title: Initial stages of calcium uptake and mineral deposition in sea urchin embryos.
abstract: Sea urchin larvae have an endoskeleton consisting of two calcitic spicules. We reconstructed various stages of the formation pathway of calcium carbonate from calcium ions in sea water to mineral deposition and integration into the forming spicules. Monitoring calcium uptake with the fluorescent dye calcein shows that calcium ions first penetrate the embryo and later are deposited intracellularly. Surprisingly, calcium carbonate deposits are distributed widely all over the embryo, including in the primary mesenchyme cells and in the surface epithelial cells. Using cryo-SEM, we show that the intracellular calcium carbonate deposits are contained in vesicles of diameter 0.5-1.5 μm. Using the newly developed airSEM, which allows direct correlation between fluorescence and energy dispersive spectroscopy, we confirmed the presence of solid calcium carbonate in the vesicles. This mineral phase appears as aggregates of 20-30-nm nanospheres, consistent with amorphous calcium carbonate. The aggregates finally are introduced into the spicule compartment, where they integrate into the growing spicule.
authors.affiliation: Department of Structural Biology, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Netta
authors.initials: N
authors.lastname: Vidavsky
authors.forename: Sefi
authors.initials: S
authors.lastname: Addadi
authors.forename: Julia
authors.initials: J
authors.lastname: Mahamid
authors.forename: Eyal
authors.initials: E
authors.lastname: Shimoni
authors.forename: David
authors.initials: D
authors.lastname: Ben-Ezra
authors.forename: Muki
authors.initials: M
authors.lastname: Shpigel
authors.forename: Steve
authors.initials: S
authors.lastname: Weiner
authors.forename: Lia
authors.initials: L
authors.lastname: Addadi
mesh_terms: D000818:Animals; D002118:Calcium / Q000737:chemistry*; D002119:Calcium Carbonate / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D022162:Cytoplasmic Vesicles / Q000737:chemistry; D005452:Fluoresceins / Q000737:chemistry; D005456:Fluorescent Dyes / Q000737:chemistry; D007477:Ions; D007814:Larva / Q000254:growth & development / Q000378:metabolism; D018613:Microscopy, Confocal; D008855:Microscopy, Electron, Scanning; D012617:Sea Urchins / Q000254:growth & development / Q000378:metabolism*; D012623:Seawater; D013050:Spectrometry, Fluorescence
chemical_list: D005452:Fluoresceins; D005456:Fluorescent Dyes; D007477:Ions; D002119:Calcium Carbonate; D002118:Calcium; C007740:fluorexon
keywords: biomineralization; intracellular mineral deposition; mineralization pathway; sea urchin embryonic spicule; transient precursor mineral phase