Differential distribution of spicule matrix proteins in the sea urchin embryo skeleton

Spicule matrix proteins are the products of primary mesenchyme cells, and a re present in calcite spicules of the sea urchin embryo. To study their pos sible roles in skeletal morphogenesis, monoclonal antibodies against SM50, SM30 and another spicule matrix protein (29 kDa) were obtained. The distrib ution of these proteins in the embryo skeleton was observed by immunofluore scent staining. In addition, their distribution inside the spicules was exa mined by a 'spicule blot' procedure, direct immunoblotting of proteins embe dded in crystallized spicules. Our observations showed that SM50 and 29 kDa proteins were enriched both outside and inside the triradiate spicules of the gastrulae, and also existed in the corresponding portions of growing sp icules in later embryos and micromere cultures. The straight extensions of the triradiate spicules and thickened portions of body rods in pluteus spic ules were also rich in these proteins. The SM30 protein was only faintly de tected along the surface of spicules. By examination using the spicule blot procedure, however, SM30 was clearly detectable inside the body rods and p ostoral rods. These results indicate that SM50 and 29 kDa proteins are conc entrated in radially growing portions of the spicules (normal to the c-axis of calcite), while SM30 protein is in the longitudinally growing portions (parallel to the c-axis). Such differential distribution suggests the invol vement of these proteins in calcite growth during the formation of three-di mensionally branched spicules.