R. B. Heap

Synthetic Capabilities of the Preimplantation Conceptus and Maternal Responses to Pregnancy

Egg transfer experiments in different species have established the relative importance of synchronization between the age of an embryo and the uterine environment of the recipient for successful pregnancy. The physiological explanation of this requirement seems to be related to the nature of the luminal environment of the uterus and its regulation by ovarian steroid hormones. These hormones, by their effects on the composition of endometrial secretions and the properties of the uterine epithelium, determine whether a blastocyst survives and implants, or whether its growth is arrested and implantation is delayed or fails. Synchronization between the age of an embryo and its subsequent environment frequently gives the best chance of success in egg transfer experiments (Chang 1950, 1968; Rowson and Moor 1966; Betteridge 1977), but transferred blastocysts also develop normally when the timing differs by about 24 h (see Hunter 1980). In rhesus monkeys greater asynchrony may be tolerated initially, since the transfer of tubal eggs to the uterus is not accompanied by an immediate loss of the egg; although pregnancy rarely proceeds to normal term, chorionic gonadotrophin production can be detected (Marston et al. 1977). In women, tubal eggs fertilized in vitro survive and implant after transfer to the uterus, and normal pregnancies arising from embryo transfer have now been reported from several clinical centres (Steptoe and Edwards 1978; Lopata et al. 1978; Wood et al. 1981). However, despite substantial technical improvements in oocyte recovery, fertilization in vitro and embryo transfer procedures, significant losses occur, apparently during the peri-implantation period.