Placentation

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.BasalParietalAxileFree centralMarginal 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. 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: 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. In the case of twins, 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. As placentation often results during the evolution of live birth, the more than 100 origins of live birth in lizards and snakes (Squamata) have seen close to an equal number of independent origins of placentation. This means that the occurrence of placentation in squamata is more frequent than in all other vertebrates combined, making them ideal for research on the evolution of placentation and viviparity itself. In most squamates two separate placentae form, utilising separate embryonic tissue (the chorioallantoic and yolk-sac placentae). In species with more complex placentation, we see regional specialisation for gas, amino acid, and lipid transport. Placentae form following implantation into uterine tissue (as seen in mammals) and formation is likely facilitated by a plasma membrane transformation. Most reptiles exhibit strict epitheliochorial placentation (e.g. Pseudemoia entrecasteauxii) however at least two examples of endotheliochorial placentation have been identified (Mabuya sp. and Trachylepis ivensi). Unlike eutherian mammals, epitheliochorial placentation is not maintained by maternal tissue as embryos do not readily invade tissues outside of the uterus. The placenta is an organ that has evolved multiple times independently, evolved relatively recently in some lineages, and exists in intermediate forms in living species; for these reasons it is an outstanding model to study the evolution of complex organs in animals. Research into the genetic mechanisms that underpin the evolution of the placenta have been conducted in a diversity of animals including reptiles, seahorses, and mammals.