Changes In The Nature Of The Cell Adhesions Of The Sea-Urchin Embryo

Reaggregation of cells from 16-cell, 100-cell, 200-cell, hatched-blastula, and gastrula stage sea urchin embryos is essentially equivalent in the absence of experimental treatments. Gentle shearing of the forming aggregates revealed that the stability of the adhesions to shearing gradually increases as the embryos develop from the 100-cell to the hatched-blastula stage. During the same developmental period, the cell adhesions become progressively more sensitive to a mixed exoglycosidase, but their sensitivity to Pronase remains constant. Both changes we detected occur at the time other investigators have observed cell junctions appearing and cellular apposition increasing. All of these changes temporally correlate with the transition from loosely associated cleavage blastomeres into the organized epithelium of the hatched blastula.