While many crystals grow from component atoms or molecules that are dissolved in liquid, sugar and salt being the most familiar examples, the team of Profs. Lia Addadi and Steve Weiner, of the Institute's Structural Biology Department, found that the sea urchin uses another strategy. The material of the spines is first amassed in a non-crystalline form, termed "amorphous calcium carbonate" (ACC). Packets of ACC are shoveled out of the cells surrounding the base of the 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.
Working with graduate student Yael Politi and Eugenia Klein and Talmon Arad of the Chemical Research Support Unit, Professors Addadi and Weiner 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 should be so difficult to observe a process that seems to be so basic. Scientists have been studying it for over a hundred 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 grow first straight out from the stump of the old 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 in the crystal, and thus controls the intricate, yet precise, growth pattern.
Though previous studies by
Contact: Jeffrey J. Sussman
American Committee for the Weizmann Institute of Science