SPONTANEOUS AND SPERM-INDUCED ACTIVATION OF OOCYTES IN LT SV STRAIN MICE/

The oocytes of LT/Sv strain mice are unique in that a high proportion of them (similar to 40% in this study) are ovulated before reaching me taphase of the second meiotic division (metaphase II). The remaining o ocytes of LT/Sv mice are ovulated at metaphase II, as in other strains of mice. When recently ovulated oocytes were cultured in vitro for 11 -12 h, those ovulated at metaphase il remained at this stage, whereas those ovulated at metaphase of the first meiotic division (metaphase I ) commonly resumed meiosis during in vitro aging. These oocytes extrud e the polar body and form a diploid pronucleus. This oocyte activation is not coupled with cortical granule exocytosis. The oocytes ovulated at metaphase II are fully capable of normal fertilization, whereas th ose ovulated at metaphase I are not. Approximately 50% of metaphase I oocytes penetrated by spermatozoa remain at this stage, and sperm nucl ei frequently undergo premature chromosome condensation. Only 13% of s perm-penetrated metaphase I oocytes formed a diploid female pronucleus and a haploid male pronucleus by 4 h after insemination. These result s demonstrate that the two types of ovulated LT/Sv oocytes have differ ent potentials to undergo either spontaneous or sperm-induced activati on.