Early Pregnancy & Placentation

Successful pregnancy depends on the complex interaction of maternal tissues and the developing conceptus. Thus the study of differentiation and disease in female reproductive tissues from conception through pregnancy to birth is of equal importance as is the investigation of the developing zygote, embryo and fetus.

The endometrium, lining the uterus and comprising epithelial and stromal cells, undergoes dramatic differentiation changes of gene expression and morphology through the cycle. Following menstruation there is approximately a 10 day phase of active stromal and epithelial proliferation (proliferative phase) predominantly under estradiol regulation. Ovulation then occurs, to be followed by the formation in the ovary of a progesterone-producing corpus leuteum. Under progesterone influence the endometrium stops proliferating and differentiates into a thick, actively secretory and well vascularized tissue (secretory or luteal phase).

If pregnancy does not occur, there is a stop in progesterone production and the endometrium is sloughed off at menstruation. During this very dynamic process of endometrial proliferation and differentiation, the uterus is receptive to a blastocyst for maximally 6 days (implantation window) in the early secretory phase.

This implantation window reflects precise changes in the differentiation status of both epithelial and stromal cells, which interact with each other in a paracrine fashion, and correlates with an increase in circulating progesterone concentration. The stromal cells play a major role in this, undergoing a switch in gene expression and morphology in the process known as decidualization.