Human sperm‐egg interaction in vitro

The early events of gamete membrane fusion and sperm incorporation are portrayed, which are possibly the earliest pictures of human conception on record. Oocytes recovered at laparoscopy from an in vitro fertilization programme using clomiphene and human menopausal and chorionic gonadotrophin stimulation were zona-punctured and examined for polyspermic penetration 3–6 hours after insemination. Three eggs were penetrated, and one donor became pregnant in the same cycle. Fusion occurred between the oolemma and the midsegment of the sperm cell membrane extending from the equatorial vestige of the acrosome to the anterior region of the postacrosomal segment. Only acrosome-reacted sperm were capable of fusing with the egg. Fusion was followed by incorporation of the spermhead into the ooplasm by a process akin to phagocytosis. Sperm chromatin decondensation occurred by progressive inflation and disorganization of the original nuclear envelope. The mechanism of gamete fusion and sperm incorporation resembles that observed during human monospermic fertilization and generally conforms to that reported for eutherian mammals.