Review: Histotrophic nutrition and the placental-endometrial dialogue during human early pregnancy.

Development of the placenta must always be in advance of that of the embryo. Evidence from domestic species demonstrates that the placenta is capable of stimulating its own development through a signalling dialogue with the endometrial glands. Placental lactogens produced by the trophoblast lead to increased expression and release of uterine secretions and mitogenic growth factors, including epidermal growth factor, that have a close temporal and spatial relationship with trophoblast proliferation. Here, we review evidence that an equivalent mechanism operates in the human. The same repertoire of receptors is present on the endometrial gland cells, and the epithelial cells have long been known to adopt a hypersecretory phenotype following an implantation. Furthermore, early pregnancy hormones stimulate the secretion of glycodelin-A and osteopontin, two 'uterine milk proteins' that have multiple potential effects at the maternal-placental interface, from organoid cultures derived from endometrial glands. Prolactin appears to be an important stimulant, but unlike in domestic species the human trophoblast does not secrete this hormone. Instead, it is a major product of decidual cells. Hence, complications of pregnancy that have their pathophysiological roots in deficient trophoblast proliferation may be due primarily to problems of decidualisation. Ensuring the endometrium is in an optimal state pre-conceptionally should therefore be a priority for women's health. Trophoblast stemness and proliferative capacity show a sharp decline at the switch from histotrophic to haemotrophic nutrition. This may reflect the increase in oxygen concentration or loss of growth factor support. Either way, there are implications for adaptive growth of the organ.