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.

December 18, 1998 -- Vol. 2 Number 51

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It would be a poor thing to be an atom in a universe
without physicists, and physicists are made of atoms.
A physicist is an atom's way of knowing about atoms.
-- George Wald

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Contents of This Issue:

1. On the Efficacies and Confusions of Alternative Medicine
2. On the Surfaces of the Terrestrial Planets
3. On the Origin of Water on Earth
4. Diffusive Contamination of Calcite Under Standard Conditions
5. Genome of the Typhus Parasite and the Origin of Mitochondria
6. On Transporters vs. Channels in Biological Membranes
7. Developmental Neurobiology: Segregation of Synapses

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1. ON THE EFFICACIES AND CONFUSIONS OF ALTERNATIVE MEDICINE
What is called "alternative" medicine, and which consists of
medical practices outside the mainstream of "official" medicine,
has always been a significant part of the health care of the
public, involving healers, herbalists, bonesetters, barbers,
shamans, spiritualists, homeopaths, psychic and magnetic healers,
and so on. It is estimated that in the US 40 percent of the
population makes use of medical procedures outside of what is
considered orthodox medical practices. ... ... In an editorial on
the subject in an important medical journal, P.B. Fontanarosa and
G.D. Lundberg make the following points: 1) There is no
alternative medicine. There is only scientifically proven,
evidence-based medicine supported by solid data, or there is
unproven medicine, for which scientific evidence is lacking.
Whether a therapeutic practice is "Eastern" or "Western", is
unconventional or mainstream, or involves mind-body techniques or
molecular genetics is largely irrelevant except for historical
purposes and cultural interest. 2) In the US and throughout the
world, most alternative therapies have not been evaluated using
rigorously conducted scientific tests of efficacy based on
accepted rules of evidence. The lack of properly designed and
conducted randomized trials is a major deficiency... However,
some advocates of alternative medicine argue that many
alternative therapies cannot be subjected to the standard
scientific method, and thus instead must rely on anecdotes,
beliefs, theories, testimonials, and opinions to support
effectiveness and justify continued use. 3) For alternative
medicine therapies that are used by millions of patients every
day and that generate billions of dollars in health care
expenditure each year, the lack of convincing and compelling
evidence concerning efficacy, safety, and outcomes is
unacceptable and deeply troubling. 4) Until solid evidence is
available that demonstrates the safety, efficacy, and
effectiveness of specific alternative medicine interventions,
uncritical acceptance of untested and unproven alternative
medicine therapies must stop. Alternative therapies that have
been shown to be of no benefit (aside from possible placebo
effects) or that cause harm should be abandoned immediately.
Physicians, insurance plans, medical centers and hospitals,
managed care organizations, and government policy-makers should
base decisions regarding incorporation of and payments for
alternative medicine therapies on evidence-based research and
objective cost-effectiveness analyses rather than on consumer
interest, market-demand or competition, well-publicized anecdotal
reports, or political pressures from well-organized and
influential interest groups. [Editor's note: The Fontanarosa-
Lundberg editorial briefed here is a commentary on 6 studies of
the efficacy of alternative medicine appearing in the same issue
of the journal.]
-----------
P.B. Fontanarosa and G.D. Lundberg (American Medical Assoc., US)
Alternative medicine meets science.
(J. Amer. Med. Assoc. 11 Nov 98 280:1618)
QY: Phil B. Fontanarosa 
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Summary by SCIENCE-WEEK http://scienceweek.com 18Dec98

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Related Background:

AN EVALUATION OF CHIROPRACTIC AND MCKENZIE TREATMENTS
In theory, "chiropractic" is a therapeutic system that uses the
recuperative powers of the body and the relationship between the
musculoskeletal structures and functions of the body,
particularly as they involve the spinal column and the nervous
system, in the restoration and maintenance of health. The most
common method of chiropractic manipulation is a short lever,
high-velocity thrust directed specifically at a "manipulable
lesion". This procedure is typically performed with the patient
lying on his or her side on a segmental table. In the US, many
individuals are licensed and certified to practice chiropractic.
There is also extant in the US the "McKenzie method of physical
therapy", in which patients are placed in one of 3 broad
categories ("derangement", "dysfunction", and "postural"
syndromes) that determine therapy. In this formulation, most
back, buttock, and leg pain is considered to result from a
derangement syndrome treated by exercises that "centralize" pain
from the feet, legs, or buttocks to the lower back. Patients are
taught to perform exercises that centralize their symptoms and to
avoid movements that peripheralize them. The McKenzie method
relies on patient-generated forces and emphasizes self-care. In
the literature at the present time, there are few data on the
relative effectiveness and costs of various treatments for low
back pain. ... ... D.C. Cherkin et al now report a study
involving random assignment of 321 adults with low back pain that
persisted for 7 days after a primary care clinic visit, the
subjects assigned to a McKenzie group, a chiropractic group, and
to a minimal intervention group that only received an educational
booklet. All McKenzie physical therapy and chiropractic patients
were treated by certified practitioners of those methods for 1
month with as many as 8 visits per patient during that interval.
The study received the cooperation and assistance of local
McKenzie therapy and chiropractic professional groups. The
authors report that for patients with low back pain, the McKenzie
method of physical therapy and chiropractic manipulation had
similar effects and costs, and that patients receiving these
treatments had only marginally better outcomes than those
receiving the minimal intervention of an educational booklet. The
mean costs of treatment (Seattle, US) were US$238 per patient for
the physical therapy group, US$226 per patient for the
chiropractic group, and US$1 per patient for the booklet group.
The authors suggest that whether the limited benefits of the
McKenzie and chiropractic treatments are worth the additional
costs is open to question.
-----------
D.C. Cherkin et al (5 authors at 2 installations, US CA)
A comparison of physical therapy, chiropractic manipulation, and
provision of an educational booklet for the treatment of patients
with low back pain.
(New England J. Med. 8 Oct 98 339:1021)
QY: Daniel C. Cherkin, Univ. of Washington Seattle 206-543-8992.
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Summary by SCIENCE-WEEK http://scienceweek.com 6Nov98

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Related Background:

SHARP CRITICISM OF NIH ALTERNATIVE MEDICINE OFFICE
In addition to its activities in the standard health-related
sciences, the U.S. National Institutes of Health (NIH) also
maintains an Office of Alternative Medicine (OAM), created in
1992, to evaluate "unconventional health practices". At $12.5
million, the budget of this NIH office is not insignificant, if
one considers that the money would provide four-year scholarships
for 200 young scientists. Recently, a number of influential
scientists have been demanding the closing of the OAM office,
which they say does little evidentiary based evaluating, and
which they say has given prestige to "dubious practices more
clearly resembling witchcraft than medicine." The OAM budget is
up for renewal this month, and its opponents hope the office will
be dismantled and disappear.
(Science 11 Jul 97) (Science-Week 18 Jul 97) 


2. ON THE SURFACES OF THE TERRESTRIAL PLANETS
Mercury, Venus, and Mars are classified as terrestrial planets
because like Earth they have rocky silicate-rich surfaces. But
each planet has distinct surface characteristics that make it
unique, and delineation of the geological forces shaping the
surfaces of the four inner Solar System planets is a major focus
of research in planetary science and geology. ... ... Thomas R.
Watters presents a review of current ideas concerning surface
formation on the inner planets, the author making the following
points: 1) The planets apparently began coalescing out of a disk
of gas and dust orbiting the Sun approximately 4.6 billion years
ago, and the accretion of silicate- and metal-rich *planetesimals
created huge amounts of heat. Gravity compression and radioactive
decay heated the interiors of the planets, and the planets melted
and differentiated: metals, particularly iron, sank into molten
cores, and the less dense silicates floated to form crust. The
subsequent cooling influenced the evolution of these crusts,
primarily by two processes, volcanism (the eruption of lava) and
tectonism (the reshaping of the surface by crust movement). 2)
The dominant cooling mechanism on Earth is plate tectonics, which
involves the movement of 8 large plates and a few dozen smaller
plates, the motion driven from beneath by convection within the
mantle. On Earth, plate tectonics concentrates most of the
volcanoes, earthquakes, and other tectonic features along plate
margins rather than scattering them evenly throughout the crust.
The San Andreas fault in California, for example, is a *strike-
slip fault at the interface between the Pacific plate (an oceanic
plate) and the North American plate (a continental plate). 3) The
extant data concerning the surface of Mercury derives primarily
from the 1974 Mariner 10 mission, which returned images of 45
percent of the surface of the planet. A prominent feature on
Mercury are the so-called "*lobate scarps". These lobate scarps
are evenly distributed on the surface of Mercury, and the even
distribution suggests that plate tectonics did not form these
features, and that instead Mercury has a single continuous
crustal plate. 4) The relatively small size of Mars, its colossal
*shield volcanoes, and the scattered nature of its tectonic
features, all suggest that Mars is also a one-plate planet, the
crust relatively thick and stationary. There is some evidence,
however, that something like plate tectonics may have formed the
lowlands of Mars, perhaps by a mechanism analogous to *sea-floor
spreading. 5) Concerning Venus, the evidence argues against plate
tectonics as the dominant heat-loss mechanism on the planet, the
evidence instead suggesting the planet cooled by conduction
through the crust, *mantle plumes, and *volcanic hot spots.
-----------
Thomas R. Watters (Smithsonian Institution Washington, US)
Planetary face-off.
(Astronomy January 1999)
QY: Thomas R. Watters, Smithsonian Institution National Air and
Space Museum, Washington, DC US 202-357-1400.
-----------

Text Notes:
... ... *planetesimals: Planetesimals 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. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk.
... ... *strike-slip fault: Geophysical faulting is a break in
rock structure that occurs when pressures in the planet's crust
are strong enough to cause fracture and displacement. A strike-
slip fault is a movement parallel to the fault plane, the two
plates shifting tangentially in opposite directions along their
interface. The 2 other major types of faults are 1) the "normal"
fault, which consists of a simple vertical shifting at the
interface, one plate moving up and the other down, and 2) the
thrust fault, which involves the edge of one plate sliding over
(overlapping) the edge of the adjacent plate.
... ... *lobate scarps: A scarp (escarpment) is a steep slope or
cliff occurring at the margin of a flat or gently sloping area,
and a "lobate" scarp is a curved fault scarp.
... ... *shield volcanoes: A shield volcano is a volcano with a
broad low-angle cone produced by highly fluid and mobile lava
(sometimes called a "Hawaiian-type" eruption). The Olympus Mons
shield volcano on Mars, the largest volcano in the Solar System,
has a height of 16 miles and a width of 300 miles.
... ... *sea-floor spreading: *sea-floor spreading is the process
whereby sea floor is continuously created as the crustal plates
move apart and continuously destroyed where the plates push
against each other.
... ... *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). 
... ... *volcanic hot spots: The term "hot spot" (also, hotspot)
refers to a long-lasting center of surface volcanism and locally
high heat flow, and about 40 locations are now so labelled. Most
hot spots are in ocean basins, and at points where the
lithosphere has apparently upswelled, elevating the denser mantle
material and creating mass anomalies. The source of a hot spot is
apparently the mantle-core boundary and a mantle plume above it,
so that if a crustal plate moves, any hot spot retains its
location with respect to the center of the Earth (or planet), the
hot spot thus not moving with the crust.
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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98

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Related Background:

LOWER LITHOSPHERE FLOW AND CONTINENTAL STRIKE-SLIP FAULTS
A geophysical fault is a break in rock structure that occurs when
pressures in the Earth's crust are strong enough to cause
fracture and displacement, and earthquakes are common at such
break points. A strike-slip fault is a movement parallel to the
fault plane, and the San Andreas fault of California is of this
type. The term "lithosphere" refers to the outer layer of the
Earth, comprising the crust and upper mantle, and extending to a
depth of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer, the system producing two forms of deformation,
namely, brittle fracture in the upper layer (accompanied by
earthquakes), and aseismic (without earthquakes) ductile flow in
the lower layer. The current consensus is that this view is
generally correct but imprecise, since the accumulated evidence
is now interpreted to indicate that frictional events along fault
lines, rather than new fractures, are the causes of earthquakes.
The essential idea is that fault lines, which are the interfaces
between the crustal plates, build up stresses resulting from the
movements of the plates, and at intervals these stresses are
suddenly relieved by interface slippages the surface manifest-
ations of which are earthquakes. One important question is how
movements of the lithosphere relate to the faults and to earth-
quakes, with one view holding that lithosphere deformations are
secondary, and the other view holding that lithosphere deform-
ations are of great significance for fault behavior. The Marl-
borough fault zone is found in the South Island of New Zealand.
Geodetic measurements, in this context, are measurements of
contemporary terrestrial structure, as opposed to determinations
based on geological (historical) analysis. ... ... Bourne et al
(University of Oxford, UK) report an investigation of the
consequences of the lithosphere deformation model. They tested
predictions of the model in two plate boundary regions, the
Marlborough fault zone and the Southern California fault zone,
and they found that slip rates on the faults predicted from
geodetic measurements are in good agreement with geologically
determined slip rates. The authors suggest their results support
the view that flow in the ductile portion of the lithosphere
drives the accumulation of strain in the brittle upper layer
where faulting occurs, and must play a role in the post-seismic
relaxation of strain resulting from slip during earthquakes, and
that the upper-crustal blocks must follow the flow of the lower
layer.
QY: S.J. Bourne 
(Nature 12 Feb 98)

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Related Background:

EARTHQUAKES AND FRICTION LAWS
The term "lithosphere" refers to the outer layer of the Earth,
comprising the crust and upper mantle, and extending to a depth
of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer, the system producing two forms of deformation,
namely, brittle fracture in the upper layer (accompanied by
earthquakes), and aseismic (without earthquakes) ductile flow in
the lower layer. The current consensus is that this view is
generally correct but imprecise, since the accumulated evidence
is now interpreted to indicate that frictional events along fault
lines, rather than new fractures, are the causes of earthquakes.
The essential idea is that fault lines, which are the interfaces
between the crustal plates, build up stresses resulting from the
movements of the plates, and at intervals these stresses are
suddenly relieved by interface slippages the surface manifest-
ations of which are earthquakes. In mechanics, "stick-slip"
friction is friction between two surfaces that are alternately at
rest and in motion with respect to each other, and in recent
years a number of laboratories have conducted model experiments
with stick-slip rock systems with the idea of obtaining a fuller
understanding of the physics of frictional phenomena occurring at
fault lines. C.H. Scholz (Columbia Univ., US), in a review of
current ideas concerning earthquake mechanics, points out that at
present the most precise and predictive model for earthquake
mechanisms is that an earthquake is a frictional rather than a
fractional phenomenon, with brittle fracture of the upper litho-
sphere layer playing a secondary role in the lengthening of
faults and frictional wear. The origin of earthquakes is evid-
ently a stick-slip frictional instability, and many of the
aspects of earthquake phenomena can apparently be explained by
the general laws applying to frictional stability regimes.
QY: Christopher H. Scholz, Columbia Univ., Dept. Earth and
Environmental Sciences, 212-854-1754 (Nature 1 Jan 98)


3. ON THE ORIGIN OF WATER ON EARTH
James F. Kasting presents a review of current ideas concerning
the origin of water on Earth, the author making the following
points: 1) Life exists on Earth because Earth has a surface that
supports liquid water, which is required by all living things...
The long molecular chains and complex branching structures of
carbon make carbon the ideal chemical backbone for life, and
water is the ideal solvent in which carbon-based chemistry can
proceed. 2) Dating of meteorites indicates that the Solar System
is approximately 4.6 billion years old, and Earth appears to be
approximately the same age. Yet the oldest sedimentary rocks --
those that formed by processes requiring liquid water -- are only
approximately 3.9 billion years old. This observation suggests
that at least some water was present on the surface of the Earth
by that time, but the early conditions remain unclear. Recently,
there has been intense interest in the discovery that one of
Jupiter's moons, Europa, apparently has an ocean of liquid water
beneath its crust of water ice. 3) If the aggregating
*planetesimals that formed the Earth resembled the most abundant
type of meteorites, the ordinary *chondrites, they contained
approximately 0.1 percent water by weight. An early Earth
consisting of 0.1 percent water would contain 4 times the amount
of water now held in the oceans. 4) The evidence indicates that
Earth (as well as the Moon) underwent relatively heavy meteorite
bombardment from approximately 4.5 billion years ago until
approximately 3.8 billion years ago. If those meteorites also
resembled ordinary chondrites, this would also provide for more
than enough water for the oceans. 5) Comets have been proposed as
another possible source of water, but recent analysis of the 3
comets Halley, Hyakutake, and Hale-Bopp indicates these comets
have a relatively high percentage of deuterium -- twice as much
in the comets as in sea water, which poses a problem for any
model involving comets as a source of early Earth's water. More
recently, it has been proposed that Earth has been continually
bombarded by small house-sized comets, enough to fill the oceans
with water over the lifetime of the Earth, but this hypothesis
remains in sharp controversy.
-----------
James F. Kasting (Pennsylvania State University, US)
The origins of water on Earth.
(Scientific American Presents Fall 1998)
QY: James F. Kasting, Pennsylvania State Univ. 814-863-8461.
-----------

Text Notes:
... ... *planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion.
... ... *chondrites: 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.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98

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Related Background:

NEW EVIDENCE FOR SUBSURFACE OCEANS IN EUROPA AND CALLISTO
The 4 largest of Jupiter's moons are called the Galilean moons,
and in order from outermost inward they are Callisto, Ganymede,
Europa, and Io. Callisto is a low-density moon 44 percent larger
than Earth's moon. Europa is denser than Callisto and slightly
smaller than Earth's moon, and it apparently contains a liquid
water mantle and a thick icy crust. Because of the presence of
large amounts of liquid water, Europa has been considered a
possible abode of life. Since 7 December 1995, the Galileo
spacecraft has been orbiting Jupiter and transmitting data
concerning the Jovian moons, and initial results from the
spacecraft's *magnetometer have indicated that neither Europa nor
Callisto have an appreciable internal magnetic field, in contrast
to Ganymede and possibly Io. ... ... K.K. Khurana et al now
report perturbations of the external magnetic fields (associated
with Jupiter's inner *magnetosphere) in the vicinity of both
Europa and Callisto. The authors interpret these perturbations as
arising from induced magnetic fields generated by the moons in
response to the periodically varying *plasma environment. The
authors suggest that *electromagnetic induction requires *eddy
currents to flow within the moons, and that their calculations
indicate that the most probable explanation is that there are
layers of significant electrical conductivity just beneath the
surfaces of both moons. The authors propose that these conducting
layers may be best be explained by the presence of salty liquid-
water oceans, for which there is already indirect geological
evidence in the case of Europa.
-----------
K.K. Khurana et al (7 authors at 3 installations, US)
Induced magnetic fields as evidence for subsurface oceans in
Europa and Callisto.
QY: K.K. Khurana 
(Nature 22 Oct 98 395:777)
-----------

Text Notes:
... ... *magnetometer: In general, any instrument used to measure
the strength and direction of a magnetic field.
... ... *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. 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 so they can 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.
... ... *electromagnetic induction: In general, this refers to
the production of an electromotive force either by motion of a
conductor through a magnetic field so as to cut across the
magnetic flux (magnetic "lines of force") or by a change in the
magnetic flux that threads a conductor.
... ... *eddy currents: (Foucault currents) In general, an eddy
current is an electrical current induced within the body of a
conductor when that conductor either moves through a nonuniform
magnetic field or is in a region where there is a change in
magnetic flux.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Nov98

-------------------

Related Background:

MOUNTING EVIDENCE FOR A SUB-SURFACE OCEAN ON JUPITER'S EUROPA
Europa is the smallest of Jupiter's so-called Galilean
satellites, with a diameter of 3138 kilometers. It has a smooth
crust of water-ice, criss-crossed by a network of light and dark
linear markings. It has been thought that beneath the crust there
may be an ocean of water, and a variety of ice tectonics has been
proposed to be operating on the surface. The Galileo spacecraft
is a US National Aeronautics and Space Administration mission to
Jupiter launched in 1989. ... ... Now Carr et al (22 authors at
12 installations, US DE) report high-resolution (54 meters per
pixel) Galileo spacecraft images of Europa that provide evidence
for mobile "icebergs", the detailed morphology of the terrain
strongly supporting the presence of liquid water at shallow
depths below the surface either at present or at some time in the
past. In a contiguous report, Pappalardo et al (11 authors at 7
installations, US) report an analysis of certain surface features
of Europa also revealed by high-resolution images from the same
spacecraft, and conclude the features are surface manifestations
of localized relatively warm ice masses that have risen through
the subsurface, and are consistent with a subsurface liquid water
ocean. Results from 2 other contiguous reports are also
interpreted as consistent with the idea of a subsurface liquid
water ocean on Europa.
QY: Michael H. Carr ; R.T. Pappalardo
; Robert Sullivan

(Nature 22 Jan 98) (Science-Week 6 Feb 98)

-------------------

Related Background:

NEW DETERMINATIONS OF INTERNAL STRUCTURE OF JUPITER'S MOON EUROPA
Data from the December 19, 1996 Galileo spacecraft encounter
with the Jovian moon Europa have now been published, and the
consensus is that Europa has a water ice-liquid outer shell
about 100 to 200 kilometers thick. Gravitational effects on the
spacecraft are consistent with two models of Europa. In one the
core is a mixture of rock and metal, and in the other it is
purely metallic. Further data concerning Europa's intrinsic
magnetic field is needed to distinguish between the two models.
Reports were provided by J. D. Anderson et al (California
Institute of Technology, Pasadena CA US; University of
California Los Angeles, CA US) and M. G. Kivelson et al
(University of California Los Angeles, CA US; Imperial College of
London UK; California Institute of Technology, Pasadena CA US).
(Science 23 May 97)

-------------------

Related Background:

POSSIBILITY OF A DEEP OCEAN ON JUPITER'S MOON EUROPA
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. 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 km wide and 1000 km 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. Until recently,
most planetary scientists apparently doubted any contemporary
existence of a liquid water mantle. But new analyses have been
appearing, and at the meeting last week of the Division for
Planetary Sciences of the American Astronomical Society in
Boston, a consensus appears to have formed that a water mantle
probably existed in the recent past, and may even exist today. In
December, the Galileo spacecraft, as part of its extended
mission, will take an even closer look at Europa, with the resol-
ution of its best images expected to improve from 70 meters to
10
meters, and planetary scientists are hopeful the question of the
state of water on Europa will be answered.
(Science 8 Aug 97)

-------------------

Related Background:

MESOSPHERIC WATER DATA SUPPORTS SMALL-COMET THEORY
Our current conceptions of the physics of the Earth's atmosphere
may be on the verge of change. The first rumbles began last May
with the report of Louis Frank (University of Iowa, US) of dark
spots in the outer atmosphere interpreted to be the result of
bombardment by house-sized water-containing small comets striking
the atmosphere at the rate of 1000 comets per hour. Many
geophysicists were dubious, but a few did support Frank's
explanation for the dark spots. Now an orbiting instrument of the
Naval Research Laboratory, a high resolution spectrograph, has
detected abundant hydroxyl radicals, a breakdown product of
water, in the mesosphere above northern latitudes. The mesosphere
is the layer from 50 to 80 kilometers out, part of the
ionosphere, and according to current views it is supposed to have
much less water than what the new measurements indicate. Many
geophysicists are still resistant to Frank's explanation, but
this new data certainly requires attention, and some geo-
physicists are saying it is time to take the idea of small comets
seriously and rethink current assumptions.
QY: Robert Conway, Naval Research Laboratory, (301) 870-0853.
(Science 22 August) (Science-Week 5 Sep 97)

-------------------

Related Background:

EVIDENCE FOR CONTINUAL BOMBARDMENT OF EARTH WITH WATER SOURCES
The water on the Earth has always been thought of as here from
the beginning, a result of the process of planetary cooling, the
seas produced by condensation of water vapor originating from
the Earth itself. Today came the startling news of evidence that
the Earth is being constantly showered with miniature comet ice
balls of 20 to 40 tons in mass at the astonishing rate of over
40,000 per day. The evidence is provided by the NASA Polar
orbiting spacecraft, the research team led by Louis A. Frank
(University of Iowa, US), who in fact first suggested this
bombardment 11 years ago to explain markings in photographs
received by NASA's Dynamics Explorer I. At that time, Frank was
heavily criticized by his colleagues and his idea called
outlandish. At this moment Frank has apparently been vindicated,
his report presented in a crowded hall yesterday at a meeting of
the American Geophysical Union convention. Calculations show
that the incoming water could account for the oceans on Earth,
and if the assessment continues to be supported, it opens new
possibilities for the understanding of early conditions on the
planet.
(New York Times 29 May 97) (Science-Week 29 May 97)


4. DIFFUSIVE CONTAMINATION OF CALCITE UNDER STANDARD CONDITIONS
Calcite is a common carbonate mineral used in analytical and
interpretive techniques in several Earth sciences. At the
resolution limits of traditional geochemical methods, the common
assumption is that dry calcite is static under the ambient
surface conditions of Earth, and that the bulk mineral behaves as
a closed system. Solid-state diffusion into calcite has been
demonstrated at elevated temperatures, but it has always been
assumed that diffusion in carbonate minerals is negligible under
standard conditions. Recently, however, there has been evidence
that cadmium and zinc ions move in calcite at room temperature.
... ... S.L.S. Stipp et al now present evidence that monovalent
ions (Na, K, Cl) originating from fluid inclusions accumulate in
*crystallites on the surface of calcite. The authors suggest
their results indicate that calcite under standard conditions
does not always behave as a closed system, which is a critical
assumption in the use of isotope ratios, trace-element
distribution, and fluid-inclusion composition for interpretations
of *paleo-climatology, *geochronology, or *petrogenesis. The
authors further suggest that the uptake capability of calcite for
contaminants in environmental systems is probably higher than
current models predict, because surface sites are constantly
renewed by ionic mobility. The authors also suggest their results
indicate that, in the context of fundamental mineral physics and
chemistry, the free-energy inhibitions to ionic movement within
a
divalent ionic solid can be overcome, probably by hydration, and
the challenge is to define the mechanism.
-----------
S.L.S. Stipp et al (5 authors at 2 installations, DK CH)
Spontaneous movement of ions through calcite at standard
temperature and pressure.
(Nature 26 Nov 98 396:356)
QY: S.L.S. Stipp 
-----------

Text Notes:
... ... *crystallites: A microscopic and often skeletal crystal
which represents the initial form of *crystalline material.
... ... *crystalline material: In general, crystalline materials
are *metamorphic or *igneous rocks formed by the process of
crystallization from solid or liquid precursors.
... ... *metamorphic: A metamorphic rock is an aggregate of
minerals formed by the recrystallization of pre-existing rocks in
response to a change of pressure, temperature, or volatile
content.
... ... *igneous rocks: Igneous rocks are rocks that have
congealed from a molten mass (magma) derived from the Earth's
crust.
... ... *paleoclimatology: The study of past climate from
geological traces.
... ... *geochronology: Determination of time intervals on a
geologic scale, through either absolute or relative dating
methods.
... ... *petrogenesis: The origin and evolutionary history of
rocks.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98


5. GENOME OF THE TYPHUS PARASITE AND THE ORIGIN OF MITOCHONDRIA
The Rickettsia are small bacteria (600 x 300 nanometers, or
spherical as "cocci"), classified as a type of proteobacteria
(i.e., "purple" bacteria, a huge phylum including many common
bacteria). The Rickettsia are obligate intracellular parasites
(i.e., they can replicate only inside living cells), and one of
the most notorious of these parasites is R. prowazekii, the agent
of epidemic louse-born typhus in humans. R. prowazekii is
estimated to have infected 20 to 30 million humans in the wake of
the First World War and killed another few million humans
following the Second World War. Because it is apparently the
descendent of free-living organisms, the genome of R. prowazekii
may provide insight into the adaptations producing an obligate
intracellular lifestyle. Also, phylogenetic analyses based on
sequences of *ribosomal RNA and *heat-shock proteins indicate
that *mitochondria may be derived from the proteobacteria, and
indeed the closest extant relatives of the ancestor to
mitochondria seem to be the Rickettsia. Finally, the genome of R.
prowazekii is a small one, making it amenable to genome
analysis.
... ... S.G.E. Andersson et al now report the complete genome
sequence (1,111,523 *base pairs) of R. prowazekii. The authors
report this genome contains 834 protein-coding genes, the
functional profiles of which show similarities to those of
mitochondrial genes: no genes required for anaerobic glycolysis
are found in either R. prowazekii or mitochondrial genomes, but a
complete set of genes encoding components of the *tricarboxylic
acid cycle and the *respiratory-chain complex is found in R.
prowazekii (and in mitochondria). In effect, *ATP production in
Rickettsia is the same as that in mitochondria. Many genes
involved in the biosynthesis and regulation of biosynthesis of
amino acids and *nucleosides in free-living bacteria are absent
from R. prowazekii and mitochondria, and such genes have
apparently been replaced by homologues in the host genome. The R.
prowazekii genome contains the highest proportion of non-coding
DNA (24 percent) detected so far in a microbial genome, and the
authors suggest such non-coding sequences may be *degraded
remnants of "neutralized" genes that await elimination from the
genome. The authors finally suggest that phylogenetic analyses
indicate that R. prowazekii is more closely related to
mitochondria than is any other microbe studied thus far.
-----------
S.G.E. Andersson et al (10 authors at 2 installations, SE US)
The genome sequence of Rickettsia prowazekii and the origin of
mitochondria.
(Nature 12 Nov 98 396:133)
QY: Charles G. Kurland 
-----------

Text Notes:
... ... *ribosomal RNA: A class of RNA molecules that have an
important role in the structure of ribosomes, the large molecular
entities that carry out protein synthesis in all cells.
... ... *heat-shock proteins: A group of specific proteins
apparently synthesized by both prokaryotes (cells without
membrane-bound organelles) and eukaryotes (cells with membrane-
bound organelles) in response to a sudden elevation of ambient
temperature.
... ... *mitochondria: Mitochondria are double-membrane enclosed
organelles of cells that are involved with several important
biochemical pathways, including electron transport and oxidative
metabolism. Various types of eukaryotic cells may contain from a
few to several thousand mitochondria in each cell type. The
mitochondria are relatively large cylindrical structures up to 10
microns long and up to 2 microns in diameter, and most biologists
believe mitochondria are cell organelles that may have originated
as separate organisms that became resident in eukaryotic cells.
Mitochondrial DNA is independent of nuclear DNA. It consists of a
circular molecule, 16,569 base pairs long in humans, with a
known
nucleotide sequence.
... ... *base pairs: The term "base pair" refers to the bases
(nucleotides) always found chemically bonded together in the DNA
double helix (adenine, for example, always bonds with thymine,
and guanine with cytosine).
... ... *tricarboxylic acid cycle: (Krebs cycle, citric acid
cycle) In aerobic respiration, the set of oxidative reactions
occurring after *glycolysis.
... ... *glycolysis: One of the 2 main energy-producing pathways
of the cell, this involves the anaerobic breakdown of glucose
with the generation of 2 molecules of adenosine triphosphate
(*ATP).
... ... *respiratory-chain complex: (electron transport chain)
Refers to a sequence of steps in the final stage of the aerobic
respiration biochemical pathway in which high energy electrons
are effectively passed through a series of membrane-bound carrier
molecules to support a proton gradient involved in energy
storage. The term "transport" here refers essentially to a
chemical flow diagram and not necessarily to an actual spatial
translocation of electrons.
... ... *ATP: (adenosine triphosphate) ATP is the most important
chemical energy source in all living cells, intimately involved
in various cell functions and cell metabolism, and an entity in
numerous cyclic chemical pathways involved in the synthesis of
components.
... ... *nucleosides: The base-sugar moieties of nucleotides.
... ... *degraded remnants: In this context, the idea is that the
Rickettsia are descendents of bacteria with substantially larger
genomes, and that both Rickettsia and mitochondria are the
products of several types of reductive evolution.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98


6. ON TRANSPORTERS VS. CHANNELS IN BIOLOGICAL MEMBRANES
The biological cell membrane, the interface between the cell and
its environment, is a complex biochemical entity one of whose
major involvements is the transport of specific substances in one
direction or another. A related major involvement of the cell
membrane is the maintenance of chemical gradients, particularly
electrochemical gradients, across the interface. These gradients
can be of high specificity (e.g., sodium vs. potassium ions) and
of great functional significance (e.g., in the production of
*action potentials in nerve and muscle cells). Across its
surface, the cell membrane is heterogeneous in both structure and
function, and during the past three decades much research has
focused on molecular analysis of this heterogeneity.
... ... Michael P. Kavanaugh presents a short review of current
models of membrane transport, with a particular focus on models
of *neurotransmitter transport. The author makes the following
points: 1) The current idea is that ions and neutral solutes can
cross biological membranes via "transporters" and "channels", and
the usual approach is to think about the difference between
transporters and channels in terms of gating mechanisms. 2) Ion
channels exhibit a wide range of selectivity properties and
permeation rates, but their gating at the most basic level can be
thought of in terms of a single barrier or gate acting as a
switch. When the gate is closed, ions cannot permeate; when the
gate is open, a permeation pathway for ions allows flux, often at
high rates (up to 10^(6) per second). 3) Transporters and "*ion
pumps", in contrast, mediate flux in a way that can be better
explained by the presence of two gates -- one gate on the
external surface of the membrane and one gate on the internal
surface of the membrane. In this general transport scheme, the
two gates are never open simultaneously, but instead they open
sequentially to allow the cytoplasmic and extracellular
compartments alternating access to the permeating pathway. Unlike
flux through an open ion channel, in transporters a gating cycle
occurs with each transport of solute, so transporters generally
mediate much slower rates of solute permeation (sometimes as slow
as 1 per second). Alternating access kinetic schemes can involve
co- or counter-transport of other ions in a transport cycle,
effectively coupling their electrochemical gradients. 4) There
are currently many questions about neurotransmitter structure and
function, but accumulating evidence suggests that transporters
and channels share some fundamental features. Apparently,
channels do not generally behave as simple singly-gated aqueous
pores, but instead they may have multiple ion-binding sites
corresponding to energy wells separated by barriers whose heights
can fluctuate. The dynamics of channel state transitions could
thus lead to a spectrum of kinetic behaviors. 5) At present, the
most compelling idea is that channel and transporter models
should not be regarded as mutually exclusive possibilities, but
rather as limiting cases of a more general mechanism.
-----------
Michael P. Kavanaugh (Oregon Health Sciences University, US)
Neurotransmitter transport: Models in flux.
(Proc. Natl. Acad. Sci. US 27 Oct 98 95:12737)
QY: Michael P. Kavanaugh, Oregon Health Sci. Univ. 503-494-2998.
-----------

Text Notes:
... ... *action potentials: In general, in this context, an
action potential is a transient change in the potential
difference across the cell membrane, the change produced by
sudden and reversed alterations in the permeability of the
membrane to ions. In most nerve cells, the absolute magnitude of
this transient change is about 80 or 90 millivolts, a pulse that
lasts about 1 millisecond before the membrane returns to its
resting state. (See also notes to report #7, this issue.) 
... ... *neurotransmitter: Neurotransmitters are chemical
substances released at the terminals of nerve axons in response
to the propagation of an impulse to the end of that axon. The
neurotransmitter substance diffuses into the synapse, the
junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *ion pumps: In general, any mechanism that establishes
ion concentration gradients across cell membranes. There are
known pumps for various ions, the pumps involving integral
membrane proteins that use a cellular energy source.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98

-------------------

Related Background:

NEUROBIOLOGY: GATING MODIFIER TOXINS AND ION CHANNELS
The venom produced by many animals has been a useful source of
ligands that interact with different types of *ion channels. In
some cases, these toxins bind to the outer vestibule of the pore
and physically block ion conduction, whereas in other cases, the
toxins modify channel gating. Toxins that act as modifiers of
channel gating have been described for *voltage-gated sodium ion
channels, voltage-gated potassium ion channels, and voltage-gated
calcium ion channels. The different effects of the pore-blocking
toxins and the gating modifier toxins arise because they interact
in specific ways with different regions of ion channels.
Hanatoxin and grammotoxin are two related protein toxins found in
the venom of the Chilean Rose Tarantula, Phrixotrichus spatulata.
Hanatoxin inhibits voltage-gated potassium ion channels and
grammotoxin inhibits voltage-gated calcium ion channels. Both
toxins inhibit their respective channels by interfering with the
normal operation of the voltage-dependent gating mechanism. The
sequence homology of hanatoxin and grammotoxin, as well as their
similar mechanisms of action, suggest the possibility that they
interact with the same region of voltage-gated calcium ion and
voltage-gated potassium ion channels. ... ... Li-Smerin and
Swartz (National Institutes of Health, US) present an
electrophysiological study of the action of hanatoxin and
grammotoxin on *genetically engineered ion channels in the
membranes of oocytes of Xenopus laevis (African clawed toad), and
the authors report that each toxin can interact with both
voltage-gated calcium ion and voltage-gated potassium ion
channels and modify channel gating. They further report that
*mutagenesis studies of voltage-gated potassium ion channels
suggests that hanatoxin and grammotoxin recognize the same
structural motif. The authors propose that these toxins recognize
a voltage-sensing domain or module present in voltage-gated ion
channels, and that this domain has a highly conserved 3-dimens-
ional structure. They conclude: "It is as if voltage-gated ion
channels follow a multi-domain architecture whereby the ion
selective pore domain has diverged to confer selectivity for
potassium ion, calcium ion, or sodium ion, and a separate
voltage-sensing domain, which is conserved in its structure, is
share by all of them."
QY: Kenton J. Swartz 
(Proc. Natl. Acad. Sci. US 21 Jul 98 95:8585)
(Science-Week 21 Aug 98)

-------------------

Related Background:

... ... *ion channels: Ion channels are protein channels in cell
membranes that allow ions to pass from extracellular solution to
intracellular solution and vice versa. Most ion channels are
selective, allowing only certain ions to pass, and an individual
cell has ion channels with various ion selectivities. The
selectivity of an ion channel can be "gated", the channel
effectively opened or closed, and ion channels are said to
voltage-gated or ligand-gated, depending on how the change in
selectivity is provoked.
... ... *voltage-gated: The "voltage" in the voltage-gating
referred to here is the potential difference across the cell
membrane, a potential difference that can be quantitatively step-
changed with experimental procedures. One then studies the force-
response characteristic (voltage-current curve) to determine ion
specificities and ion permeabilities. Under natural conditions,
changes in potential difference (and consequent voltage-gating)
are a result of the electrical behavior of regions of the
membrane contiguous to the channel.
... ... *genetically engineered: In this report, the oocytes were
genetically engineered to express specific ion channels in their
membranes. The experimental advantage of the oocytes used is that
genetic engineering methods are easily applied, and the cells are
large enough to facilitate electrical measurements. The toad
oocytes, then, are merely a vehicle for the production of the ion
channels, which in this case were derived genetically from
mammalian (rat) brain and muscle cells.
... ... *mutagenesis: In general, mutagenesis is any alteration
of the genome. In this report, genetic engineering methods were
used to produce specific protein residue changes in ion channels,
so that the electrical behavior of such specifically mutated ion
channels could be studied.

-------------------

Related Background:

STRUCTURAL ANALYSIS OF VOLTAGE-GATED CHLORIDE ION CHANNELS
In the context of studies of biological cell membranes, the term
"ion selectivity" refers to the ability of all cell membranes to
distinguish between various ions such as Na(+), K(+), Ca(++),
Cl(-), etc. There is much evidence that this ion selectivity
involves specific pores or channels in the cell membrane, with
certain channels specific for certain ions, the channels capable
of being opened or closed (gated) depending on conditions and
various interactions with ligands binding to receptors. These
receptors are in some cases part of the channel itself and in
other cases neighboring entities that control channel dynamics.
Since the movement of ions is by definition an electric current,
ion-selective channels can be viewed as involved in "conductance"
pathways. Fahlke et al (5 authors at Vanderbilt Univ., US) report
they have identified regions of a human skeletal muscle chloride
channel that contribute to formation of its anion-selective
conduction pathway. A core structural moiety evidently spans the
cell membrane and is conserved in evolution. The authors suggest
the conserved motif may be a critical sequence for anion
selectivity of ion selective membrane pores.
QY: Alfred L. George Jr. 
(Nature 4 Dec 97) (Science-Week 26 Dec 97)


7. DEVELOPMENTAL NEUROBIOLOGY: SEGREGATION OF SYNAPSES
The signaling units of the nervous system are the neurons (nerve
cells), and the connections between neurons are in most cases
plastic, changing during developmental processes and as a result
of experience. Throughout the developing nervous system,
competition between nerve cell *axons apparently causes the
permanent removal of some synaptic connections, changing the
number and strength of axonal connections with target cells. ...
... W-B. Gan and J.W. Lichtman report a study of synaptic
connections at *neuromuscular junctions in the mouse. The authors
report that in mouse neuromuscular junctions at birth, terminal
branches of different axons are intermingled. However, during the
several weeks after birth, these branches progressively
segregated into non-overlapping compartments before the complete
withdrawal of all but one axon. Segregation was caused by
selective branch atrophy, detachment, and withdrawal: the axon
branches that were nearest to the competitor's branches were
removed before the more distant branches were removed. The
authors propose this progression suggests that the signals that
mediate the competitive removal of synapses must decrease in
potency over short distances. The authors further suggest the
following: 1) Even within the confines of a neuromuscular
junction 10 to 50 microns long, some branches of a single axon
are maintained while others are lost. Thus, the elimination of
different branches of an axon is regulated independently at the
level of each individual branch. 2) Departing axonal branches
undergo a stereotyped process of atrophy, detachment, and
retraction within a neuromuscular junction. 3) The local
regulation of axon branch removal does not support the idea that
signals that maintain their strength over distance (such as
*action potentials) are mediators of synaptic competition.
-----------
W-B. Gan and J.W. Lichtman (Washington University St. Louis, US)
Synaptic segregation at the developing neuromuscular junction.
(Science 20 Nov 98 282:1508)
QY: Jeff W. Lichtman 
-----------

Text Notes:
... ... *axons: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. In some types of nerve cells,
axons are extensively branched into a multitude of fine fibers
that make contact (synapses) with other nerve cells. The general
input extensions of nerve cells are called "dendrites", and they
may also be extensively branched. In the context of this report,
the fine terminal branches of axons are making contact with
specialized junctions of muscle cells.
... ... *neuromuscular junctions: (myoneural junctions). The
neuromuscular junction is an anatomically specialized area of
contact between the axon terminal of a motor neuron and the
surface membrane of a muscle fiber (muscle cell). In general, a
"motor" neuron is a neuron that conveys nerve impulses (action
potentials) from the central nervous system to either muscles or
glands.
... ... *action potentials: (nerve impulse) A rapid propagating
wave (approximately 1 millisecond in duration) of depolarization
followed by repolarization. In the language of physics, the
neuron axon is an electrical transmission line with a transverse
time-variant negative conductance element in parallel with a high
capacitance. In fact, the equations describing the propagation of
neuron action potentials derive from the classical equations for
wave propagation along electrical transmission lines developed by
Maxwell and Kelvin. Action potentials also occur in certain types
of muscle cells, the impulses propagated along the length of the
muscle fiber.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Dec98

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CORRECTION NOTE:

In report #1 of the issue of December 11, 1998, "nemotode" should
be "nematode".

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