Reduction in fertilization rate in vitro of oocytes from immature rats induced to superovulate.

Immature female rats (65-70 g) were injected with 4 i.u. PMSG (control) or superovulated with 8, 16, 24 or 40 i.u. PMSG and were killed 68-70 h later, shortly after the normally expected time of ovulation. Oocytes were recovered from the oviducts and inseminated in vitro. After 18 h oocytes were counted and classed as degenerate or 1-cell. Mean numbers of oocytes recovered were 8.2, 26.8, 50.7, 38.7 and 38.5 for each dose of PMSG respectively. The 1-cell oocytes were assessed for sperm penetration of the vitellus and pronuclear development and later for development to the 2-cell stage. Fertilization rates at the 1-cell stage were 76.8, 62.9, 53.6, 52.2 and 44.5% for the rats treated with 4, 8, 16, 24 and 40 i.u. respectively (P less than 0.001). On average, 91% of fertilized 1-cell oocytes developed to the 2-cell stage and there was no difference between treatments in this respect. Significantly more of the unfertilized oocytes were degenerate in the rats treated with 24 or 40 i.u. PMSG (34.6 and 50.4%) than in those treated with 4, 8 or 16 i.u. (7.0, 13.9, and 7.5%) (P less than 0.001). When rats were killed 63-65 h after PMSG, just before the normally expected time of ovulation, some of the rats treated with 24 and 40 i.u. PMSG had partly ovulated: of the oocytes recovered from the oviducts only 12.3% (24 i.u.) and 26.6% (40 i.u.) were fertilized. These results demonstrate that proportionately fewer oocytes recovered from superovulated rats are competent to undergo in-vitro fertilization than are oocytes recovered from control rats.