Effect of Calcium on the Stability of the Vitelline Envelope of Surf Clam Oocytes.

Fertilization and parthenogenic activation of oocytes of the surf clam, Spisula solidissima, require the presence of calcium in the extracellular medium. Here we report that the depletion of calcium causes a dramatic increase in the stability of the vitelline envelopes (VE). On the basis of this effect, we have developed a method of isolating intact VE and have studied their morphology, composition, and properties. Experiments using 45Ca2+ have revealed that isolated VE bind calcium in a weak, but specific way. These findings suggest that the function of calcium may be to maintain the oocyte surface in a fertilization-competent state, while the reactions subsequent to the initial activation event, and leading to nuclear envelope breakdown (NEBD), may not require calcium. In support of this hypothesis, we have demonstrated that hypertonic conditions induce the oocytes to undergo NEBD in the absence of extracellular calcium.