Effects of Delayed Ovulation on Pregnancy in the PMSG-Treated Immature Rat 1

Abstract

The effects of phenobarbital (PB)-delayed ovulation on embryonic development, implantation, and serum concentrations of progesterone (P), androstenedione (A), and estradiol (E₂) were ascertained in the pregnant mare serum gonadotropin (PMSG)-treated immature rat. Two and three days of ovulatory delay retarded embryonic development as evidenced by the presence of 70-80% of the embryos in the two-cell stage on Day 3 (Day 1 = sperm positive); whereas, nondelayed controls exhibited 40-50% of the embryos in the two-cell stage and 30-40% in the four-cell stage. Ovulatory delay of 3 days induced a higher percentage of abnormal embryos when compared with controls. After 2 days of delay, implantation was inhibited in ~40% of the rats by Days 6 and 8. In controls ~80% of rats exhibited implantation sites on Day 6 and all control rats exhibited implantation sites on Day 8. Serum concentrations of P, A, and E₂ were similar to nondelayed controls on Day 3 of pregnancy suggesting that the retardation in development and inhibition of implantation are consequences of intrafollicular aging and/or to an altered uterine environment. However, the possibility remains that some of these alterations may be related to the effects of PB on the reproductive tract and/or embryo. On Day 14 of pregnancy, an increase in resorption sites was observed after 2 and 3 days of ovulatory delay. This correlated with a significantly lower level of serum P in rats with delayed ovulation indicating that ovulatory delay not only alters embryonic development by altering the oocyte and uterine environment but also may induce abnormalities in the ability of corpora lutea to secrete P and/or alter the clearance rate of P.