In biology, placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, 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.[1] This definition is modified from the original Mossman (1937)[2] definition, which constrained placentation in animals to only those instances where it occurred in the uterus.
| Placentation | |
|---|---|
 Placentation resulting from cleavage at various gestational ages |
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:
- endotheliochorial placentation
- 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)
- epitheliochorial placentation
- Chorionic villi, growing into the apertures of uterine glands ( epithelium). (e.g. in ruminants, horses, whales, lower primates, dugongs)
- hemochorial placentation
- In hemochorial placentation maternal blood comes in direct contact with the fetal chorion, which it does not in the other two types.[3] 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.[4] (e.g. in higher order primates, including humans, and also in rabbits, guinea pigs, mice, and rats)[5]
During pregnancy, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessels in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.
In humans, the placenta develops in the following manner. Chorionic villi (from the embryo) on the embryonic pole grow, forming chorion frondosum. Villi on the opposite side (abembryonic pole) degenerate and form the chorion laeve (or chorionic laevae), a smooth surface. The endometrium (from the mother) over the chorion frondosum (this part of the endometrium is called the decidua basalis) forms the decidual plate. The decidual plate is tightly attached to the chorion frondosum and goes on to form the actual placenta. Endometrium on the opposite side to the decidua basalis is the decidua parietalis. This fuses with the chorion laevae, thus filling up the uterine cavity.[6]
In the case of twins, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentationoccurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in pre-eclampsia.
https://en.wikipedia.org/wiki/Placentation
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