The fertilizability of human oocytes at different stages of meiotic maturation.

Human oocytes were collected from small follicles during natural menstrual cycles. The oocytes were cultured from six to 36 hours and were inseminated for four hours before termination of the maturation culture. Maturation and fertilizability was assessed from serial sections of individual oocytes. Germinal vesicle breakdown commenced after six hours. Metaphase I was attained by 40% of oocytes by 12 hours and by 60% at 18 hours. Metaphase II was observed in over 40% of oocytes after 24 hours of culture. Both the zona pellucida and vitellus were penetrable to sperm at all stages of meiosis. However, the penetrability of the vitellus increased significantly with advancing stages of meiotic maturity. The chromatin of sperm within germinal vesicle oocytes was not undergoing decondensation. Sperm chromatin was partially decondensed in metaphase I and completely swollen in metaphase II oocytes. Immature oocytes penetrated by sperm had retained their cortical granules whereas these granules were absent from fertilized, mature oocytes.