ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

August 20, 1999 -- Vol. 3 Number 34

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It is impossible to dissociate language from science or science
from language, because every natural science always involves
three things: the sequence of phenomena on which the science is
based; the abstract concepts which call these phenomena to mind;
and the words in which the concepts are expressed. To call forth
a concept, a word is needed; to portray a phenomenon, a concept
is needed. All three mirror one and same reality. Words are thus
required to preserve and transmit ideas, so that it is clear that
the advancement of a science and the improvement of its technical
vocabulary go hand in hand. No matter how certain we are of the
phenomena, no matter how adequately our concepts reflect them, we
cannot help perpetuating wrong ideas unless we have a precise
terminology in which to express ourselves.
-- Antoine Laurent Lavoisier (1743-1794)

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Contents of This Issue:

1. On Ammonia and the Population Explosion
2. On the Natural Occurrences of Diamond
3. On Rotating Superfluid Helium-3
4. Polarized Starlight and Amino Acid Homochirality
5. Cell Biology: Protein Sorting and Golgi Compartments
6. On Human Embryonic Stem Cell Research

In Focus: On Horizontal Gene Transfer

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1. ON AMMONIA AND THE POPULATION EXPLOSION
Ammonia [NH(sub3)], a nitrogen hydride, is a colorless gas with a
rather interesting human history that ranges from its discovery
by the remarkable chemist Joseph Priestley (1733-1804) to the
first large-scale synthetic production and use of ammonia in
synthetic fertilizers and explosives in the 20th century. The
human requirement for synthetic fertilizers and explosives is an
instance of irony in the application of science, since the major
use of synthetic fertilizers is in the production of crops to
feed people, and the major use of explosives is in the production
of weapons to kill people. Nitrogen compounds are essential to
fertilizers and explosives, but in the early 20th century the
best large-scale source of such compounds was in the nitrate
deposits of Chile [*Note #1], which at that time was quite remote
from Europe. Another possible source of nitrogen compounds, only
theoretical at the time, was Earth's atmosphere, since the
atmosphere is mostly nitrogen gas and therefore constitutes an
inexhaustible supply. If atmospheric nitrogen could be converted
to ammonia, the ammonia could be used in the synthesis of various
nitrogen compounds, including fertilizers and explosives. Fritz
Haber (1868-1934) and Carl Bosch (1874-1940) are credited with
the discovery of the Haber-Bosch process for the synthesis of
ammonia from its elements, a discovery that literally altered the
course of 20th century history. The basis of the process is the
combining of nitrogen and hydrogen at high pressure over a
catalyst. Haber, who first demonstrated the synthesis in 1909,
received the Nobel Prize for Chemistry in 1918; Bosch, who
engineered the application of the method to the large-scale
production of ammonia, received the Nobel Prize for Chemistry in
1931 [*Note #2]. ... ... Vaclav Smil (University of Manitoba, CA)
presents an historical essay on the Haber-Bosch discovery, the
author making the following points:
     1) The author poses the question: What is the most important
invention of the 20th century? The usual answers include
airplanes, nuclear energy, space flight, television, and
computers, but none of these are critical to human well-being.
The synthesis of ammonia from its elements, however, is critical:
the world's population could not have grown from 1.6 billion in
1900 to the 6 billion of today without the Haber-Bosch process.
     2) The synthesis of ammonia belongs to that special group of
discoveries -- including Edison's light bulb and the Wright
brothers' flight -- for which we can pinpoint the date of the
decisive breakthrough. The archives of Badische Anilin-Und Soda-
Fabrik (BASF) contain a letter from Haber, at that time Professor
of Physical Chemistry at Technische Hochschule in Karlsruhe, to
the company directors, a letter in which Haber recounts how the
previous day the first demonstration to company scientists of the
synthesis of ammonia from nitrogen and hydrogen was made: "All
parts of the apparatus were tight and functioned well, so it was
easy to conclude that the experiment could be repeated."
     3) Although a number of company officials lacked confidence
in the application of Haber's method because of the high pressure
(over 100 atmospheres) required, Carl Bosch, who managed the BASF
nitrogen-fixation research, was apparently confident: "I believe
it can go. I know exactly the capability of the steel industry.
It should be risked." It was Bosch who was responsible for the
development of the proper steel housing necessary for large-scale
ammonia production.
     4) The present world output of ammonia amounts to
approximately 130 million metric tons per year, and 80 percent of
this goes into fertilizers, of which urea is the most important.
The ammonia is absolutely essential to sustain today's
population: rich countries might fertilize much less by cutting
excessive food production and by eating fewer animals, but even
the most assiduous recycling of organic wastes and the widest
planting of *nitrogen-fixating legumes could not supply enough
nitrogen for land-scarce, poor and populous nations. For several
decades now, virtually all the fixed nitrogen added to the fields
of China, Egypt, and Indonesia has come from synthetic
fertilizers.
     5) The author concludes: "Without this [the Haber-Bosch
process], almost two-fifths of the world's population would not
be here -- and our dependence will only increase as the global
count moves from 6 to 9 or 10 billion people."
-----------
Vaclav Smil: Detonator of the population explosion.
(Nature 29 Jul 99 400:415)
QY: Vaclav Smil, Dept. of Geography, University of Manitoba,
Winnipeg, CA.
-----------
Text Notes:
... ... *Note #1: During World War I (1914-1918) access to the
Chilean nitrate deposits by Germany was almost impossible, with
imports of nitrates blocked by the British navy. The German
military needed explosives, which required nitrates, which
required a source of usable nitrogen. This was the main impetus
for the development of the large-scale production of ammonia by
Bosch and BASF. Many historians believe that if Germany had had
to depend only on Chilean nitrates for explosives, World War I
would have ended in 1916, with several million lives saved.
... ... *Note #2: The personal story of Fritz Haber is
interesting. Haber became a prominent chemist following his
discovery of the synthesis of ammonia from nitrogen and hydrogen.
He was extremely patriotic, and during the war he devoted great
efforts to the development of gas warfare, directing the first
warfare use of chlorine gas in 1915, and of mustard gas in 1917.
In the history of war, the beginning of gas warfare is dated as
April 22, 1915, "the day at Ypres when Haber's gas blowing
process surprised and overpowered the enemy lines for the first
time." Because of his work in gas warfare, there were many
protests when Haber was awarded the Nobel Prize after the war
ended. Following the war, and the huge reparations demanded from
Germany by the Allies, Haber worked to isolate gold from seawater
in order to pay the reparations. The yield was too small and
research failed. In 1933, when the Nazis came to power in
Germany, Haber's patriotic services in ammonia synthesis for
explosives, gas warfare, and the attempted isolation of gold from
seawater were dismissed as irrelevant because Haber was a Jew,
and Haber was forced to give up his post and flee Germany. He
went first to England, then decided to go to Palestine, but he
died in Switzerland on his way south. Carl Bosch had a different
fate: Bosch, who was not a Jew, remained in Germany as a
prominent scientist. In 1933, Bosch actually cautioned Hitler
against the policy of dismissing non-Aryan scientists, pointing
out to Hitler the severe damage which this policy threatened to
inflict on the pursuit of chemistry and physics in Germany.
Hitler's response: "Then we'll just get along without physics and
chemistry for a hundred years!" In 1935, as the Nazi era
continued, Bosch succeeded Max Planck as head of the Kaiser
Wilhelm Society (now called the Max Planck Society).
... ... *nitrogen-fixating legumes: In leguminous plants such as
beans and peas, the symbiotic bacteria Rhizobium form
characteristic root nodules, the bacteria supplying the plant
with usable nitrate obtained from atmospheric nitrogen, while the
bacteria obtain carbohydrates from the plant. In general, the
term "nitrogen-fixation" refers to any fixation of nitrogenous
compounds from atmospheric nitrogen. In nature, this is achieved
by the normal metabolism of specialized soil bacteria (e.g.,
Rhizobium), and also by the electric discharges of lightening in
the atmosphere. The Haber-Bosch process is industrial nitrogen-
fixation.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99


2. ON THE NATURAL OCCURRENCES OF DIAMOND
In general, in this context, the term "metamorphism" refers to
the mineralogical and structural changes of solid rock in
response to environmental conditions at depth in the Earth's
*crust. Diamond is a good example of the metamorphic formation of
a rock crystal. The most common naturally occurring forms of pure
carbon are graphite and diamond. If diamond is heated to
approximately 1200 degrees centigrade, it slowly transforms to
graphite, the rate of transformation increasing with temperature.
At 2000 degrees centigrade the transformation is extremely rapid
and the crystal shatters into a heap of graphite powder. It has
been found, however, that diamond can survive these high
temperatures if it is subjected to a pressure of several thousand
atmospheres. Conversely, diamond can be produced by heating
graphite to approximately 2500 degrees centigrade under a
pressure in excess of 100,000 atmospheres, and it is this process
that is responsible for the production of diamonds in nature.
Before the middle of the 19th century, virtually all diamonds
were found in or near surface streams, and most of these diamonds
were discovered either in India or Brazil. In the year 1866, the
children of a Boer farmer, Daniel Jakobs, while playing on the
bank of the Orange River at Hopetown near Kimberley in South
Africa, found a diamond weighing 21.25 *carats (slightly more
than 4 grams). Subsequent exploration in the area demonstrated
that the diamond-bearing deposit was a roughly circular region a
few hundred meters in diameter, effectively a cylindrical "pipe"
that had been pushed upwards from a region deep in the Earth's
crust. Such a geological feature is now known as a "Kimberlite
pipe". At present, the main source of diamonds is the *igneous
rock "kimberlite", in which diamonds are found as scattered
crystals. Kimberlite pipes are believed to be the remains of
magma plugs that have been forced up to the Earth's surface, and
there are known Kimberlite pipes with diameters up to a
kilometer. There is evidence that in some cases the magma plugs
may derive from *mantle plumes. The largest gem quality diamond
ever found was the 621-gram (3106 carats) Cullinan crystal
discovered in 1905. Diamonds have also been found in meteorites
as microcrystalline clumps up to approximately a millimeter in
size, the meteoritic diamonds evidently formed from graphite
nodules as *iron meteorites were subjected to intense high
pressures by the shock of impact on the surface of the Earth
(shock metamorphism). Concerning the use of diamond in industry,
diamond is the hardest substance available. In addition, diamond
has the highest thermal conductivity of any known substance (five
times the thermal conductivity of copper at room temperature),
which means that a diamond cutting tool transfers heat quickly
and does not become hot while in use. ... ... Stephen E. Haggerty
(University of Massachusetts Amherst, US) presents a review of
the natural occurrences of diamond and the implications of
diamond for our knowledge of Earth and Solar System processes.
The author makes the following points:
     1) Carbon is the fourth most abundant element in the Solar
System after hydrogen, helium, and oxygen. As a nucleosynthetic
bridge to the heavy elements, to stellar evolution, and to
biosynthesis, carbon holds an interesting position in the
periodic system of elements. Diamond is pure carbon and is an
impervious time capsule: Some diamonds are pre-Solar and have
recorded such extraordinary astrophysical events as *supernovae
explosions; other diamonds bear witness to Solar System
formation; and diamonds from our planet are a window to the
geodynamic evolution of Earth's deep interior.
     2) The study of diamond and its various crystal forms
(polymorphs) (e.g., *lonsdaleite, *fullerene, and graphite) has
seen a recent burst of activity in geochemistry and geophysics,
in novel methods of synthesis, and in the development of useful
applications that take advantage of its covalent bonding,
clarity, extreme hardness, high thermal conductivity, and high
electrical resistance.
     3) Diamond is now recognized as an extraordinary recorder of
astrophysical and geodynamic events that extend from the far
reaches of space to Earth's deep interior. Many diamonds are
natural antiques that formed a) in presolar supernovae by carbon
vapor deposition, b) in asteroidal impacts and meteorite craters
by shock metamorphism, and c) from fluids and melts in Earth's
mantle 1 to 2 billion years after *planetary accretion. The
carbon in diamond is primordial, but there are unexplained
isotopic fractionations and uncertainties in heterogeneity.
-----------
Stephen E. Haggerty: A diamond trilogy: Superplumes,
supercontinents, and supernovaes.
(Science 6 Aug 99 285:851)
-----------
Text Notes:
... ... *crust: The crust is the outermost of the 3 major layers
of the planet (core, mantle, crust), its depth varying from
approximately 5 kilometers to as much as 60 to 80 kilometers.
... ... *carats: 1 carat = 0.2 grams
... ... *igneous rock: Igneous rocks are rocks that have
congealed from a molten mass.
... ... *mantle plumes: Mantle plumes are thin vertical conduits
of molten rock material from the core-mantle boundary to the
crust. Seismic studies indicate the interior of the Earth
consists of three parts: a metallic core, a dense rocky mantle,
and a thin low-density crust. The central part of the core is
solid, but the outer part of the core is evidently liquid.  The
mantle, the layer of dense rock and metal oxides between the
molten part of the core and the surface, has plastic properties
(i.e., it is a solid capable of flow under pressure). 
... ... *iron meteorites: Meteorites composed mostly of iron,
with less than 30 percent nickel, and with only a small
proportion of silicate minerals. They form only a few percent of
meteorite falls.
... ... *supernovae explosions: Supernovae are stellar explosions
in which virtually an entire star is disrupted. The estimate is
that in our own Galaxy approximately 1 supernova occurs every 30
years, with most of the supernovae obscured by galactic dust.
... ... *lonsdaleite: A form of carbon found in meteorites. It
can also be formed synthetically by ballistically accelerating
metal projectiles into carbon targets.
... ... *fullerene: Fullerenes are large molecules composed
entirely of carbon, with the chemical formula C(n), where n is
any even number from 32 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.
... ... *planetary accretion: Planetesimals, present in the
vicinity of a young star, are bodies with dimensions of 10^(-3)
to 10^(3) meters that are believed to form planets by a process
of accretion. The term "accretion" refers to an aggregation, an
increase in the mass of a body by the addition of smaller bodies
that collide and adhere to it, provided the relative velocities
are low enough for coalescence.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99


3. ON ROTATING SUPERFLUID HELIUM-3
     In general, a "superfluid" is a fluid that flows without any
resistance. "Superconductivity" is sometimes considered as a
special case of superfluidity in which the "fluid" components
(electrons) are charged. But more conventionally, superfluidity
is considered a property of liquid helium at extremely low
temperatures, a property that enables liquid helium to flow
without friction.
     Both helium isotopes (sup4)He (the common isotope, often
denoted as helium-4) and (sup3)He (the rare isotope, often
denoted as helium-3) possess superfluidity under special
circumstances: helium-4 becomes a superfluid below 2.172 degrees
kelvin, while helium-3 becomes a superfluid only below 0.00093
degrees kelvin.
     The superfluidity of liquid helium is apparently due to the
weakness of the attractive force between two helium atoms and to
the small atomic mass, which according to laws of quantum
mechanics make the atoms difficult to *localize. One interesting
difference between helium-4 and helium-3 is that the helium-4
atom, with two protons, two neutrons, two electrons, and an
*intrinsic spin of 0, is subject to *Bose-Einstein statistics,
while the helium-3 atom, with two protons, one neutron, two
electrons, and an intrinsic spin of 1/2, is subject to *Fermi-
Dirac statistics.
     There are at least two distinct superfluid states in helium-
3, the two most common denoted as helium-3A and helium-3B. Aside
from the fascinating macroscopic behaviors of superfluid liquid
helium, it is a system that provides a window for the
understanding of certain important quantum behaviors, including
microscopic quantum vortices (which are also displayed by
electrons in superconductors). A quantum vortex is a type of flow
pattern exhibited by superfluids under certain experimental
conditions, e.g., liquid helium in a rotating container. The term
"vortex" designates the familiar whirlpool pattern where the
fluid moves circularly around a central line and the velocity
decreases in inverse proportion to the distance from the center.
A superfluid is considered to be characterized by a macroscopic
quantum-mechanical wave function that locks the superfluid into a
*coherent state. This forces certain mathematical constraints on
the wave function, so that for a superfluid in a rotating
container, the system (the wave equation for the system) produces
a lattice of quantized vortex lines, each line the axis of a
microscopic vortex, with the entire array of vortex lines
rotating rigidly with the container. The essential idea is that
when superfluid helium is in a rotating container, the
mathematics of the system wave function are such that a set of
discrete microscopic vortex states are produced by each
particular set of boundary conditions, and these microscopic
vortex states are experimentally observable [*Note #1]. In short,
the result is a system where the "quantum world" becomes visible
on a macroscopic scale.
     The superfluidity of liquid helium was first discovered in
1938, and the quantized vortex lines were first detected in the
mid-1950s. Experimentally, quantized vortex lines are usually
produced by rotating a vessel containing superfluid helium, but
other methods for producing vortex lines have been used.
... ... O.V. Lounasmaa et al (3 authors at Helsinki University of
Technology, FI) present a review of recent work on rotating
superfluid helium-3, the authors making the following points:
     1) Superfluid helium-3 exhibits a multitude of different
types of topological defects, such as point singularities, vortex
lines, *domain walls, and 3-dimensional textures. This behavior
allows investigation of general principles, such as topological
stability and confinement, nucleation of singularities (i.e.,
seeded growth of the number of singularities), and interactions
between objects (localized discrete patterns; a vortex is an
"object" in this context) of different topologies. There are
promising analogies to *quantum field theory, elementary particle
physics, and cosmology.
     2) Superfluid helium-3 exhibits the most complicated vortex
states that exist in nature. Experiments have revealed 7
different kinds of vortices in the two superfluid phases
helium-3A and helium-3B. Many interesting properties of the
vortex structures have been discovered, and frequently these are
understood in detail because quasi-classical quantum theory forms
a reliable foundation for theoretical studies.
     3) Most of the knowledge of quantum vorticity in helium-3
originated from the Finnish-Soviet ROTA project started in 1978.
These studies have concentrated on identifying the topology and
structure of the different objects in the rotating superfluid
helium-3. Work using the first ROTA machine quickly resulted in
the discovery of vortices both in the A and B phases, which was
expected, but the great variety of vortex phenomena was a
surprise.
     4) The ROTA experimental method typically involves a
cylindrical container 7 millimeters long and 5 millimeters in
diameter, which is rotated around its axis up to 3 radians per
second. To study the superfluid state, the liquid must be cooled 
to a temperature of a few millikelvins or less. In addition to
the ROTA apparatus in Helsinki, rotating cryostats for
investigation of superfluid helium-3 have been used at the
University of Manchester (UK), Cornell University (US), and
University of California Berkeley (US).
     5) Concerning analogy applications to cosmology, the authors
report experiments in which superfluid helium-3 was heated
locally by absorption of single neutrons, and the authors suggest
the resulting events can be used to test theoretical models of
the Big Bang at the beginning of our Universe.
     6) The authors suggest that helium-3 provides a well-
understood model for analyzing other systems where theory and/or
experiments are less developed: superfluid helium-3 is the most
complex coherent many-body system available that can be described
by quasi-classical quantum theory, a theory which has been highly
successful in explaining laboratory observations.
-----------
O.V. Lounasmaa et al: Vortices in rotating superfluid (sup3)He.
(Proc. Natl. Acad. Sci. US 6 Jul 99 96:7760)
QY: Olli V. Lounasmaa [olli.lounasmaa@hut.fi]
-----------
Text Notes:
... ... *localize: In this context, a "localized" atom is an atom
that has a greater probability of being in some region of space
than in others.
... ... *intrinsic spin: In this context, the term "intrinsic
spin" refers to the intrinsic (virtual) angular momentum of an
atom. In general, angular momentum is a property of any rotating
or revolving system, its value dependent on the distribution of
mass and velocity about the axis of rotation or revolution.
... ... *Bose-Einstein statistics: Bose-Einstein statistics is
the statistical mechanics of a system of indistinguishable
particles for which there is no restriction on the number of
particles that may simultaneously exist in the same quantum
energy state. Bosons are particles that obey Bose-Einstein
statistics, and they include photons, pi mesons, all nuclei
having an even number of particles, and all particles with
integer spin.
... ... *Fermi-Dirac statistics: The statistics of an assembly of
identical half-integer spin particles. Such particles satisfy the
Pauli exclusion principle, i.e., no two particles of the same
kind in the system may simultaneously occupy the same quantum
state.
... ... *coherent state: In quantum physics, coherence is matter
of locking of phase differences between wave functions. The wave
functions of two or more particles are said to be coherent if the
phase difference between their wave functions remains constant.
In general, a perfectly coherent system of particles can be
described by a single macroscopic wave function.
... ... *Note #1: Below a certain rotation speed threshold, no
vortices exist, and the superfluid remains at rest while the
container rotates (the Landau state). At the threshold speed, the
first vortex appears and corresponds to the first excited
rotational state of the system. If the container continues to
accelerate, additional quantized vortices appear, and at any
given speed the vortices form a regular array that rotates with
the vessel.
... ... *domain walls: (Bloch walls) In general, a Bloch wall is
a transition layer, with a finite thickness of a few hundred
lattice constants, between adjacent ferromagnetic domains with
opposite spin directions. A "lattice constant" is any one of
several possible parameters defining the unit cell of a lattice.
A "ferromagnetic domain" is a region of a ferromagnetic material
(e.g., various forms of iron, steel, cobalt, nickel, and their
alloys) within which atomic or molecular magnetic moments are
aligned parallel.
... ... *quantum field theory: In general, a quantum field
theory is a theory that describes the quantum effects of a
classical system of fields defined on space-time and satisfying
various partial differential equations.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99


4. POLARIZED STARLIGHT AND AMINO ACID HOMOCHIRALITY
One of the great puzzles of biology is the homochirality of most
amino acids and sugars present in biological systems. In general,
chirality is a property of certain asymmetric objects such that
the object and its mirror image cannot be superimposed one on the
other while both objects are restricted to the same plane (e.g.,
a left- and right-handed glove). In chemistry, chiral molecules
are optically active, a phase of each form rotating the plane of
incident polarized light. The two possible forms are called
"optical isomers", and each form is called an "enantiomer". An
equal mixture of the two forms is called a "racemic mixture".
Homochirality is the preference of a process or system for a
single optical isomer in a pair of isomers. Optically active
substances are termed "dextrorotatory" (the D- form; the + form)
or "levorotatory" (the L- form; the - form) according to whether
the plane of polarization of the incident polarized light is
rotated to the right or to the left with respect to the direction
of incidence of the light. The great puzzle in biology is that
nearly all amino acids in biological systems are of the L- form,
while nearly all sugars are of the D- form [*Note #1]. The
question is how did this arise? Is the basis terrestrial or
extraterrestrial? ... ... Stuart Clark (University of
Hertfordshire, UK) presents a review of recent astronomical
observations that suggest a solution to the puzzle, the author
making the following points:
     1) The author suggests that the nature of the starlight
present when the Earth first formed may be responsible for the
asymmetry of the amino acids now found in all living systems. In
general, the idea is that bombardment of comets, large and small,
brought water, gases, and a number of other volatile compounds to
Earth billions of years ago, and the rain of this material from
space may have seeded the young planet with a preponderance of L-
amino acids.
     2) The author points out that the decades old controversy
concerning the *Murchison meteorite was finally settled in 1997
when it was confirmed that L-alanine is twice as abundant in the
meteorite as its optical isomer, that L-glutamic acid is 3 times
as abundant. The previous ambiguities that had caused some
researchers to consider the amino acids in the meteorite in
racemic mixture are now resolved, and the author suggests that
the asymmetries in the meteorite make it almost certain that the
Solar System formed with an excess of amino acids (see background
material below).
     3) The author delineates some of the observational evidence
concerning circularly polarized infrared starlight associated
with certain young stars (see background material below).
Circularly polarized ultraviolet light is of special significance
in this context, since if a racemic mixture is subjected to
circularly polarized ultraviolet light, under the proper
conditions, one of the isomers can be preferentially destroyed
(asymmetric photolysis) (see background material below).
Theoretical work suggests that dust grains associated with stars
can indeed generate circularly polarized radiation at both
visible and ultraviolet wavelengths, as well as in the infrared
part of the spectrum. The general idea is that circularly
polarized light forms when initially unpolarized or plane
polarized starlight is scattered by a group of spinning dust
particles oriented by a local magnetic field.
     4) The author suggests that planets forming around stars in
regions bathed in ultraviolet light with high circular
polarization will naturally incorporate amino acids with
enantiomeric excesses of one sort or the other, and that the same
probably held true for Earth approximately 5 billion years ago.
The author suggests that one consequence of this hypothesis is
that if carbon-based life exists on extrasolar planets, a
biochemistry based on D- amino acids could well be the rule.
-----------
Stuart Clark: Polarized starlight and the handedness of life.
(American Scientist Jul/Aug 99 87:336)
QY: Stuart Clark [sclark@star.herts.ac.uk]
-----------
Text Notes:
... ... *Note #1: Glycine is the only biological amino acid that
is not chiral.
... ... *Murchison meteorite: A *carbonaceous chondrite that fell
in 1969 near Murchison, Australia. Analysis of the interior of
the meteorite revealed evidence of amino acids. 
... ... *carbonaceous chondrite: "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.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99
-------------------
Related Background:
STELLAR CIRCULAR POLARIZATION AND BIOMOLECULAR HOMOCHIRALITY
Electromagnetic radiation involves the propagation of both
electric and magnetic forces, and at each point in a light beam,
there is a component electric field and a component magnetic
field, both of which oscillate in all directions perpendicular to
each other and to the direction in which the beam is propagated.
In plane-polarized light, the component electric field oscillates
as in ordinary light except that the direction of oscillation is
contained within a plane. Likewise, in plane-polarized light, the
component magnetic field oscillates within a plane, the planes in
question being perpendicular. Circularly polarized light has a
component electric field that varies in direction but not in
magnitude, so that the field traverses a spiral path in either a
clockwise or counterclockwise direction. In the laboratory, high
levels of *enantiomeric excess in *racemic substances can be
produced by asymmetric photolysis by circularly polarized light. 
Biological molecules exhibit extensive *homochirality (e.g.,
living systems use almost exclusively L-amino acids and D-
sugars), and this has been a puzzle since its discovery in the
19th century. It has been suggested that homochirality may be a
prerequisite for the origin of life, and a number of processes
have been proposed that may have produced enantiomeric excess in
prebiotic organic molecules, including the action of circular
polarization from the daylight sky, but these effects are
considered too small to account for homochirality. Another view
is that the origin of homochirality is extraterrestrial, and
apparent support for this view has been provided by the recent
discovery of an excess of L-amino acids in the *Murchison
meteorite. ... ... Bailey et al (8 authors at 4 installations, AU
UK FR JP) now report strong infrared circular polarization
resulting from dust scattering in *reflection nebula in an Orion
star formation region (OMC-1), and the authors suggest that
circular polarization at shorter wavelengths might have been
important in inducing chiral asymmetry in interstellar organic
molecules that could be subsequently delivered to the early Earth
by comets, interplanetary dust particles, or meteors. The authors
suggest this could account for the excess of L-amino acids found
in the Murchison meteorite, and could explain the origin of the
homochirality of biological molecules. The authors conclude:
"Whether a very high enantiomeric excess is a prerequisite for
the origin of life, or a very small effect is amplified in the
process, the ultimate source [of the excess] is likely to be of
extraterrestrial origin."
QY: Jeremy Bailey, Anglo-Australian Observatory, PO Box 296,
Epping, New South Wales 2121, AU.
(Science 31 Jul 98 281:672) (Science-Week 21 Aug 98)
-------------------
Related Background:
... ... *enantiomeric: In chemistry, an enantiomer is a compound
whose structure is not superimposable on its mirror image, the
compound being one of a pair of optical isomers, each of which
interacts differently with polarized light (i.e., shows optical
activity). A mixture of two optical isomers in equal amounts is
called a *racemic mixture, and racemic mixtures do not show
optical activity. A reactant or process that produces an
enantiomeric excess is simply a reactant or process that produces
or selects one enantiomer in excess over the other enantiomer.
... ... *racemic: (see *enantiomeric)
... ... *homochirality: Chirality is a property of certain
asymmetric molecules (or of any object), the property being that
the mirror images of the molecules cannot be superimposed one on
the other while facing in the same direction. Homochirality is
the preference of a process or system for a single optical isomer
in a pair of isomers.
... ... *Murchison meteorite: This meteorite, which fell near
Murchison AU in 1969, contains 8 amino acids and the nucleotide
bases adenine, guanine, and uracil.
... ... *reflection nebula: A bright cloud of interstellar gas
and dust that lies in the vicinity of a star or stellar group,
with the starlight scattered in all directions by the dust
grains, and the scattered light visible because of the high
density of the dust.
-------------------
Related Background:
POSSIBLE EXPLANATION FOR LEVOROTATORY AMINO ACIDS IN BIOlOGY
A chiral molecule is an asymmetric molecule that cannot be super-
imposed on its mirror image. The molecule has two forms, called
enantiomorphs, that are mirror images of each other, the
solutions of which rotate the plane of polarized light in
different directions, left or right, making the enantiomers
levorotatory or dextrorotatory. Only levorotatory amino acids are
present in biological systems, and the reason for this is
unknown. M. H. Engel and S. A. Macko (University of Oklahoma, US;
University of Virginia, US) now report that individual amino-acid
enantiomers from the Murchison meteorite contain higher
concentrations of the nitrogen isotope N(15) compared to similar
materials on Earth. The authors suggest this confirms that the
asymmetries in enantiomorph concentrations in the meteorite are
extraterrestrial in origin, and indicates an extraterrestrial
source for an L-enantiomeric excess in our solar system that may
predate the origin of life on Earth.
QY: M. H. Engel [ab1635@ou.edu]
(Nature 18 Sep 97) (Science-Week 3 Oct 97)


5. CELL BIOLOGY: PROTEIN SORTING AND GOLGI COMPARTMENTS
     The historical differences between physics and biology in
the first half of this century are instructive. In 1950,
physicists could not see the various constituents of matter, but
they had physical theories that produced very good predictions of
the behavior of these constituents. In contrast, in 1950,
biologists could see many constituents of the biological cell,
but the cell in its ensemble of parts appeared so complex that
how these constituents behaved was a mystery. At that time, one
controversial constituent of the biological cell was the so-
called "Golgi apparatus", first discovered in 1898 by Camillo
Golgi (1843-1926). Golgi was the first to introduce the use of
silver salts in staining cells (he received the Nobel Prize for
Physiology and Medicine in 1906), and with this silver stain
method cellular components were revealed that were previously
invisible when cells were treated with organic dyes (the standard
classical method of staining cells). For half a century, however,
many biologists considered the Golgi apparatus a staining
artifact, and it was not until the 1950s and the use of the
electron microscope in biology that the Golgi apparatus was
finally confirmed as a real structure in cells. Still, at that
time there was no clear detailed idea concerning how the Golgi
apparatus contributed to the functioning of the cell, although it
did seem to be involved in secretion, since it appeared more
pronounced in secreting cells than in other cells. It took the
remaining decades of this century for the story of the Golgi
apparatus to be unfolded.
     The Golgi apparatus (Golgi complex) is a collection of
organelles (Golgi bodies) in eukaryotic cells (i.e., cells with
internal membrane-bound organelles) that essentially function as
a collecting and packaging center for substances that the cell
manufactures for export. Golgi bodies are thus particularly
abundant in secretory cells. They consist of stacks of membranous
sacs that are pinched off as Golgi vesicles for delivery to the
exterior of the cell.
     In this context, a "vesicle" is a small intracellular
membrane-bound volume in which substances are stored or
transported. Another cellular structure of importance in
understanding the operation of the Golgi apparatus is the so-
called "endoplasmic reticulum" (ER), which was first identified
with the use of the electron microscope in the 1950s. In general,
the endoplasmic reticulum is an extensive system of flattened
membranous sacs traversing the cytoplasm of all eukaryotic cells
and continuous with the envelope that surrounds the nucleus.
"Rough" endoplasmic reticulum (called rough because of its
electron-microscopic appearance) is covered with ribosomes. A
ribosome is a small particle, a complex of various ribonucleic
acid component subunits and proteins that functions as the site
of protein synthesis in the cell. The rough endoplasmic reticulum
essentially provides a transportation system for the delivery of
newly synthesized proteins to other parts of the cell, or for
secretion to the exterior (exocytosis) via the Golgi apparatus.
The other type of endoplasmic reticulum is "smooth" endoplasmic
reticulum, which lacks ribosomes, and which is involved in lipid
synthesis.
     The essentials of the operation of the endoplasmic
reticulum-Golgi apparatus system are as follows:
     The endoplasmic reticulum in effect divides the cytoplasm
into two compartments: the cytosol (the non-membranous part of
the cytoplasm outside the ER) and the cisternal space (the
connected lumens of the ER sacs). The cytosol contains enzymes
involved in metabolic pathways, whereas the ER cisternal space
provides a route for the movement of materials through various
intracellular compartments and, in some cases, to the cell
exterior.
     The rough endoplasmic reticulum plays a central role in the
synthesis of secretory proteins, integral membrane proteins, and
proteins destined to reside in the lumen of the endoplasmic
reticulum. Proteins synthesized in the rough endoplasmic
reticulum are routed to the Golgi apparatus by membrane vesicles
that shuttle back and forth between the two structures. Proteins
passing through the Golgi complex contain specific chemical
markers that target them to various locations, including the
endoplasmic reticulum, the Golgi apparatus itself, and secretory
vesicles.
     Cells exhibit two types of secretory processes: a) in
"constitutive secretion", protein products are moved to the cell
surface in a continuous fashion by nonselective bulk flow; b) in
contrast, "regulated secretion" occurs only in response to
external stimuli. In both types of secretion, membrane vesicles
fuse with the plasma membrane, discharging their contents into
the extracellular space (exocytosis), and after this process has
occurred, the membrane components are recycled back to the Golgi
apparatus by vesicles that bud from the plasma membrane.
     Formulating the above account of the operations of the
endoplasmic reticulum and Golgi apparatus, broad and brief as it
is, has required 50 years and the labor of thousands of
biologists. The outlined dynamics are literally a major part of
the workings of the eukaryotic biological cell.
... ... B.B. Allen and W.E. Balch (Scripps Research Institute,
US) present a review of current research concerning the operation
of the Golgi apparatus, the authors making the following points:
     1) Movement of cargo between cell compartments requires
transiently *coated vesicle carriers. Biosynthetic cargo exiting
the endoplasmic reticulum includes the newly synthesized proteins
and lipids that are moved to distinct cellular and extracellular
destinations. Other cargo incorporated into vesicles includes
proteins that are continuously recycled between compartments.
These components encompass the transport machinery involved in
cargo selection, vesicle formation, and targeting and fusion of
vesicles.
     2) A fundamental principle of membrane traffic is that
vesicle formation is initiated by the selection and concentration
of cargo. This occurs through interactions between sorting
determinants (markers) on the cargo and cytosolic coat components
that direct cargo to the forming vesicle. Soluble cargo (cargo
found in the lumen of the ER compartment) will necessarily
require sorting receptors to couple the protein to the cytosolic
coat machinery. A variety of coat complexes participate in
vesicle formation.
     3) The authors pose the question: How does the Golgi stack
of cisternae mediate transport of cargo from the endoplasmic
reticulum to the cell surface? The authors suggest a possibility
is that cargo-containing vesicles derived from the endoplasmic
reticulum form early Golgi compartments that then mature by
retrieval of processing enzymes from later Golgi compartments.
Maturation continues at terminal Golgi compartments by retrieval
of transport components from the *endocytic pathway to promote
sorting of cargo to multiple destinations. Thus, the authors
suggest, retrograde movement may integrate exocytic (secretory)
and endocytic (material uptake) pathways in eukaryotic cells and
coordinate membrane flow and cargo transport through the Golgi
stack.
-----------
B.B. Allan and W.E. Balch: Protein sorting by directed maturation
of Golgi compartments.
(Science 2 Jul 99 285:63)
QY: William E. Balch [webalch@scripps.edu]
-----------
Text Notes:
... ... *coated vesicle carriers: Coated vesicles are observed in
the cytoplasm of many eukaryotic cells. They measure 50 to 250
nanometers in diameter, and are characterized by a coat made up
of a polyhedral lattice of clathrin subunits together with
smaller amounts of other proteins. Coated vesicles are concerned
with the rapid and continuous transport of molecules between
specific membranous organelles of the cell and to and from the
cell membrane. (See background material below.)
... ... *endocytic pathway: In general, the term "endocytosis"
refers to the uptake of external materials by cells by means of
phagocytosis (uptake of particulate material) or pinocytosis
(uptake of liquid material). In both cases, the cell surface
membrane literally folds completely around the entity to be taken
up, and the membrane-bound is in effect pulled into the cell.
(See background material below.)
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99
-------------------
Related Background:
STRUCTURAL MECHANISMS OF ENDOCYTOSIS
The term "endocytosis" refers in general to any process in which
materials are taken into a biological cell by membrane-bound
vesicles that pinch off from the plasma membrane. When the
material taken up consists of large fragments or whole organisms,
the process is called "phagocytosis". "Receptor-mediated
endocytosis" is a specialized type of endocytosis that brings
specific macromolecules into the cell. Many hormones, *growth
factors, *lymphokines, and nutrients, enter the cell in this
manner. During receptor-mediated endocytosis, the external ligand
first binds to its corresponding plasma membrane receptor; the
receptor-ligand complex then becomes concentrated in specific
regions of the plasma membrane, regions called "coated pits".
Each coated pit is an infolding of the plasma membrane whose
cytoplasmic surface is coated with a polyhedral lattice
constructed from the protein clathrin. The clathrin molecule
consists of 3 large polypeptide chains and 3 small polypeptide
chains organized into a 3-pronged structure, a "triskeleton", and
clathrin triskeletons polymerize with one another to form the
polyhedral lattice. After ligand-receptor complexes have become
clustered within a coated pit, the invaginated membrane pinches
off and becomes internalized as a coated vesicle. This coated
vesicle is initially surrounded by a cage of clathrin molecules,
but this clathrin coat is quickly shed, and vesicles soon
accumulate in what is known as the "endosome compartment" of the
cell. Nearly all the details of the above brief description of
receptor-mediated endocytosis were completely unknown several
decades ago; our current picture is a result of intensive
research in many laboratories, research combining electron
microscopy, biochemistry, and molecular biology.
... ... M. Marsh and H.T. McMahon (2 installations, UK) present a
review of recent research on endocytosis, with a focus on
developments in the clathrin-mediated endocytic pathway. The
authors make the following points:
     1) The uptake, or endocytosis, of extracellular material
into cells in membrane-bound vesicles has been of great interest
to cell biologists for most of this century. The many functions
in which endocytosis plays a role include *antigen presentation,
nutrient acquisition, *clearance of apoptotic cells, pathogen
entry, receptor regulation, and *synaptic transmission.
     2) Concerning clathrin-mediated endocytosis, a high-
resolution 3-dimensional view of the clathrin coat is beginning
to emerge. Clathrin-coated vesicle formation is a complex process
dependent on, and regulated by, the activities of a set of
intracellular proteins that are recruited through various
protein-protein and protein-lipid interactions. *Phosphorylation
and dephosphorylation are apparently key regulators of these
interactions and of the activities of the involved proteins, but
the precise order in which the different components act at each
step of the process remains to be solved.
     3) Of all the coat-mediated transport events characterized
so far in cell biology, endocytic clathrin-coated vesicles are
unique in their degree of complexity. This may reflect a need for
a higher order of control to coordinate clathrin-mediated
endocytosis in various important processes: e.g., the rapid
recovery of synaptic vesicles membranes, or cellular responses to
environmental stimuli.
     4) The authors conclude: "The developments seen over the
past couple of years will continue; new insights and structures
will be published soon. The challenge for the new century will be
to understand how these structures interact to drive endocytosis.
-----------
M. Marsh and H.T. McMahon: The structural era of endocytosis.
(Science 9 Jul 99 285:215)
QY: M. Marsh [m.marsh@ucl.ac.uk]
-----------
Text Notes:
... ... *growth factors: Growth factors are peptide hormones that
regulate the growth of cells and tissues.
... ... *lymphokines: (interleukins) Hormones secreted by certain
antigen-processing cells of the immune system, the hormones
causing immune cells specific for the antigen to proliferate.
... ... *antigen presentation: In general, "antigen presentation"
refers to the presentation of antigens on the surfaces of
antigen-presenting cells of the immune system. In order for an
antigen to be presented on the surface of such a cell, the
antigen must first by taken up by the cell via endocytosis. [See
report #5, this issue of SW.]
... ... *synaptic transmission: This is a general term referring
to the events mediating the membrane-to-membrane interaction
between a neuron and another neuron, or a neuron and a muscle or
gland cell, or a neuron and a sensory receptor cell. The junction
is called a "synapse", and in many cases junction transmission
involves release and uptake of "transmitter" substances.
... ... *Phosphorylation: In general, the process of introducing
a phosphoric acid group into a molecule. Biochemical
phosphorylation reactions are of importance in the trapping of
energy, in the formation of biosynthetic intermediates during
metabolic processes, and in the control of the activity of many
enzymes and other proteins.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 6Aug99
-------------------
Related Background:
FLUORESCENT PROTEIN ANALYSIS OF EUKARYOTIC CELL SECRETION
At all levels of complexity, biological cells are involved in
both the synthesis and secretion of specific chemical substances,
and one important research problem is to unravel the step-by-step
pathway from the production of new material to its export. The
endoplasmic reticulum, visible with the electron microscope and
first described in the 1950s, is an extensive system of flattened
membranous sacs in the cytoplasm of eukaryotic cells. The system
is continuous with the envelope of the cell nucleus and part of
it is covered with ribosomes, the particles responsible for
protein synthesis. This part, called the "rough endoplasmic
reticulum" because of the way it looks in electron micrographs,
is responsible for transporting synthesized material to other
parts of the cell, or to a structure called the Golgi apparatus
if the material is to be secreted into the solution outside the
cell. The Golgi apparatus, visible in the light microscope and
first described in the 19th century, is a collective term for a
group of organelles present in eukaryotic cells that function as
accumulation and packaging centers for substances destined for
export. These Golgi organelles, or Golgi "bodies", consist of
stacks of membranous sacs that are pinched off for delivery to
the exterior of the cell. Last week John F. Presley et al
(National Institutes of Health, US; Johns Hopkins University, US)
reported the use of fluorescent protein tags to follow the
movement of synthesized protein from the endoplasmic reticulum to
the Golgi apparatus, recording fluorescent images at 3.6 second
intervals, and then preparing a "quicktime" movie. This is
apparently the first time the movement of material from the
endoplasmic reticulum to the Golgi apparatus has been completely
visualized, and the evidence now seems firm that microtubules are
the tracks along which such unidirectional translocation occurs.
QY: Jennifer Lippincott-Schwartz [jlippin@helix.nih.gov]
(Nature 4 Sep 97) (Science-Week 19 Sep 97)


6. ON HUMAN EMBRYONIC STEM CELL RESEARCH
In a multicellular living organism such as a human or a mouse,
what differentiates one cell type from another is apparently not
the genome, since the genome is the same in every cell, but which
parts of the genome are operational. In other words, each cell
type, skin cell, muscle cell, etc., has a particular gene profile
characteristic of that cell type. Cells of a particular cell type
are said to be "differentiated". Stem cells, present in all early
embryos and in some tissues, are undifferentiated cells that in
response to appropriate signals differentiate and give rise to a
variety of cell types. Embryonic stem cells are "totipotent",
i.e., they have the potential to differentiate into any type of
tissue cell. These cells form at a very early stage in human
development and remain in an undifferentiated state for only a
short period of time. They are first clearly recognizable
approximately 5 to 7 days after fertilization, when a human
embryo forms a structure called a "blastocyst", a hollow fluid-
filled sphere consisting of only 140 cells. There are two types
of cells in the blastocyst at this stage: a) "trophoblast cells",
which form the wall of the sphere, and which will become
supporting tissues of the fetus (e.g., the placenta); b) "inner-
cell-mass cells", a clump of cells located at one end within the
blastocyst interior, and which are the undifferentiated cells
(stem cells) that will divide and develop into the individual.
The expected future medical applications of stem cells,
particularly embryonic stem cells, are extremely promising, but
because of the involvement of embryos and certain other
considerations, basic stem cell research has provoked intense
controversy. ... ... Shirley J. Wright (University of Dayton, US)
presents a review of those aspects of human embryonic stem cell
research that have been the focus of science policy controversy,
the author making the following points:
     1) Human blastocysts -- each capable of developing into a
complete human being -- are a potential source of embryonic stem
cells, undifferentiated cells with the potential to develop into
any cell type in the body. These cells have enormous therapeutic
potential for the replacement of damaged or diseased tissues, but
current legal and ethical concerns limit the nature of the
research that can be performed with these cells because of their
source.
     2) At the 5 day stage, the human blastocyst is approximately
200 microns in diameter. Cells of the inner cell mass can give
rise to all 3 germ layers -- the ectoderm, mesoderm, and endoderm
-- which in turn give rise to all the tissues in the body. The
ectoderm cells develop into skin, nerves and eyes; the mesoderm
cells develop into bone, blood, and muscles; the endoderm cells
develop into the lungs, liver, and the lining of the intestines.
At the 5 to 7 day stage, the inner cell mass can be removed from
the blastocyst and cultured in a dish as embryonic stem cells.
     3) Early human embryos can also provide undifferentiated
pluripotent cells (i.e., cells capable of differentiating into
certain cell types but not all cell types) in the form of
primordial germ cells, the precursors of eggs and sperm cells.
The primordial germ cells do not differentiate early, remaining
in the yolk sac until approximately the 6th to 8th week of
development, when they migrate to the developing gonads in the
embryo. These primordial germ cells may be extracted as
pluripotent embryonic germ cells beginning approximately 24 days
after fertilization.
     4) Embryonic stem cells obtained from the inner cell mass of
a blastocyst can be grown in a culture dish on a layer of
"feeder" cells derived from irradiated mouse *fibroblasts. The
layer of feeder cells arrests the differentiation of the stem
cells by releasing various inhibitory factors. Cell lines derived
in this manner are immortal -- they can divide indefinitely to
form more undifferentiated cells, thus providing a ready source
for future research.
     5) Fusing a human somatic cell (i.e., any human non-germ
cell) with an enucleated egg cell allows the creation of person-
specific embryonic stem cells, thus avoiding the complications of
tissue incompatibility. In this technique, a patient's somatic
cell is placed next to an enucleated egg cell, and the two cells
are fused by application of an electric current, the somatic cell
nucleus entering the egg cytoplasm. The egg is then activated and
develops into a blastocyst embryo, and the blastocyst can now
provide embryonic stem cells compatible with the patient. This is
the technique that was used Ian Wilmut and his group to produce
the cloned sheep Dolly.
     6) Transfer of a human somatic-cell nucleus (such as a cheek
*epithelial-cell nucleus) to an enucleated bovine egg cell
produces a "*chimera" that could be the source of embryonic stem
cells. Such an experiment was successfully performed by Robl and
Cibelli in 1996. The embryo developed to the 32-cell stage, but
was not allowed to develop further.
     7) Production of human replacement tissue (e.g., neural
cells, pancreatic cells, or heart-muscle cells) in a culture dish
is one of the important potential clinical applications of
embryonic stem-cell technology. Once cultured, the differentiated
cells would be injected into the damaged organ, where they would
replace the damaged tissue. But this has not yet been achieved,
and the clinical technology will require years of development.
     8) The author concludes: "As a society we must identify the
ethical, social, legal, medical, theological, and moral issues
that surround this research. People from all walks of life --
scientists, lawyers, ethicists, clergy, and the general public --
should be involved in making the decision. We are also at the
crossroads where further scientific evidence is needed to explore
the full potential of these cells, and yet many of the necessary
experiments raise further ethical issues. The question of how we
should use these powerful cells remains a challenging problem for
the next century."
-----------
Shirley J. Wright: Human embryonic stem-cell research: Science
and ethics.
(Amer. Scientist Jul/Aug 99 87:352)
QY: Shirley J. Wright [wrights@neelix.udayton.edu]
-----------
Text Notes:
... ... *fibroblasts: A type of connective tissue cell, secreting
structural proteins (e.g., collagen) that form certain tissue
components, including the extracellular matrix.
... ... *epithelial-cell: In animals, epithelial cells
(epithelium) compose the cell layers that form the interface
between a tissue and the external environment, for example, the
cells of the skin, the lining of the intestinal tract, and the
lung airway passages.
... ... *chimera: In general, a "chimera" is any cell or organism
with genetic material from two or more genotypes (e.g., two or
more species). 
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Aug99
-------------------
Related Background:
PATENT ON UMBILICAL-CORD STEM CELLS REJECTED IN EUROPE
Stem cells are undifferentiated cells that give rise to all the
varieties of cells. Bone marrow stem cells are undifferentiated
cells in bone marrow that give rise to all the varieties of blood
cells, including the various leukocytes (white blood cells) of
the immune system: B-cells, T-cells, and macrophages. Umbilical
cord stem cells can produce red and white blood cells and
platelets, and their transplantation is more effective and
cheaper than conventional methods involving stem cells from bone
marrow donors. Umbilical cord stem cells, for example, have lower
immunogenicity, which reduces the risk of rejection by the
patient. A group of international researchers and international
biotechnology companies have won a legal challenge against a
European patent on the use of stored umbilical cord stem cells.
The patent was granted 3 years ago to the US company Biocyte, but
it has now been rejected in Europe, with the primary reason for
the rejection apparently the existence of previous use of such
cells by others. Researchers in the field say the winning of this
legal battle lifts the threat of expensive patent infringement
litigation, a threat that has intimidated the exploration of new
uses for umbilical cord blood cells.
(Nature 99/399:626) (SW Bulletin 25 Jun 99)
-------------------
Related Background:
CONVERSION OF NEURAL STEM CELLS INTO BLOOD CELLS
... Stem cells are common in embryos, but they have also been
identified in adult tissues that undergo extensive cell
replacement due to physiological turnover or injury, e.g., the
*hematopoietic, intestinal, and *epidermal systems. Stem cells
have also been found in the central nervous system, a tissue
believed to be capable of only extremely limited self-repair.
Central nervous system stem cells can generate the 3 major cell
types found in the adult brain: *astrocytes, *oligodendrocytes,
and neurons. This is consistent with the view that the
developmental potential of stem cells is restricted to the
differentiated elements of the tissue in which they reside. But
some developmental peculiarities suggest certain cells may be
able to differentiate into cell types that are not of the same
origin. ... ... C.R.R. Bjornson et al (5 authors at 4
installations, CA IT) now report an investigation to determine
whether stem cells are restricted to produce specific cell types,
namely, those from the tissue in which they reside. The authors
report that after transplantation into *irradiated host mice,
genetically labelled mouse neural stem cells were found to
produce a variety of blood cell types, including *myeloid and
*lymphoid cells, as well as early hematopoietic cells. The
authors suggest that neural stem cells appear to have a wider
differentiation potential than previously thought, and that if
they behave similarly to their mouse counterparts, human neuronal
stem cells may provide a renewable and characterized source of
cells that could be used in approaches aimed at hematopoietic
reconstitution in various blood diseases and disorders.
-----------
C.R.R. Bjornson et al: Turning brain into blood: A hematopoietic
fate adopted by adult neural stem cells in vivo.
(Science 22 Jan 99 283:534)
QY: Christopher R.R. Bjornson [adanac@u.washington.edu]
-----------
Text Notes:
... ... *hematopoietic: From hematopoiesis (hemopoiesis,
hematogenesis) Refers to the formation and development of the
various types of blood cells. 
... ... *epidermal: The term "epidermal" refers to the
superficial epithelial portion of the skin. In animals,
epithelial cells compose the cell layers that form
the interface between a tissue and the external environment, for
example, the cells of the skin, the lining of the intestinal
tract, and the lung airway passages.
... ... *astrocytes: (astroglia, macroglia) Glial cells are more
numerous than neurons in the brain, but their function has been
generally characterized as "metabolic" or "supportive", without
much discussion of details. Astrocytes are the largest glial
cells, with many extensions radiating outward like a starburst,
and at least one of their functions is apparently to maintain the
so-called "blood-brain barrier" effectively separating neural
tissue from blood.
... ... *oligodendrocytes: (oligodendroglia) Glial cells
characterized by sheet-like processes that are wrapped around
individual neuron axons to form the myelin sheath of nerve fibers
in the central nervous system. (The myelin sheath of a nerve
fiber is effectively a periodically interrupted insulation which
increases the propagation velocity of nerve impulses.)
... ... *irradiated host mice: In this investigation, host
animals were radiated before transplantation in order to reduce
the population of immune system blood cells, this reduction
apparently intensifying the signals resulting in donor stem cell
differentiation.
... ... *myeloid: Refers to bone marrow cells or cells derived
from bone marrow cells.
... ... *lymphoid cells: Refers to cells of the lymphatic system.
The lymphatic system is a complex network for the distribution of
lymph fluid (which is similar to blood plasma -- blood without
red cells).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 2Apr99
-------------------
Related Background:
RESEARCH USE OF STEM CELLS APPROVED BY NIH
It is probable that one of the major public debates in the coming
months will concern the use of human stem cells in research (see
related background report below). On 19 January 1999, the US
Department of Health and Human Services announced that a
Congressional ban on human embryo research does not apply to
human stem cells, and federally financed researchers will soon
be able to work on these cells. In its ruling, the Office of
the General Counsel of the US health department said that because
stem cells by themselves do not have the capacity to develop into
a human being, they cannot be considered embryos. Deriving the
cells from human embryos is legal, provided it is not done with
federal money. Harold E. Varmus, director of the US National
Institutes of Health, has stated it would still be illegal for
researchers to use federal money to derive their own stem cells
from human embryos, but they can now use federal money to work on
the cells obtained by others, in particular the stem cells
obtained by James Thompson (University of Wisconsin, US) from
human embryos created in surplus amounts in a fertility clinic.
The new US government ruling on the use of human stem cells is
opposed by the US National Conference of Catholic Bishops, and
the ruling is still subject to modification or elimination by the
new US Congress.
(New York Times 20 Jan 1999)
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 29Jan99
-------------------
Related Background:
EMBRYONIC STEM CELLS DERIVED FROM HUMAN BLASTOCYSTS
*Embryonic stem cells are derived from *totipotent cells of the
early mammalian embryo and are capable of *unlimited and
undifferentiated proliferation in vitro. In *chimeras with intact
embryos, mouse embryonic stem cells contribute to a wide range of
adult tissues, including *germ cells, providing a powerful
approach for introducing specific genetic changes into the mouse
*germ line. ... ... J.A. Thompson et al now report the production
of human *blastocyst-derived *pluripotent cell lines that have
normal chromosome characteristics, express high levels of
*telomerase activity, and express *cell surface markers that
uniquely characterize primate embryonic stem cells. The authors
report that after undifferentiated proliferation in vitro for 4
to 5 months, these cells still maintained the developmental
potential to form *trophoblast, and to form derivatives of *all 3
embryonic germ layers, including gut *epithelium (mesoderm) and
neural epithelium, embryonic *ganglia, and *stratified squamous
epithelium (ectoderm). The authors suggest these cell lines
should be useful in human developmental biology, drug discovery,
and transplantation medicine. ... ... In a related commentary in
the same journal, J. Gearhart makes the following points: 1) A
renewable tissue-culture source of human cells capable of
differentiating into a wide variety of cell types would have
broad applications in basic research and transplantation
therapies. A major step in realizing this goal has now been taken
with the demonstration that human embryonic stem cells can be
grown in culture. 2) In the work of J.A. Thompson et al, four
cell lines tested produced *teratomas when grown in
*immunosuppressed mice. Histology of the tumors revealed
differentiated cells derived from all 3 embryonic germ layers
(ectoderm, mesoderm, and definitive endoderm) -- a result
consistent with pluripotency. 3) The derivation of human
embryonic stem cells now raises a whole new set of expectations.
On the basis of the already completed use and study of mouse
embryonic stem cells, the research and clinical potential for
human embryonic stem cells is enormous. They will be important
for in vitro studies of normal human embryogenesis, abnormal
development (through the generation of cell lines with targeted
gene alterations and engineered chromosomes), human gene
discovery, drug and *teratogen testing, and as a renewable source
of cells for tissue transplantation, cell replacement, and gene
therapies. These latter applications could eventually make
unnecessary the direct use of fetal tissue in transplantation
therapies [*Note #1].
-----------
J.A. Thompson et al (7 authors at 2 installations, US IL)
Embryonic stem cell lines derived from human blastocysts.
(Science 6 Nov 98 282:1145)
QY: James A. Thompson, University of Wisconsin 608-262-3961.
-----------
J. Gearhart (Johns Hopkins University, US)
New potential for human embryonic stem cells.
(Science 6 Nov 98 282:1061)
QY: John Gearhart [gearhart@jhmi.edu]
-----------
Text Notes:
... ... *Embryonic stem cells: In general, the term "stem cells"
refers to undifferentiated cells that upon differentiation can
give rise to various specialized cell lines such as blood cells,
skin cells, nerve cells, etc. Adult bone marrow, for example,
contains stem cells that are the precursors of the various
specialized types of blood cells. "Embryonic" stem cells are
specifically stem cells derived from the embryo only.
... ... *totipotent cells: Cells that have the ability to
differentiate into any type of cell and thus form a new organism
or regenerate any part of an organism.
... ... *unlimited and undifferentiated proliferation in vitro:
In general, differentiated "normal" cells in tissue culture
produce a limited number of replications. In contrast, embryonic
stem cells and many types of cancer cells in tissue culture show
unlimited replications, and are thus called "immortal" cell
lines. In this context, "undifferentiated" proliferation is
simply proliferation without cell differentiation
(specialization).
... ... *chimeras: In this context, an animal that has received a
transplant of genetically and immunologically different tissue.
In this report, the transplant involves the injection of human
cultured stem cells into mice.
... ... *germ cells: In general, reproductive cells. All other
cells are "somatic" cells.
... ... *germ line: In general, this refers to the line of
differentiated germ cells.
... ... *blastocyst: A mammalian egg in the later stages of
*cleavage but before implantation in the uterus. The blastocyst
consists of a hollow fluid-filled ball of cells and an inner cell
mass (embryonic stem cells) from which the embryo develops.
... ... *cleavage: The early and rapid division stage that
divides the fertilized egg into smaller and smaller cells
(blastomeres) while retaining the same overall size of the
embryo.
... ... *pluripotent cell: A cell that has the potential,
depending on conditions, to give rise to many differentiated cell
lines but which lacks complete totipotency.
... ... *telomerase: Telomeres are defined ends of chromosomes
that contain specific repeated DNA sequences. They are essential
for normal chromosome replication, and since their length
shortens a bit with each replication, they are believed to be
involved in the aging of the cell. Telomerase is an enzyme that
repairs damage to telomeres, and it is thought by some that
cancerous cells may have mutant telomerase, the mutant enzyme
conferring immortality on the cancer cell.
... ... *cell surface markers: Cell surface proteins or protein
components that can be chemically identified.
... ... *trophoblast: In the early vertebrate embryo, the outer
ectodermal cell layer of the blastocyst. In mammals, it is the
trophoblast that attaches to the uterus and forms the placenta.
... ... *all 3 embryonic germ layers: In the embryos of higher
animals, there occurs the transformation of a single-layer
blastula into a 3-layered gastrula consisting of ectoderm,
mesoderm, and endoderm 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.
... ... *epithelium: In animals, epithelial cells (epithelium)
compose the cell layers that form the interface between a tissue
and the external environment, for example, the cells of the skin,
the lining of the intestinal tract, and the lung airway passages.
... ... *ganglia: (singular: ganglion) In the context of cells,
the original meaning of "ganglion" was any cluster of nerve cell
bodies in the central or peripheral nervous system. Currently,
the term "ganglion" refers to a aggregation of nerve cell bodies
located in the peripheral nervous system. Unfortunately, many
neuroanatomy texts still label certain neuron clusters in the
central nervous system in the old way (e.g., basal ganglia).
... ... *stratified squamous epithelium:  The cells of the
epithelium are for the most part closely packed cells with little
extracellular material between adjacent cells, the cells arranged
in continuous sheets in either single or multiple layers. The
cells may be flat, cubelike, columnar, or a combination of
shapes, and "squamous" cells are flattened and scalelike. In this
context, "stratified squamous epithelium" refers to a structure
consisting of distinctly layered epithelial cells (layers varying
in size and shape of cells), the top layer of which are squamous
cells.
... ... *teratomas: A teratoma is a neoplasm (tumor) composed of
multiple tissues, including tissues not normally found in the
organ in which it arises. A teratoma in the adult human ovary,
for example, can contain hair, teeth, skin, heart muscle, nerve
cells, and so on -- all a result of "wild" cellular
differentiation of neoplastic cells, but with enough regulation
so that distinct tissues are formed. In the context of this
report, the teratomas occurred in mice after injection of
cultured human stem cells, thus demonstrating the ability of
those stem cells to differentiate into organized specific tissue-
producing cells.
... ... *immunosuppressed mice: In general, this refers to mice
whose immune system response has been suppressed by chemical,
biological, or physical means. In this report, the purpose of the
immunosuppression was to allow the development of a mouse
teratoma provoked by injection of human stem cells. Without
immunosuppression, the human stem cells would be immediately
attacked and possibly destroyed by the mouse immune system before
the stems cells could differentiate.
... ... *teratogen: Any drug or other agent that causes abnormal
fetal development.
... ... *Note #1: We repeat here a quotation that appeared at the
head of a recent issue of SW: "Between the fifth and tenth days
the lump of stem cells differentiates into the overall building
plan of the mouse embryo and its organs. It is a bit like a lump
of iron turning into the space shuttle. In fact it is the
profoundest wonder we can still imagine and accept, and at the
same time so usual that we have to force ourselves to wonder
about the wondrousness of this wonder." -- Miroslav Holub
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 27Nov98


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IN FOCUS: ON HORIZONTAL GENE TRANSFER
[Editor's note: Biologists recognize two types of gene transfer
from one organism to another: vertical and horizontal. Vertical
gene transfer occurs between parents and offspring, and
horizontal gene transfer is the transfer that may occur (by
various possible mechanisms [*Note #1]) between organisms
otherwise, including transfer between organisms of different
species. It is in bacteria that horizontal gene transfer has been
studied most extensively, particularly in the last decade. The
following extract summarizes current work in this field.]
----------------
"It is becoming increasingly apparent that many genes within
eukaryotes and prokaryotes have been acquired by horizontal
transfer, but not all genes are equally likely to be transferred.
The preferential horizontal transfer of genes in both eukaryotes
and prokaryotes is strongly correlated with gene function.
Specifically, genes participating in *transcription,
*translation, and related processes (informational genes) are far
less likely to be horizontally transferred than genes
participating in *housekeeping functions (operational genes).
Furthermore, the frequency of horizontal transfer in prokaryotes
is not related to evolutionary rates (nucleotide substitution
rates) because evolutionary rates for operational and
informational genes have not differed significantly since the
*cyanobacteria and *proteobacteria diverged. Two alternative
hypotheses have been proposed to explain the previously observed
patterns of horizontal transfer. The first, the continued
horizontal transfer hypothesis, proposes that horizontal transfer
of operational genes is a continual process in prokaryotes. This
hypothesis implies that horizontal gene transfer of operational
genes is a far more important factor in prokaryotic evolution
than previously thought. The second, or early massive horizontal
transfer hypothesis, proposes that one, or a few, massive ancient
exchanges of (operational) genes occurred early in prokaryotic
evolution, before the diversification of modern prokaryotes. This
hypothesis explains the observed similarity of evolutionary rates
for operational and informational genes since cyanobacteria and
proteobacteria diverged. It supports the idea that massive
horizontal exchanges could have created modern prokaryotes...
We propose that a major factor in the more frequent horizontal
transfer of operational genes is that informational genes are
typically members of large complex systems, whereas operational
genes are not, thereby making horizontal transfer of
informational gene products less probable (the complexity
hypothesis)."
-----------
-- R. Jain et al: "Horizontal gene transfer among genomes: The
complexity hypothesis." (Proc. Natl. Acad. Sci. US 1999 96:3801)
-----------
Text Notes:
... ... *Note #1: Three types of horizontal gene transfer exist:
conjugation, transduction, and transformation. Conjugation is a
type of sexual reproduction exhibited by some bacteria, the
process involving the exchange of genetic material by means of a
tube or bridge, the transfer of DNA occurring either in one
direction or in both directions. Transduction involves the
transfer of genetic material from one bacterium to another with
the intermediation of the virus bacteriophage. Essentially, when
the virus infects one bacterium, it often carries away pieces of
that bacterium's genome, and those pieces, upon the infection of
a new bacterium, become incorporated into another genome.
Finally, transformation is the process involving the uptake or
incorporation of DNA fragments (or plasmids) by a bacterium,
first observed in 1944 by Oswald Avery. Transformation is a
common laboratory technique used in genetic engineering. One
significant aspect of these modes of horizontal bacterial gene
transfer is that in principle they make possible the horizontal
gene transfer of genetic material from genetically engineered
bacteria to other organisms without predictable results. So if
genetically engineered bacteria are to be widely used in
industry, for example, it is of some importance to have a
thorough understanding of the mechanisms in order to limit any
harmful ecosystem transfer effects.
... ... *eukaryotes and prokaryotes: The term "eukaryotes" refers
to cells (or organisms composed of such cells) that have internal
membrane-bound organelles such as a nucleus. The term
"prokaryotes" refers to cells that do not possess such internal
organelles (e.g., bacteria).
... ... *transcription: In this context, "transcription" is the
process by which genetic information in DNA is converted into
RNA.
... ... *translation: In this context, "translation" is protein
synthesis, the process during which polypeptides are synthesized
on ribosomes in accordance with RNA code.
... ... *housekeeping functions: So-called "housekeeping" genes
are genes coding for proteins involved in essential functions
such as metabolic cycles.
... ... *cyanobacteria: (formerly misclassified as blue-green
algae). Photosynthetic bacteria containing chlorophyll.
... ... *proteobacteria: (purple bacteria) A general category
comprising a large number of diverse forms (e.g., Escherichia
coli).


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