placenta(redirected from membranous placenta)
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afterbirth,organ that develops in the uterus during pregnancy. It is a unique characteristic of the higher (or placental) mammals. In humans it is a thick mass, about 7 in. (18 cm) in diameter, liberally supplied with blood vessels. Composed mainly of tissue that develops from the embryo beginning early in pregnancy, the placenta is attached to the uterus, and the fetus is connected to the placenta by the umbilical cord. The placenta acts as an interface between the mother and fetus, drawing nourishment and oxygen, which it supplies to the fetus, from the maternal circulation. In turn, the placenta receives the wastes of fetal metabolism and discharges them into the maternal circulation for disposal. It also acts as an endocrine gland, producing estrogen, progesterone, gonadotrophin, and serotonin, and works to prevent the mother's immune system from rejecting the fetus. Shortly after delivery of the fetus the placenta is forced out by contractions of the uterus. Severe hemorrhage may occur if the placenta does not emerge in its entirety or if the uterus fails to contract properly.
in humans and almost all other mammals, some chordates, and some invertebrates, the organ that unites the fetus and the maternal uterus and provides for the exchange of substances between the two during intrauterine development. Through the placenta the embryo receives oxygen and nutrients from the maternal blood; in turn, the embryo eliminates wastes and carbon dioxide into the placenta. The organ also performs a barrier function, regulating the entry of various substances into the embryo. The placenta contains vitamins and enzymes that participate in the embryo’s metabolism.
Hormones (chorionic gonadotropin), acetylcholine, and other substances that act on the mother’s body are synthesized in the placenta. In humans and other mammals the placenta is formed by some type of union between the chorion (external embryonic membrane) and the uterine wall. In the early stages of embryonic development, processes known as villi form over the entire surface of the chorion. The villi subsequently develop into depressions of the mucous membrane of the uterus. Blood vessels of the yolk sac or allantois usually grow into the villi. Accordingly, vitelline and chorioallantoic placentas are distinguished. A vitelline placenta characterizes most marsupials and some fishes (Selachii), amphibians, and reptiles (a chorioallantoic placenta forms in some reptiles).
Among viviparous invertebrates, a placenta characterizes some onychophores and salpae. However, neither in structure nor in origin is the placenta of these animals comparable to the placenta of vertebrates. In onychophores, the yolk sac forms a placental relationship with the uterine wall. In salpae, the placenta is formed with the participation of follicular epithelial cells that intermix with rudiments of embryonic organs and function as an intermediary between the organs and the maternal organism. In higher mammals, the vitelline placenta functions first and, some time later, gives way to the chorioallantoic placenta. Both types of placentas function in rabbits, moles, horses, camels, and other animals.
Several placental structures are distinguished in mammals according to the location of the villi on the chorion and crypts in the maternal mucosa. In a diffuse placenta, short bushy villi are formed over the entire surface of the chorion. The villi do not unite with the maternal mucosa but only enter into its crypts. Cetaceans, swine, camels, and horses are characterized by a diffuse placenta. Ruminants have cotyledonary placentas, whose long branching chorionic villi are arranged in the form of tufts called cotyledons. The villi enter the crypts of caruncles, that is, the thickenings of the maternal mucosa. Carnivores have an annular placenta, in which the chorionic villi form a belt around the interior of the uterus. In a discoid placenta, which characterizes rodents, some insectivores, bats, and primates, part of the disk-shaped placenta is covered with villi, whereas the remaining surface of the chorion is smooth.
Placentas are also classified according to the number of layers of tissue separating the vascular systems of the mother and fetus. For example, the epitheliochorial placenta of some marsupials, swine, tapirs, cetaceans, camels, horses, and lemurs consists of villi and crypts that are covered with epithelium which survives throughout pregnancy. When the afterbirth is expelled, the villi are readily drawn out of the crypts. In the desmochorial placenta of many ruminants, the epithelium lining the depressions in the maternal mucosa is destroyed by enzymes of the epithelium of the ingrowing villi. In endotheliochorial placentas, which characterize all carnivores, not only the epithelium but also the connective tissue dissolves. The villi grow deep into the maternal mucosa, and their epithelium lies next to the endothelium of the uterine blood vessels. A hemochorial placenta, found in rodents, some insectivores, bats, and primates, is one in which the endothelium of the uterine blood vessels is destroyed as well, and the chorionic villi come into contact with the maternal blood. An achorial (nonvillous) placenta does not have villi, and there is no close connection between the fetal and maternal placentas. Epitheliochorial and syndesmochorial placentas are said to be non-deciduate, because at birth the chorionic villi emerge from the depressions in the maternal mucosa without injuring it. Since the casting off of the vasochorial and hemochorial placentas is accompanied by part of the maternal mucosa, such placentas are called deciduate. The structure of the placental tissues varies with the stage of embryonic development.
K. M. KURNOSOV
In humans. The human placenta is formed when the external embryonic villous membrane unites with the uterine wall toward the end of the third month of pregnancy. In a full-term fetus, it resembles a flat disk measuring 15 cm wide, 20 cm long, and about 3 cm thick; its weight is about 500 g. The embryo is joined to the placenta by the umbilical cord. The placenta has a maternal surface that adjoins the uterus and fetal plate to which the umbilical cord and its blood vessels are attached. There is an intensive exchange of substances through the placenta where the blood vessels of the mother and fetus come into close contact but do not coalesce: O2 and nutrients enter the fetal bloodstream, and CO2 and waste products enter the maternal blood vessels. All exchanges between the mother and the fetus take place across the surface of the chorionic villi, which cover an area of 6,000 to 10,000 sq cm by the end of pregnancy. The total length of the villi is 50 km. The placenta contains enzymes and vitamins and synthesizes hormones and mediators, which help the mother’s body adjust to pregnancy. The placenta also functions as a physiological barrier that protects the fetus against injurious influences from the mother (selective retention by the placenta of certain harmful substances circulating in the maternal blood). On the other hand, some chemical compounds, drugs in particular, that are not toxic to the mother but are harmful to the fetus owing to their teratogenic action freely pass through the placenta. Because of this, the USSR developed standard methods of testing drugs for teratogenic activity. Impairment of placental function may give rise to a variety of complications, for example, premature birth, slow placental separation, and toxicosis of pregnancy.
REFERENCESGarmasheva, N. L. Platsentamoe krovoobrashchenie. Leningrad, 1967. Garmasheva, N. L. Zhenshchine 0 vnutriutrobnom razvitii rebenka, 2nd ed. Moscow, 1973.
In plants. The placenta in seed plants is a swelling, projection, or process in internal tissues of the ovary; it has a vascular bundle. Seed rudiments, such as ovules and megasporangia, are attached to the placenta. In ferns, the placenta is a prominence or tubercle with a vascular bundle bearing sporangia. In brown algae, it is a complex of cells under the sporangia, and in red algae, it is the widened base of the cystocarp.