Transport of Embryos to the Uterus: Normal and Abnormal Timing

Abstract

The fact that the timing of entry of embryos into the uterus from the Fallopian tubes is critical for subsequent survival in most species suggests that the mechanisms regulating this transport process must themselves be programmed with accuracy. Although the primate embryo may be a notable exception in this context of critical timing, evidence from farm and laboratory species indicates that premature or belated entry into the uterus compromises survival of the embryo. Some of the most persuasive findings are drawn from embryo transplantation studies, such as those of Chang (1950) and Adams (1979) in rabbits involving the use of donors and recipients at differing stages after ovulation. When seeking control systems that underlie timing of such remarkable sensitivity, it is worth recalling that (a) the egg is released from the ovary at approximately the start of corpus luteum formation — this gland being the principal ovarian source of progesterone — and (b) there is abundant evidence dating back to the 1930s indicating that the rate of passage of eggs or embryos from the tubes to the uterus can be grossly disturbed by systemic treatment with oestrogens or progestagens or, alternatively, by means of bilateral ovariectomy. In the first instance, therefore, a reasonable assumption might be that mechanisms that regulate passage of embryos to the uterus are strongly influenced by the prevailing secretion of ovarian steroid hormones.