Hamster oocyte penetrability during preovulatory maturation

In vitro fertilization techniques were used to analyze the penetrability of preovulatory hamster oocytes. The zonas of granulosa cell-free primary (GV) oocytes were penetrated in vitro in 2-3 h as readily as those of ovulated secondary oocytes (80% vs. 88%), whether inseminated separately or as mixed oocyte groups. In fact, a significantly higher (P less than 0.05) mean number of perivitelline spermatozoa was present in immature (3.6) compared with secondary (1.9) oocytes, primarily reflecting a lack of the zona block to polyspermy in the immature population. By contrast, when granulosa cells remained around GV oocytes, zona penetration was low and more were penetrated, with more spermatozoa incorporated into the vitellus as a function of increasing time of oocyte recovery after hCG. We conclude, contrary to previous reports, that the zona pellucida of the hamster GV oocyte is readily penetrable by spermatozoa in vitro. However, the resumption of meiosis brings an increase in the penetrability of the granulosa cell vestment as well as the capacity for cortical granule exocytosis and the ability to decondense and transform the fertilizing sperm nucleus. The fact that the zona pellucida of the immature oocyte has proved to be penetrable in vitro and/or in vivo in all the mammals studied in this respect is discussed with particular reference to the situation in man.