Regulative specification of ectoderm in skeleton disrupted sea urchin embryos treated with monoclonal antibody to PI-nectin

PI-nectin is a glycoprotein first discovered in the extracellular matrix (E CM) of Paracentrotus lividus sea urchin embryo, apically located on ectoder m and endoderm cells, The molecule has been described as functioning as an adhesive substrate for embryonic cells and its contact to ectoderm cells is essential for correct skeletogenesis. The present study was undertaken to elucidate the biochemical characteristics of PI-nectin and to extend knowle dge on its in vivo biological function. Here it is shown that the binding o f mesenchyme blastula cells to PI-nectin-coated substrates was calcium depe ndent, and reached its optimum at 10mM Ca2+. Perturbation studies using mon oclonal antibody (McAb) to PI-nectin, which prevent ectoderm cell-PI-nectin contact, show that dorsoventral axis formation and ectoderm differentiatio n were retarded. At later stages, embryos recovered and, even if growth and patterning of the skeleton was greatly affected, the establishment of dors oventral asymmetry was reached. Similarly, the expression of specific ectod erm and endoderm territorial markers was achieved, although occurring with some delay. Endoderm differentiation and patterning was not obviously affec ted. These results suggest that both endoderm and ectoderm cells have regul ative capacities and differentiation of territories is restored after a lag period. On the contrary, failure of inductive differentiation of the skele ton cannot be rescued, even though the ectoderm has recovered.