Echistatin, a disintegrin, inhibits sperm-oolemmal adhesion but not oocyte penetration

Objective: To study the effects of echistatin, a disintegrin known to block the binding of fibronectin (FN) and vitronectin to their respective integrin receptors, α5β1 and αvβ3, on the adhesion of human spermatozoa to the oolemma of zona-free hamster eggs and their subsequent penetration. Design: Motile capacitated human spermatozoa and zona-free hamster eggs were coincubated in the presence of echistatin or in its absence and observed at short serial intervals. Whole mounts of these eggs, washed out of sperm suspension and stained with acridine orange, were scored for numbers of oolemmal adherent and penetrating sperm. Setting: University Hospital laboratories. Patients: Known fertile semen donors. Main Outcome Measures: Numbers of spermatozoa adherent to the oolemma and those penetrating the oocyte. Result: Sperm adherence to the oolemma was reduced significantly at micromolar concentrations of echistatin, in a concentration-dependent manner. In contrast, echistatin did not inhibit the penetration of oocytes by sperm that had become adherent to the oolemma despite the presence of echistatin. Conclusions: We propose that two processes occur in the binding of sperm to the oolemma, one that is echistatin sensitive and possibly involving the integrin receptors that recognize FN and vitronectin, and a second process, resistant to echistatin, that leads to gamete membrane fusion.