Induction of the acrosome reaction on the surface of de-jellied sea urchin eggs

The vitelline coat of sea urchin eggs was disrupted by DTT and trypsin after removal of the jelly layer. Thereafter the percentage of acrosome reaction was determined and the fertilization rate was estimated, employing the treated eggs. Electron microscopical investigation of these eggs showed that the vitelline coat was disrupted but no morphological difference was observed between eggs treated with DTT and those treated with trypsin. However, the fertilizability of the eggs was markedly decreased by the treatment with trypsin. In contrast, DTT treatment did not affect the fertilizability of the eggs, indicating that some surface substance(s) necessary for fertilization which were not eliminated by DTT were digested by trypsin. At the same time, the percentage of acrosome reaction of supernumerary spermatozoa in the presence of variously treated eggs was estimated as an index of the acrosome reaction-inducing activity of the egg surface. The acrosome reaction of spermatozoa actually occurred at the surface of de-jellied and DTT-treated eggs. However, the eggs treated with trypsin lost the capacity to induce the acrosome reaction. The surface substance which induces the acrosome reaction and renders the eggs fertile was removed by trypsin and found in the supernatant fraction. The necessity of an acrosome reaction for fertilization was demonstrated by the fact that the low fertilizability of trypsin-treated eggs was brought back to the control level by insemination with spermatozoa previously treated with egg water to evoke the reaction of the acrosomes.