Maternal Surface: Has a rough, spongy appearance with 15-20 lobules called cotyledons. These are separated by decidual septa. This surface is artificial, created upon separation from the uterine wall after birth. It is composed of the decidua basalis, which is the modified endometrial layer of the uterus.
Fetal Surface: Appears smooth and shiny because it is covered by the amnion. The umbilical cord inserts into this side, and fetal blood vessels (chorionic vessels) can be seen branching out across the surface. Underlying the amnion is the chorion.
Key Microscopic Structures
Chorionic Villi: The functional units of the placenta, responsible for exchange. They are finger-like projections from the chorion that provide a large surface area for metabolic exchange.
Core: Contains fetal capillaries and connective tissue (Wharton’s jelly).
Trophoblast Layers: The villi are covered by two layers of trophoblast cells which are of fetal origin.
Cytotrophoblast (Inner Layer): Mitotically active cells that are considered stem cells which differentiate to form the syncytiotrophoblast.
Syncytiotrophoblast (Outer Layer): A multinucleated cell layer that is in direct contact with maternal blood. It is responsible for invading the uterine wall and producing hormones like hCG, hPL, progesterone, and estrogen.
Placental Circulation
There are two distinct circulatory systems: maternal and fetal. Maternal and fetal blood do not mix.
Maternal (Uteroplacental) Circulation:
Maternal blood enters the intervillous space via the spiral arteries of the uterus.
This oxygen- and nutrient-rich blood bathes the chorionic villi.
Deoxygenated blood is then drained by the endometrial veins.
Fetal Circulation:
Deoxygenated blood flows from the fetus to the placenta via two umbilical arteries.
These arteries branch into capillaries within the chorionic villi, where gas and nutrient exchange occurs with the maternal blood in the intervillous space.
Oxygenated blood returns to the fetus via one umbilical vein.
Placental Barrier
The structure that separates maternal and fetal blood is the placental membrane.
It is composed of the syncytiotrophoblast, cytotrophoblast, the villous connective tissue, and the endothelium of the fetal capillaries.
