Human Early Placental Development: Potential Roles of the Endometrial Glands

Diagrammatic representation of the relationship of the conceptus to the uterine glands during the first trimester. A) The blastocyst attaches and implants between openings of the uterine glands. The prevailing oxygen concentration will be low as the superficial decidua is oedematous and supplied by a capillary plexus arising from the spiral arteries, favouring trophoblast proliferation. B) As the conceptus enlarges the syncytiotrophoblast will invade into the superficial capillaries and the uterine glands, releasing the contents of both into the lacunae. EGF from the glands will stimulate trophoblast proliferation in the earliest stages, aiding development of the cytotrophoblastic shell. hCG and hPL from the syncytiotrophoblast, and prolactin from the decidual cells may in turn stimulate the glandular cells. C) As the conceptus enlarges the syncytiotrophoblast will encroach on the tips of the spiral arteries. It is essential that the cytotrophoblastic shell is well-formed by this stage, in order that sufficient endovascular extravillous trophoblast cells (EVT) are available to plug the spiral arteries beneath the conceptus. Incomplete plugging of the arteries in the periphery of the normal placenta is associated with early onset of the maternal circulation and villous regression to form the chorion laeve.

Summary of the potential servomechanism by which the human conceptus may stimulate activity in the glandular epithelial cells to meet its requirements.