Changes in morphology, sperm penetration and fertilization of ovulated hamster eggs induced by oviductal exposure

In the human, mature eggs at the pre-ovulatory follicular stage placed into the oviduct via gamete intra-Fallopian transfer (GIFT) establish more implantations and pregnancies than do those for standard in-vitro fertilization and embryo transfer (IVF). Previous studies in the hamster have shown that mature follicular eggs are less readily penetrated by spermatozoa than oviductal eggs. This study examines whether ovulation or pre-fertilization exposure to the oviduct per se affects sperm penetration and fertilization of mature ova. Three types of eggs were used: pre-ovulatory, follicular [12 h post-human chorionic gonadotrophin (HCG), 1-1.5 h pre-ovulation], and ovulated (bursal and oviductal, both 15 +/- 0.5 h post-HCG). Bursal eggs were obtained by ligating the infundibulum on one or both sides of the tract. The morphological changes in eggs due to oviductal exposure were quantified using computerized image analysis. Cumulus-free follicular and bursal eggs were significantly less penetrated than oviductal eggs 1 h post-insemination (36, 39 and 62%, respectively). Cumulus-intact oviductal compared to bursal eggs, paired within females, were fertilized at a significantly higher rate (4 h post-insemination; 89 and 58%, respectively). Fresh oviductal and bursal eggs had equivalent cell diameters (79 microns) and zona thickness (15-15.8 microns), but oviductal compared with bursal eggs had larger zonae (119 and 116 microns, respectively) and perivitelline volumes (107 and 47 pl). Oviductal, but not bursal, zonae had the oviductal glycoprotein, oviductin, bound to them. We conclude that prefertilization oviductal exposure and not ovulation or time post-HCG alters the morphology and fertilizability of eggs.