How the sea urchin grows new spines

The sea urchin’s 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 of Science has shown 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, 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 the Weizmann group have shown the same strategy is used by immature sea urchins and mollusks in the larval stage to build internal skeletons, this is the first time that the process was observed in adult marine animals. It is far from obvious that larva and adult would use the same methods – their lifestyles are very different. For instance, the tiny sea urchin larva is transparent and swims around, while the round, spiky adult lives on the sea floor. This can translate into differences in biological processes, as well.

Because it works for both, Addadi and Weiner believe this method is probably a basic strategy used not only by 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. In addition, the idea of growing single crystals by first creating the material in an amorphous phase might prove useful to material scientists and engineers who want to produce and shape sophisticated synthetic materials that have the properties of single crystals.

Prof. Addadi’s research is supported by the J & R Center for Scientific Research, the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research, the Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly, the Philip M. Kltuznick Fund for Research, the Minerva Stiftung Gesellschaft fuer die Forschung m.b.H., the Women’s Health Research Center and the Ziegler Family Trust, Encino, CA. She holds the Dorothy and Patrick Gorman Professional Chair.