Skeletogenesis, a key morphogenetic event in the embryonic development of vertebrates, is also a very important, albeit a transient, milestone in the development of a marine invertebrate, the sea urchin. In recent years this biological system, in which the biomineralization process can be investigated in a well-defined, single cell type (the primary mesenchyme cell), has received considerable study. The objective of this review is to summarize our current knowledge about skeletogenesis in the sea urchin embryo and highlight the outstanding questions that remain to be answered. It will become apparent that there are two broad sets of questions: one set pertains to cell differentiation and morphogenesis; the other is concerned with basic cellular processes that are common to most cell types in a developing organism, such as cell-cell interactions, membrane fusion, ion transport and exocytosis. The former set of questions has been reviewed on numerous occasions, most recently by Morrill...

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