Micromere Differentiation in the Sea Urchin Embryo: Immunochemical Characterization of Primary Mesenchyme Cell‐Specific Antigen and Its Biological Roles: sea urchin/primary mesenchyme cell/monoclonal antibody/spicule formation/cell migration

Primary mesenchyme cell (PMC)‐specific antigens in developing sea urchin embryos of five different species have been studied by using two different monoclonal antibodies, P4 and B2C2. Like B2C2 in Strongylocentrotus purpuratus (Anstrom et al., 1987) P4 reacted with the N‐linked carbohydrate in Strongylocentrotus intermedius embryo. Although both antibodies recognize the same group of glycoproteins in S. intermedius, P4 epitopes appeared earlier than B2C2 epitopes in Clypeaster japonicus embryo. PMCs of Anthocidaris crassispina blastulae raised in sulfate‐deficient sea water were immuno‐reactive with P4 but not with B2C2, although the embryos raised in normal sea water reacted with both antibodies at similar intensity. These results suggest that the epitopes of P4 and B2C2 are formed by glycosylation and sulfation, respectively. PMCs may display differential modification in their surface glycoprotein synthesis during differentiation. Furthermore, P4 inhibited cultured micromere descendant cells of Hemicentrotus pulcherrimus from attaching to the plastic dishes and forming spicules in vitro without detectable cytotoxic effect. P4‐reactive glycoproteins may play important roles in cell‐substrate interaction and spicule formation.