Studies on the role of exogenous calcium in fertilization, activation and development of the sea urchin egg.

Both isotopic and microelectrode studies reveal a significant Ca2+ influx at fertilization. The role, if any, of this influx is disputed. An attempt to reevaluate contradictory findings by others on this role was made and the results with S. purpuratus and L. pictus eggs, using acrosome reacted sperm and EGTA- buffered media (free [Ca2+] less than 10(-11M), indicate that exogenous Ca2+, and hence the Ca2+ influx, is not required for fertilization, activation and subsequent cleavage. Blastomere adhesion, however, is affected. The contradictory findings by others may have resulted from reduced fertilizability in Ca2+-free seawater, which can be circumvented by higher sperm concentration and by a sensitivity to temperature in Ca2+-free medium, which can be bypassed by carrying out fertilization at lower temperatures. Under the assay conditions, there is also no effect of Ca2+-free media on the kinetics of the cortical reaction or polyspermy. The data support the hypothesis that sperm induce release of Ca2+ from intracellular stores, perhaps by affecting an equilibrium between Ca2+ sequestration and Ca2+ release.