In vitro maturation and fertilization of preovulatory dog oocytes.

Mature beagle bitches were injected with exogenous gonadotrophins to induce superovulation. Preovulatory oocytes were collected at 72 h after intravenous injection of human chorionic gonadotrophin and cultured for 72 h in a modified Krebs-Ringer bicarbonate solution containing 10% fetal calf serum and 30 mg gentamicin sulfate l-1. Control oocytes were collected from ovaries of anoestrous bitches and cultured for 72 h. All oocytes were at the germinal vesicle stage when collected. At 72 h of culture, a significantly larger population (31.9%) of preovulatory oocytes reached metaphase II, compared with the control group (12.1%). Ejaculated beagle spermatozoa were washed and preincubated for 4 h, then capacitated spermatozoa were added to the oocytes. The kinetics of sperm penetration into the ooplasm did not differ between the two groups of oocytes, but the development of male and female pronuclei was observed more frequently in oocytes collected from preovulatory follicles. These results suggest that preovulatory maturation is required for the normal development of canine oocytes, although they are ovulated at the germinal vesicle stage.