The sea urchin's crystal secret
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Judy Siegel-Itzkovich, THE JERUSALEM POST Nov. 28, 2004
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The sea urchin has a lowly name, but its tough, brittle spines are an
engineering wonder.
Composed of a single crystal from base to needle-sharp tip, they grow
back within a few days after being broken off. Now a team of scientists
at the Weizmann Institute in Rehovot has an explanation for how they do
it.
While many crystals grow from component atoms or molecules that are
dissolved in liquid - sugar and salt being the most familiar examples -
professors Lia Addadi and Steve Weiner of the institute's structural
biology department found that the sea urchin uses another strategy: The
material that comprises the spines is first amassed in a non-crystalline
form, called amorphous calcium carbonate (ACC). Packets of ACC are
shoveled out of the cells surrounding the base of a broken spine and up
to the growing end. Within hours of arriving in place, the amorphous
material, which is composed of densely packed but disorganized
molecules, turns to calcite crystal in which the molecules line up
evenly in lattice formations.
In addition to increasing scientific understanding of these species, the
idea of growing single crystals by first creating the material in an
amorphous phase might prove useful to material scientists and engineers
wanting to produce and shape sophisticated synthetic materials that have
the properties of crystals.
Working with graduate students Yael Politi and Eugenia Klein, and Talmon
Arad of the chemical research support unit, they used four different
methods of investigation, including two kinds of electron microscopy, to
look for the ACC as it was being deposited and turning to crystal.
"The question," says Weiner "is why it is so difficult to observe a
process that seems to be so basic. Scientists have been studying it for
over a 100 years. In fact, because the ACC is a transient phase, we had
to develop new methods to catch it while it exists."
The captured images show microscopic needles that first grow straight
out from the stump of the spine, and then branch out to form a lacy
structure that is hard but light. The crystalline structure of the old
spine provides the template for the alignment of the molecules, and thus
controls the intricate yet precise growth pattern.
Though previous studies by the Weizmann group have shown that the same
strategy is used by immature sea urchins and mollusks in the larval
stage to build internal skeletons, this is the first time the process
was observed in adult marine animals. It is far from obvious that larvae
and adult creatures would use the same methods; their lifestyles are
very different, and this can translate into differences in biological
processes. For instance, the tiny sea urchin larva is transparent and
swims around, while the round, spiky adult lives on the sea floor.
Because it works for both, Addadi and Weiner believe this method is
probably a basic strategy used by not only close relatives of the sea
urchin such as sea stars, but by a wide variety of spiny and shelled sea
creatures like mollusks and corals.
FRANCO-ISRAELI RESEARCH
A cooperation agreement between the Hebrew University of Jerusalem and
the French National Scientific Research Organization (Le Centre National
de la Recherche Scientifique, or CNRS) has been signed via a video
conference linking Jerusalem and Paris. The agreement will establish a
joint lab for research in neurophysiology and neurophysics. The
agreement represents a further effort by the governments and scientific
communities of France and Israel to promote scientific cooperation.
CNRS is a large basic research organization operating throughout France.
Over 1,200 CNRS units are functioning in conjunction or association with
institutes of higher education and other French research bodies.
Sitting in the Mount Scopus campus, HU president Prof. Menachem Magidor
was a signatory, while in Paris, the agreement was signed by CNRS
director-general Dr. Bernard Larrouturou, president of the University of
Bordeaux, Prof. Bernard Begaud and University of Paris president Prof.
Jean-Francoise Dhainaut.
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