranged, none of the "out-of-frame" triplets must be meaningful,
and that a message can be read in only one way. Assuming this is
the basis for the code, the largest number of meaningful triplets
was shown to be precisely 20. With caution, Crick et al
concluded: "We present the solution here because it gives the
'magic number' 20, so that our answer may perhaps be of
biological significance."
     2) But despite the caution of Crick et al, in 1957 and for
several following years, nearly the entire scientific community
acclaimed the proposal as one of extreme elegance and
significance. No matter the elegance, the idea had to be
completely abandoned in the face of subsequent evidence. We now
know that of the 64 possible triplets, 61 triplets do specify an
amino acid with enough redundancies so that only 20 amino acids
are actually entrained. We also know that the reading of the
coded nucleotide sequence does indeed start at the beginning of a
gene and effectively "count off in threes". Ironically, the
evidence against the 1957 Crick et al proposal was provided
shortly afterward in 1961 by Crick himself and another group of
co-authors, who postulated:
... ... a) A group of 3 bases codes one amino acid.
... ... b) The code is not of the overlapping type.
... ... c) The sequence of the bases is read from a fixed
starting point.
... ... d) One particular amino acid can be coded by one of
several triplets of bases.
The 1957 proposal of Crick et al involved assumptions and
arguments contrary to items (c) and (d) above, assumptions and
arguments that produced an elegant theory predicting the coding
of precisely 20 amino acids, but a theory which turned out to be
completely wrong.
     3) John Maynard Smith is not the first to focus on the idea
of the Crick et al 1957 proposal as an example of elegance
leading theory astray. As Smith notes, Horace Freeland Judson, in
his classic history of molecular biology, _The Eighth Day of
Creation_, writes of the comma-free code: "An idea of Crick's
that was the most elegant biological theory ever to be proposed
and proved wrong." [Judson, p.315].
-----------
John Maynard Smith: Too good to be true.
(Nature 15 Jul 99 400:223)
QY: John Maynard Smith, School of Biological Sciences, University
of Sussex, Falmer, Brighton BN1 9QG UK
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 6Aug99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BEAUTY AND TRUTH IN SCIENTIFIC THEORIES
There is an old adage, particularly in the physical sciences,
that of two theories arising from the same set of facts, one
theory beautiful and the other theory ugly, the beautiful theory
is more likely to be correct. There are indeed theories that are
difficult or impossible to test that have extensive and expansive
lives because of their aesthetic appeal, and that are discarded
with great reluctance when testing of the theory does eventually
become possible. ... ... J. McAllister (University of Leiden,
NL), in a review of the relation between the aesthetic properties
of scientific theories and their acceptance by the scientific
community, notes that many scientists claim to be able to tell by
means of aesthetic judgment how close a theory is to the truth,
but that in fact it often happens that a theory that is
aesthetically innovative strikes most scientists as ugly when it
is first put forward. For example, Kepler's theory of planetary
motions was initially considered ugly because it involved
ellipses rather than circles; Newton's theory of gravitation was
considered ugly because it postulated action at a distance;
quantum electrodynamics was first considered ugly for relying on
nonstandard mathematical operations for renormalization; and,
indeed, there is the famous rejection of quantum theory by
Einstein because he felt it lacked aesthetic appeal. Noting that
what is called beautiful changes as society and science change,
McAllister concludes the evidence that any aesthetic property of
theories is a sign of truth is at present scarce.
QY: James W. McAllister [mcallister@rullet.leidenuniv.nl]
(American Scientist Mar/Apr 1998)
For more information: http://scienceweek.com/swfr.htm


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