ECTODERM CELL-ECM INTERACTION IS ESSENTIAL FOR SEA-URCHIN EMBRYO SKELETOGENESIS

Paracentrotus lividus sea urchin nectin (Pl-nectin) is an extracellula r matrix (ECM) protein of the sea urchin embryo on the apical surface of the ectoderm and has been shown to be an adhesive substrate for emb ryonic cells. A monoclonal antibody (McAb) to Pl-nectin was generated that inhibits the adhesion of blastula cells to Pl-nectin-coated subst rates in an in vitro functional assay. To examine for possible in vivo functions of Pl-nectin, Fab fragments (Fabs) of Pl-nectin McAb were a dded to early blastulae. Ingression of primary mesenchyme cells was no t affected by Fabs. As control embryos reached the pluteus stage, trea ted embryos showed a severe inhibition of skeletal elongation and patt erning. When the Fabs were injected directly into the blastocoel, even at higher concentration than was applied externally, skeletogenesis w as normal. Therefore, the effect of the antibody on spiculogenesis was indirect. The treatment was partially reversible as embryos eventuall y seemed to recover and elongate spicules, although with an incorrect patterning. Migration of pigment cells was also affected by the Fabs, since they did not disperse throughout the ectoderm but remained clust ered in ectopic areas. Ln contrast, the development of endoderm struct ures Tvas not affected. Our results indicate that II the sea urchin em bryo the appropriate contact of ectodermal cells with outer ECM compon ents is essential for the correct morphogenesis of inner mesodermal st ructures. (C) 1998 Academic Press.