Cellular Interactions During Implantation in Ruminants

Abstract

All mammals so far investigated are similar in that there is a strictly limited period during which the hormonally primed uterine epithelium (UE) will form an intimate association with the blastocyst and initiate implantation. However, the subsequent cellular modifications differ enormously in the various groups, largely reflecting the range of definitive placental structures produced (1). This variety probably reflects the difficulty of balancing the conflicting requirements of fetal nutrition and camouflage; that is, between getting the conceptus as close to the maternal blood as possible for adequate nutrition while maintaining a sufficient barrier to protect its status as an immunologically foreign body. On the fetal side of the placenta, the blood vessel endothelium and covering trophectoderm or chorion always survives, and indeed, the latter may be duplicated (rabbit) or triplicated (rats and mice); the survival of the maternal layers—UE, endometrium, and endothelium—varies from complete loss (humans and rodents) to complete retention (lemurs, camels, and pigs). The cellular and/or syncytial layers formed or lost as a result of implantation are characteristic for any particular species, largely following the updated Grosser classification of epitheliochorial, synepitheliochorial, endotheliochorial, or hemochorial (1).