Structural Characterization of the Transient Amorphous Calcium Carbonate Precursor Phase in Sea Urchin Embryos


  • We thank Prof. Matthias Epple and Dr. Alexander Becker for their valuable assistance; Wolfgang Caliebe from the NSLS beam line X19 at BNL for providing technical support; Prof. Aldo Shemesh and Dr. Ruth Yam for the mass spectrometer determinations; and Prof. Muki Spiegel, Sharon Marziano, and Hamutal Krugoliac from the Israel Oceanographic and Limnological Research for providing fertile sea urchins. We thank the Kimmelman Center for Biomolecular Structure and Assembly, Weizmann Institute, for financial support. F. W. is grateful to support of the National Institutes of Health and the National Science Foundation (USA). L. A. is the incumbent of the Dorothy and Patrick Gorman professorial chair of Biological Ultrastructure, and S. W. is the incumbent of the Dr. Walter and Dr. Trude Burchardt professorial chair of Structural Biology. I. S. is the incumbent of the Pontecorvo professorial chair of cancer research. Supporting Information is available online from Wiley InterScience or from the authors.


Sea urchin embryos form their calcitic spicular skeletons via a transient precursor phase composed of amorphous calcium carbonate (ACC). Transition of ACC to calcite in whole larvae and isolated spicules during development has been monitored using X-ray absorption spectroscopy (XAS). Remarkably, the changing nature of the mineral phase can clearly be monitored in the whole embryo samples. More detailed analyses of isolated spicules at different stages of development using both XAS and infrared spectroscopy demonstrate that the short-range order of the transient ACC phase resembles calcite, even though infrared spectra show that the spicules are mostly composed of an amorphous mineral phase. The coordination sphere is at first distorted but soon adopts the octahedral symmetry typical of calcite. Long-range lattice rearrangement follows to form the calcite single crystal of the mature spicule. These studies demonstrate the feasibility of real-time monitoring of mineralized-tissue development using XAS, including the structural characterization of transient amorphous phases at the atomic level.