Placentae

Placentation
Placentation resulting from cleavage at various gestational ages
Details
Latin	placentatio
Anatomical terminology []

In biology, placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gasses, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange. This definition is modified from the original Mossman (1937) definition, which constrained placentation in animals to only those instances where it occurred in the uterus.

In live bearing mammals, the placenta forms after the embryo implants into the wall of the uterus. The developing fetus is connected to the placenta via an umbilical cord. Mammalian placentas can be classified based on the number of tissues separating the maternal from the fetal blood. These include:

In this type of placentation, the chorionic villi are in contact with the endothelium of maternal blood vessels. (e.g. in most carnivores like cats and dogs)

Chorionic villi, growing into the apertures of uterine glands ( epithelium). (e.g. in ruminants, horses, whales, lower primates, dugongs)

In hemochorial placentation maternal blood comes in direct contact with the fetal chorion, which it does not in the other two types. It may avail for more efficient transfer of nutrients etc., but is also more challenging for the systems of gestational immune tolerance to avoid rejection of the fetus.