Development and Function of Placenta

The placenta is a crucial organ for embryonic development. Its formation begins with implantation, where the blastocyst attaches to the endometrium. Trophoblast cells of the blastocyst release digestive enzymes that break down the endometrium's vascular and connective tissue, aiding the embryo's embedment and forming chorionic villi. These villi become vascularized with embryonic blood vessels, connecting to the umbilical arteries and veins within the umbilical cord. The breakdown of maternal blood vessels by these enzymes leads to the pooling of maternal blood around the chorionic villi, facilitating exchange between maternal and fetal blood.

The placenta comprises the chorion (from the trophoblast), chorionic villi, ruptured maternal blood vessels forming blood pools, and the umbilical cord. The umbilical cord contains the umbilical vein and artery, along with extraembryonic membranes like the allantosis and yolk sac. The chorion is the third extraembryonic membrane.

The placenta has a vital endocrine function. Early in pregnancy, trophoblast cells release Human Chorionic Gonadotropin (HCG), stimulating the corpus luteum to produce progesterone and estrogen, maintaining the endometrium. Later, the fully formed placenta produces its own estrogen and progesterone. Towards the end of pregnancy, the placenta releases Corticotropin-Releasing Hormone (CRH), which prompts the pituitary gland to release ACTH. ACTH then stimulates the adrenal gland to produce a precursor molecule, which the placenta converts into more estrogen, leading to uterine contractions and childbirth.

A primary function of the placenta is gas and nutrient exchange. Within the chorionic villi, a network of fetal blood vessels is surrounded by a semi-permeable membrane. This membrane allows the exchange of nutrients (like glucose, water, minerals, and amino acids) and gases (oxygen and carbon dioxide) between maternal and fetal blood without direct mixing. This is crucial as fetal lungs are not fully developed until after birth.

The semi-permeable placental membrane acts as a barrier, preventing large substances like bacterial cells and red blood cells from crossing between mother and fetus. However, tiny antibodies, specifically Immunoglobulin G (IgG), can pass into the fetal blood system, providing passive immunity to the developing embryo whose immune system is not yet fully functional. Additionally, the placenta facilitates the removal of fetal waste products such as carbon dioxide, urea, uric acid, and creatinine, transferring them to the maternal blood for excretion by the mother's kidneys.