The placenta is a disc shaped organ that forms the communication between the fetus and the mother. The umbilical cord inserts on the chorionic plate of the placenta. The maternal portion of the placenta is called decidua basalis. It shows lobules called cotyledons while the fetal side looks smooth and shiny from the amnion. Chorionic villi are the basic functional unit of the placenta. The villi go through different stages in their development. They float in maternal blood contained in the surrounding lacunar spaces. The outer layer of the villi is made of syncytiotrophoblast. It is rich in actin, tubulin and intermediate filament proteins such as cytokeratins; arranged as microtubules and microfilaments. The cytotrophoblast layer is present under the syncytiotrophoblast. It shows Langhans’ cells which are stem cells. Under this layer is a basement membrane. The core of the villi is formed of connective tissue stroma and fetal blood vessels. In the larger villi the core carries fetal arteries and veins while the smaller and peripheral villi carry fetal capillaries and sinusoids.
The development of placenta begins following 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 for establishing 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 to form the syncytiotrophoblast. Maternal blood supplies the placenta and fetus with oxygen and nutrients, and allows the transfer of the 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 and can be seen on the fetal side of the placenta. The chorionic arteries further branch out 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 and finally the uterine veins.
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