The placenta is a disc-shaped organ that forms the communication between the fetus and the mother. The umbilical cord inserts into the chorionic plate of the placenta. The maternal portion of the placenta is called the decidua basalis. It shows lobules called cotyledons, while the fetal side looks smooth and shiny because it’s covered by the amnion.
Chorionic villi are the basic functional unit of the placenta. The villi pass through different stages during development. They float in maternal blood contained in the surrounding lacunar spaces.
The outer layer of the villi is made of syncytiotrophoblast. It’s rich in actin, tubulin, and intermediate filament proteins such as cytokeratins, which are arranged as microtubules and microfilaments. Beneath the syncytiotrophoblast is the cytotrophoblast layer. It contains Langhans’ cells, which are stem cells. Under this layer is a basement membrane.
The core of the villi is formed by connective tissue stroma and fetal blood vessels. In larger villi, the core carries fetal arteries and veins, while smaller and more peripheral villi carry fetal capillaries and sinusoids.
The development of the placenta begins after implantation of the blastocyst. The trophoblast layer of the blastocyst eventually forms the placenta. The syncytiotrophoblast (outer layer) invades the uterine wall and spiral arteries to establish placental circulation. Abnormal implantation can cause eclampsia, IUGR, and other disorders.
The placenta secretes progesterone and leptin in addition to human chorionic gonadotropin (hCG) and human placental lactogen (HPL). The cytotrophoblast is considered a source of stem cells that form the syncytiotrophoblast.
Maternal blood supplies the placenta and fetus with oxygen and nutrients, and it also allows transfer of metabolic waste from placental and fetal tissue through the umbilical cord to the maternal kidney for elimination.
Maternal blood from the spiral arteries flows into the lacunar and intervillous spaces. It supplies oxygen and nutrients, which are absorbed by the villi and reach the fetus through the umbilical veins. At the placenta, the umbilical arteries branch into chorionic arteries, which can be seen on the fetal side of the placenta. The chorionic arteries further branch to form capillaries.
The umbilical arteries carry deoxygenated blood to the fetal core vessels in the villi. The deoxygenated blood is pushed back into the endometrial veins and finally into the uterine veins.
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