The Secret of Seawater Spines

Researchers from the Weizmann Institute of Science have discovered how urchins get the calcium they need to build hard shells and spines, possibly altering the way scientists view the process of biomineralization.

Professors Lia Addadi and Steve Weiner of the Weizmann Institute of Science's Structural Biology Department have published a study in the Proceedings of the National Academy of Sciences that show how sea urchins construct their spines with packets of "unorganized" material that solidify into crystal when laid in place.

"So the question went back a step: How do they get the calcium ions they need to make this material in the first place?" said Addadi. "Free calcium is not abundant in sea water," adds Weiner, "so they need an efficient way to extract and concentrate the ions."

With the help of Dr. Andreas Schertel of Carl Zeiss Microscopy in Germany and Dr. Sefi Addadi of the Weizmann Institute of Science's Life Sciences Core Facilities, the researchers were able to observe the urchin's cells as they are in life, including their life underwater. Using new and advanced techniques to observe thin slices of the cells in sea urchin embryos and then to reconstruct three-dimensional images of these cells and their intake of labeled calcium ions, the researchers discovered that sea urchin larval cells actually "drink" seawater.

The cells they observed contained bubbles called vacuoles that worked to collect the calcium ions to create concentrated blocks of calcium to construct the spines. This theory is a complete contrast to the prevailing idea that the urchin's outer membranes cells take in only ions, one at a time, through special channels. With this discovery came a new question: how did the cells absorb the calcium but expel other ions in the sea water, specifically the sodium and chloride?

"Researchers may be busy for years to come figuring out how these cells manipulate the ions in the seawater they drink," said Weiner.