Mechanism of soybean trypsin inhibitor induced polyspermy as determined by an analysis of refertilized sea urchin (Arbacia punctulata) eggs

Soybean trypsin inhibitor (SBTI) retards the separation of the vitelline layer from the surface of Arbacia eggs and the formation of the fertilization membrane. Numerous loci of vitelline layer (cortical projections) remain associated with the plasma membrane of SBTI-treated zygotes immediately after insemination. Gradually the attached portions of vitelline layer decrease in number and by approximately 15 min postinsemination the cortical projections are sparse or nonexistent. Monospermic SBTI-treated zygotes show a corresponding decline in both the incidence and degree of refertilization (polyspermy) upon reinsemination during this period. Ultrastructural analysis demonstrates that refertilizing spermatozoa are able to fuse with and penetrate SBTI-treated zygotes only at the apical surface of the cortical projections, i.e., where the vitelline layer remains attached to the plasma membrane of the zygote. These data indicate that the vitelline layer lining the apical surface of the cortical projections retains structural and functional properties characteristic of the surface of the unfertilized ovum.