An investigation of the latency period between sperm oolemmal adhesion andoocyte penetration

In clinical studies of the ability of capacitated human sperm to penetrate zona-free hamster eggs, we have previously observed that the ratio of oolem mal adherent to penetrating sperm varied between men. Sperm incorporation d id not occur immediately following gamete adhesion and not all adherent spe rm penetrated the egg. To further investigate this phenomenon, comparisons were made of the kinetics of gamete adhesion, membrane fusion, and sperm in corporation of capacitated mouse and human spermatozoa by zona free hamster eggs and of mouse sperm by zona-free mouse and hamster eggs. Eggs were ins eminated with either capacitated human or mouse sperm or combinations of bo th, washed out of sperm suspension after initial gamete adherence, and furt her incubated in sperm-free medium. Gamete membrane fusion was judged by dy e transfer of Hoechst 33342 and sperm entry of the cortical ooplasm by obse rvation of expanded sperm heads within acridine orange stained eggs. Oolemm al adherent mouse and human sperm fused with and penetrated zona-free hamst er eggs at different times whether eggs were inseminated in parallel or wit h combinations of sperm of both species. Oolemmal adherent mouse sperm pene trated zona-free hamster eggs prior to their penetration of zona-free mouse eggs. Ultrastructural studies of zona-free human eggs inseminated with hum an sperm confirmed prior observations with hamster eggs that only acrosome- reacted human sperm adhere to the oolemma. These results have lead us to po stulate that sperm entry into the egg may occur through a "zipper" mechanis m involving the ligation of local gamete receptors similar to the incorpora tion of target particles by phagocytes and suggest that not all oolemmal ad herent human sperm are capable of being incorporated although they have und ergone an acrosome reaction. Mel. Reprod. Dev. 52.319-327, 1999. (C) 1999 W iley-Liss, Inc.