Placentation 3845750 199713966 2008-03-21T00:02:07Z 68.101.70.212 /* Placentation in mammals */ In biology, '''placentation''' refers to the formation, type and structure, or arrangement of [[placenta]]s. The function of placentation is to transfers nutrients from maternal tissue to a growing [[embryo]]. Placentation is best known in pregnant female mammals ([[eutheria]]), but also occurs in other animals, eggs ([[yolk sac]] placentation) and [[flowering plant]]s. ==Placentation in mammals== {{main|Placenta}} In placental mammals, the placenta forms after the embryo implants into the wall of the [[uterus]]. The developing [[fetus]] is connected to it via an [[umbilical cord]]. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are: * '''endotheliochorial placentation''' (e.g. in most [[carnivore]]s like [[cat]]s and [[dog]]s) * '''epitheliochorial placentation''' (e.g. in [[ruminant]]s, [[horse]]s, [[whale]]s) * '''hemochorial placentation''' (e.g. in most [[primate]]s, including [[human]]s) 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 vessel]]s in order to supply the needed amount of blood. In humans, placentation takes place 7-8 days after fertilization. In the case of [[twin]]s, '''dichorionic placentation''' refers to the presence of two placentas (in all [[dizygotic]] and some [[monozygotic]] twins). '''Monochorionic placentation''' occurs 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]]. ==Placentation in plants== In [[flowering plant]]s, placentation occurs where the [[ovule]]s are attached inside the [[ovary (plants)|ovary]]. The ovules inside a [[flower]]'s ovary (which later become the [[seed]]s inside a [[fruit]]) are attached via [[funiculus|funiculi]], the plant equivalent of an umbilical cord. The part of the ovary where the funiculus attaches is referred to as placenta. In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include: *'''Basal placentation''': The placenta is at the base (bottom) of the ovary. *'''Apical placentation''': The placenta is at the apex (top) of the ovary. *'''Parietal placentation''': The placentas are in the ovary wall within a non-sectioned ovary. *'''Axile placentation''': The ovary is sectioned by radial spokes with placentas in separate [[locule]]s. *'''Free central placentation''': The placentas are in a central column within a non-sectioned ovary. ==See also== * [[Carpel]] * [[Ovule]] * [[Placenta]] [[Category:Plant reproduction]] [[Category:Embryology]]