Sperm binding and fertilization envelope formation in a cell surface complex isolated from sea urchin eggs

An isolated surface complex consisting of the vitelline layer, plasma membrane, and attached secretory vesicles has been examined for its ability to bind sperm and to form the fertilization envelope. Isolated surface complexes (or intact eggs) fixed in glutaraldehyde and then washed in artificial sea water are capable of binding sperm in a species-specific manner. Sperm which bind to the isolated surface complex exhibit the acrosomal process only when they are associated with the exterior surface (vitelline layer) of the complex. Upon resuspension of the unfixed surface complex in artificial sea water, a limiting envelope is formed which, based on examination of thin sections and negatively stained surface preparations, is structurally similar to the fertilization envelope formed by the fertilized egg. These results suggest that the isolated egg surface complex retains the sperm receptor, as well as integrated functions for the secretion of components involved in assembly of the fertilization envelope.