Reproduction – fetal development, placenta and maternal physiology

Rudolf Cardinal, 22/24 Nov 1998 Implantation and the decidual response • Implantation in humans and some other species is invasive – the uterine surface epithelium is breached and the underlying stroma is invaded. (In yet other species, the epithelium remains intact with the fetal and maternal epithelia in close contact.) • The blastocyst makes a signal (unknown) which induces a stromal response called decidualization. The stroma increases its vascular permeability, becomes oedematous, the matrix and stroma change composition and morphology and new capillaries grow. This prepares the major endometrial contribution to the placenta – the decidua. • The decidual response depends on a uterus that has been exposed to progesterone (to synthesize secretory products) and also oestrogen (to stimulate the actual secretion of those products, and to make the uterine epithelium responsive to the blastocyst’s signal). • Remember that the blastocyst also makes hCG to maintain the corpus luteum.