placentáció

Placentation is the formation and development of the placenta, the transient organ that links the developing fetus to the maternal circulation. Through the placenta, nutrients and oxygen are supplied to the fetus and waste products removed, while hormones produced by the placenta support pregnancy.

During placentation, trophoblast cells from the outer blastocyst invade the maternal endometrium (decidua), form chorionic villi, and remodel uterine blood vessels to secure a blood supply. The placenta consists of fetal tissue (chorion and villous structures) and maternal tissue (decidua basalis). The maternal and fetal blood do not mix directly in most placentation types; instead, exchange occurs across a thin barrier formed by trophoblast and endothelium.

Placental organization varies across mammals. Types of placentation include epitheliochorial (all maternal barriers intact; observed in horses, pigs and some ruminants), endotheliochorial (uterine epithelium and blood vessels eroded; common in dogs and cats), and hemochorial (maternal blood contacts the chorion; humans, mice). The placenta's surface can be discoid, cotyledonary, or diffuse, among other patterns.

Functions extend beyond nutrient and gas exchange. The placenta synthesizes hormones such as human chorionic gonadotropin, progesterone, estrogens, and placental lactogens that support pregnancy, regulate maternal physiology, and modulate immune interactions to tolerate the fetus.

Disorders of placentation can contribute to complications such as intrauterine growth restriction, preeclampsia, and preterm birth.