Human Placenta: Formation, Structure, and Function
The Placenta: Structure and Function
The placenta is composed of maternal and, especially, fetal tissues. The structure facilitates contact between maternal and fetal blood circulations across the largest possible area, while maintaining their independence, at least until delivery. The human placenta exhibits the following characteristics:
- Hemochorial: The egg implants in the uterine lining, with the trophoblast penetrating the endometrium to erode the endothelium of uterine vessels, establishing direct contact with maternal blood.
- Decidual: Trophoblast penetration leads to the destruction of uterine connective tissue. Expulsion of the placenta causes hemochorial bleeding and the shedding of the decidua, the region of the mucosa from which the placenta detaches at birth.
- Discoid and Pseudocotyledonous: The human placenta is discoid, unlike diffuse placentas that occupy the entire uterine cavity. Villi are grouped into small clusters (cotyledons) separated by incomplete septa.
Early Development (Weeks 1-3)
From the morula stage, smaller peripheral cells (micromeres) differentiate into trophoblast cells, distinct from the bulkier central cells (macromeres) that form the yolk sac and embryo. At the blastula stage, the trophoblast consists of a single peripheral cell layer. By the beginning of the 2nd week, the trophoblast differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast.
The syncytiotrophoblast proliferates and infiltrates between the epithelial cells of the uterine lining, destroying them and creating a gap through which the blastocyst penetrates the endometrium. By the middle of the 2nd week, the implanted egg’s syncytiotrophoblast continues to proliferate, developing a series of vacuoles that progressively enlarge and merge, forming lacunae. The syncytiotrophoblast erodes endometrial capillaries, allowing maternal blood to enter the lacunae.
Cytotrophoblastic cords then grow between the syncytiotrophoblast trabeculae, forming primary villi. The lacunae converge into a single cavity bounded by the syncytiotrophoblast: the intervillous chamber. By the middle of the 3rd week, primitive vasculosanguineous islets differentiate within the mesenchymal axis of secondary villi, which then become tertiary villi. Fetal placental circulation is established around the 20th day. By the end of the 3rd week, all elements necessary for embryonic and maternal circulation are in place. The embryo’s blood, flowing through the villi’s vessels, is separated from maternal blood in the intervillous chamber by the endothelium of embryonic capillaries.
Placental Development: Day 20 to End of Month 4
The placenta undergoes a series of transformations to reach its final form.
Obliteration of the Uterine Cavity
By the end of the 2nd month, chorionic villi oriented towards the abembryonic pole degenerate, leaving a smooth, avascular chorion. The smooth chorion consists of the extraembryonic mesenchymal chorionic plate, which limits the extraembryonic coelom, and a cell layer of cytotrophoblast. The chorionic villi oriented towards the basal plate continue to grow, forming the villous chorion, the future discoid placenta.
By the end of the 3rd month, the abembryonic pole obliterates against the uterine wall. The placenta becomes a discoid organ attached to the uterine wall via the maternal side, or basal plate. The fetal side, or chorionic plate, receives the insertion of the umbilical cord. At its periphery, the placenta is continuous with the membranes, formed from the amniotic cavity into the myometrium, by the amnion, the chorion, and the fusion of the decidua parietalis.