The micromeres that arise at the fourth cell division in developing sea urchin embryos give rise to primary mesenchyme, which in turn differentiates and produces calcareous endoskeletal spicules. These spicules have been isolated and purified from pluteus larvae by washing in combinations of ionic and nonionic detergents followed by brief exposure to sodium hypochlorite. The spicules may be demineralized and the integral matrix dissolves. The matrix is composed of a limited number of glycoproteins rich in aspx, glux, gly, ser, and ala, a composition not unlike that found in matrix proteins of biomineralized tissues of molluscs, sponges, and arthropods. There is no evidence for collagen as a component of the matrix. The matrix contains N-linked glycoproteins of the complex type. The matrix arises primarily from proteins synthesized from late gastrulation onward, during the time that spicule deposition occurs. The mixture of proteins binds calcium and is an effective immunogen. Electrophoresis of the glycoproteins on SDS-containing acrylamide gels, followed by blotting and immunocytochemical detection, reveals major components of approximately 47, 50, 57, and 64 kD, and several minor components. These same components may be detected with silver staining or fluorography of amino acid-labeled proteins. In addition to providing convenient molecular marker for the study of the development of the micromere lineage, the spicule matrix glycoproteins provide an interesting system for investigations in biomineralization.

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