Follicle-oocyte-sperm interactions in vivo and in vitro in pigs.

In vitro culture has provided new information on the mechanisms involved in fertilization where two completely different cells fuse together. At the same time, results obtained in vitro have led to new questions. Does the follicle influence the final maturation process of the oocyte and does the oviduct regulate the normal behaviour of spermatozoa? Recent studies indicate a critical influence of both the follicular compartment and the oviduct on the normal fertilization process. Oocytes matured in vivo are more competent to induce pronuclear formation, but are still susceptible to polyspermy. Oocytes matured in vitro can develop fully after fertilization, but require the presence of follicular factors during culture to enhance their developmental competence. Fresh or frozen spermatozoa can penetrate oocytes in vitro, but their conditioning by differential centrifugation or oviductal cells influences the rate of polyspermy. The understanding of these influences is a prerequisite to enhancing in vitro production of pig embryos.