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Imagine a construction site submerged under water: Building blocks are floated to a facility where the scaffolding is being put together. That is how the skeleton and other mineral structures are created in the embryos of numerous animals. In sea creatures the building blocks are of minerals derived from sea water; in developing humans and other mammals, the minerals are supplied by the mother’s blood via the food she eats.


Live sea urchin embryo grown in sea water labeled with a green fluorescent dye; dye-labeled calcium carbonate granules are observed all over the embryo. (A) Fluorescence and bright-light images superimposed (B) Fluorescence image alone

Starting with fertilization, a Weizmann Institute-led team has traced the initial steps in the construction of the skeleton in live embryos – those of sea urchins. As reported recently in the Proceedings of the National Academy of Sciences, USA, they have gained surprising new insights into this elaborate process. 

A scanning electron microscope image showing a vesicle containing calcium carbonate nanospheres, 20 to 30 nanometers across, in a flash-frozen sea urchin embryo


To accumulate sufficient calcium and other minerals, each sea urchin embryo, which is about the size of a cross section of a human hair, needs to use all the calcium contained in hundreds of times its own volume of seawater. For decades, scientists had thought that the take-up of calcium and carbonate from the water and the actual building of the skeleton were performed exclusively by specialized embryonic cells. But in the new study, when the researchers observed the growth of the sea urchin embryo in sea water containing calcium ions labeled with green fluorescent dye, they were amazed to discover that the entire embryo was soon lit up in green: Evidently, many of its cells had absorbed the calcium ions.

To make sure the finding had not been produced accidentally, the scientists confirmed the wide distribution of the mineral using several advanced technologies: In addition to observing live embryos with a light laser microscope, they investigated frozen embryonic samples with a scanning electron microscope. Moreover, they used an innovative Israeli system – a multi-modal imaging station featuring fluorescence microscopy, elemental mapping and a scanning electron microscope that allows the examination of samples in open air rather than in a vacuum. The study was performed by Prof. Lia Addadi, Prof. Stephen Weiner and graduate student Netta Vidavsky, all of the Weizmann Institute’s Structural Biology Department, together with Weizmann Institute graduate Dr. Sefi Addadi of B-nano Ltd., Dr. Julia Mahamid, a Weizmann graduate currently in Germany, and Dr. Eyal Shimoni of Weizmann’s Chemical Research Support Department, as well as David Ben-Ezra and Prof. Muki Shpigel of the Israel Oceanographic and Limnological Research Institute in Eilat.    
(l-r) Dr. Eyal Shimoni, Dr. Sefi Addadi, Prof. Lia Addadi, Netta Vidavsky and Prof. Stephen Weiner
The study also revealed that when calcium carbonate gets into the embryo’s cells, it forms solid granules composed entirely of nanospheres. This texture is characteristic of many amorphous minerals and is known to be an intermediate stage in the formation of the skeletal tissue of the sea urchin, as revealed in studies by Weiner and Addadi more than a decade ago. The granules, in turn, were found to be stored in vesicles, spherical container-like structures.
A scanning electron microscope image of a flash-frozen sea urchin embryo, showing a large number of intra-cellular vesicles


These findings point to new avenues for studying skeleton formation in a wide range of living organisms, including humans. The fact that the entire embryo is mobilized for construction of the skeleton suggests that in humans, contrary to accepted belief, in addition to the specialized osteoblasts, other cells might be involved in the construction of bones and teeth. Moreover, the temporary storage of calcium carbonate in vesicles might be applicable to organisms other than sea urchins.

Elucidating these mechanisms is of enormous importance for understanding biological mineralization – an understanding that in turn could be crucial for future investigations into various diseases and structural abnormalities affecting bones and teeth.
Prof. Lia Addadi's research is supported by the Gerhardt M.J. Schmidt Minerva Center on Supramolecular Architecture; the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; and the Carolito Stiftung. Prof. Lia Addadi is the incumbent of the Dorothy and Patrick Gorman Professorial Chair.
Prof. Stephen Weiner's research is supported by the Helen and Martin Kimmel Center for Archaeological Science, which he heads; the J & R Center for Scientific Research; the Maurice and Vivienne Wohl Charitable Foundation; the Exilarch's Foundation; the estate of Hilda Jacoby-Schaerf; the estate of George and Beatrice F. Schwartzman; and the European Research Council. Prof. Weiner is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.