plazenta

Plazenta is a temporary organ that develops during pregnancy in placental mammals, including humans. It connects the developing fetus to the uterine wall and serves as the site of nutrient, gas, and waste exchange between mother and fetus. It also functions as an endocrine gland, producing hormones that support pregnancy.

Development and structure: It forms from fetal tissues (chorion) and maternal tissues (decidua basalis) shortly after implantation, and by the end of the first trimester the placental villi are well established. The functional unit is the villous tree, with fetal blood in capillaries within villi surrounded by maternal blood in the intervillous space. In humans, the placenta has a hemochorial type, where maternal blood contacts the chorionic epithelium. The placental barrier is a layer of trophoblast and fetal endothelium that limits but does not completely prevent the transfer of substances.

Function: Nutrient and gas exchange occur by diffusion, active transport, and facilitated diffusion; waste products and carbon dioxide move in opposite directions. The placenta secretes hormones such as human chorionic gonadotropin (hCG), progesterone, estrogens, placental lactogen, and relaxin, supporting corpus luteum function and fetal growth. It also provides immunological protection and acts as a barrier to some pathogens, though many substances can cross.

Clinical and species notes: Abnormal placentation can lead to conditions such as placenta previa, placental abruption, or placental insufficiency, contributing to fetal growth restriction. In nonhuman mammals, placental structure varies; humans have a discoid, hemochorial placenta, while other species show diffuse or cotyledonary placentation types.