Acceleration of embryo transport in superovulated adult rats

The etiology of reduced fertility in rodents and humans after exogenous gonadotropin-induced superovulation is unknown. This study examines implantation failure in adult rats induced to superovulate by gonadotropin treatment. After multiple injections of pregnant mare serum gonadotropin (PMSG) followed by human chorionic gonadotropin (hCG), superovulation in adult rats was confirmed on day-1 of pregnancy, however, on day-13, no implantation sites were observed. Daily flushing of oviducts and uteri revealed that the number of embryos recovered from superovulated rats on day-2 was only one-third of that retrieved on day-1. The numbers of embryos retrieved on these days from the oviducts of control rats, however, were almost equivalent. On day-3, no embryos were retrieved from either the oviducts or uteri of superovulated rats, while all embryos from control rats were found in the oviducts. Most embryos recovered from superovulated rats showed normal development. Compared to levels in control rats, serum estradiol (E2) in superovulated rats increased significantly on days-2 and -3 of pregnancy, whereas, serum progesterone (P) levels remained unchanged. Thus, E2/P ratio increased on days-2 and -3 of pregnancy in superovulated adult rats coinciding with the timing of embryo transport acceleration. Our results suggest that superovulated oocytes can be fertilized and reach an early stage of development within the oviduct. Implantation failure in superovulated rats may be due to the accelerated embryo transport resulting from elevated E2/P ratio.