Cellular control over spicule formation in sea urchin embryos: A structural approach

The spicules of the sea urchin embryo form in intracellular membrane-deline ated compartments. Each spicule is composed of a single crystal of calcite and amorphous calcium carbonate. The latter transforms with time into calci te by overgrowth of the preexisting crystal. Relationships between the memb rane surrounding the spiculogenic compartment and the spicule mineral phase were studied in the transmission electron microscope (TEM) using freeze-fr acture. In all the replicas observed the spicules were tightly surrounded b y the membrane. Furthermore, a variety of structures that are related to th e material exchange process across the membrane were observed. The spiculog enic cells were separated from other cell types of the embryo, frozen, and freeze-dried on the TEM grids. The contents of electron-dense granules in t he spiculogenic cells were shown by electron diffraction to be composed of amorphous calcium carbonate. These observations are consistent with the not ion that the amorphous calcium carbonate-containing granules contain the pr ecursor mineral phase for spicule formation and that the membrane surroundi ng the forming spicule is involved both in transport of material and in con trolling spicule mineralization. (C) 1999 Academic Press.