Information AboutPlacenta |
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The placenta is an Ephemeral (temporary) Organ present in Female Placental Vertebrates , such as some Mammal s and Shark s during Gestation ( Pregnancy ), but a placenta has evolved independently also in other animals as well, for instance scorpions, and velvet worms. Placentophagy (from 'placenta' + Greek φαγειν, to eat) is the act of mammals eating the placenta of their young after childbirth. The placenta is a primary site of nutrition and gas exchange between the mother and fetus. The placenta is a fetomaternal organ that has two components, the fetal part (Chorion Frondosum) and the maternal part (Decidua Basailis). DEVELOPMENT OF PLACENTA Decidua The wall of body of uterus consists of three layers
Endometrium consists of three layers
The compact and spongy layers disintegrate and are shed during menstruation and after parturition. So they are collectively called as functional layer of endometrium. Gravid endometrium or decidua (deciduus L- falling off). The term decidua is appropriate because this part of endometrium separates or falls away from the remainder of uterus after child birth. The decidua basalis is the part of decidua deep to the conceptus at the embryonic pole. Decidua basalis forms the maternal part of placenta. The decidua capsularis is the superficial part of decidua overlying the conceptus at the abembryounic pole. The decidua paraitalis is the rest of decidua. Decidual reactions In response to increasing levels of progesterone the decidual cells enlarge due to accumulation of fat and glycogen, these pale staining cells are now called decidual cells. The cellular and vascular changes in decidual cells are called decidual reactions. VILLI Primary villi appear at day 13. Secondary villi appear at day 16. Tertiary villi appear at the end of 3rd week. Chorionic villi cover the entire chorionic sac until the beginning of 8th week. Afterwards, as the chorion grows the villi covering the deciduas capsularis are compressed, reducing the blood supply to them. These villi degenerate and produce a bare avascular area known as Smooth Chorion or Chorion Leave. The villi associated with decidua basalis rapidly increase in number and branch, giving rise to Villous Chorion or Chorion Frondosum or Bushy Chorion. It is the fetal part of placenta. AMNIOCHORIONIC MEMBRANE The amnion enlarges faster than the chorion so that the amnion and chorion fuse and form the amniochorionic membrane. It is the membrane that ruptures spontaneously or by artificial means during labor (breaking of water). With growth of amnion and chorion the decidua capsularis becomes stretched and degenerates. So, the smooth chorion lies in contact with decidua parietalis. STRUCTURE OF PLACENTA By the beginning of 4th month the placenta has two components.
Placenta is bordered on the Fetal side by Chorionic plate Maternal side by Decidual plate (decidua basailis) Between the chorionic and decidual plates are intervillous spaces which are filled with maternal blood. During the fourth and fifth months the decidua forms a number of decidual septa, which project into intervillous spaces but do not reach the chorionic plate. As a result of this septum formation the placenta is divided into a number of compartments or cotyledons. Since the decidual septa do not reach the chorionic plate, so contact between cotyledons is maintained. PLACENTAL CIRCULATION Maternal Placental Circulation The maternal blood enters the intervillous space through endometrial arteries (spiral arteries). 80 – 100 in number. They pierce the decidual plate and then pass through the gaps in cytotrophoblastic shell. As the artery enters it is under high pressure because it enters through the small gap, this pressure forces the blood deep into intervillous spaces and bathes the villi. Exchange of gases takes place. As the pressure decreases, the deoxygenated blood flows backwards to the decidua and enters the endometrial veins. Fetoplacental Circulation Deoxygenated fetal blood pass through umbilical arteries to placenta. At the junction of umbilical cord and placenta, they branch radially to form chorionic arteries. Chorionic arteries also branch before they enter into the villi. In the villi they form an extensive arteriocapillary venous system, that bring the fetal blood extremely close to the maternal blood but normally no intermingling of fetal and maternal blood occurs. FUNCTIONS Filtration and transfer The placenta receives nutrients, oxygen, antibodies and hormones from the mother's Blood and passes out waste. It forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. The placental barrier does NOT allow the two bloods from the mother and embryo to mix because if the blood types don't match then the baby will be destroyed. Many substances are not filtered out, however, including Alcohol and some chemicals associated with smoking Cigarettes . Several types of Virus , such as Human Cytomegalovirus , may also cross this barrier; this often leads to various degrees of birth defects in the infant. Metabolic and endocrine activity In addition to the transfer of gases and nutrients, the placenta also has metabolic and Endocrine activity. It produces, amongst other Hormones , Progesterone , which is important in maintaining the pregnancy; Somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; Estrogen ; Relaxin , and human chorionic gonadotrophin HCG . This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy. The site of the former umbilical cord attachment in the center of the front of the Abdomen is known as the Umbilicus , navel, or belly-button. DELIVERY When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the fetus the umbilical cord is usually clamped and severed prior to the delivery of the placenta. The placenta is delivered mainly by uterine contractions, but the umbilical cord can be gently pulled while pressure is placed on the abdomen, to speed its delivery. Usually the placenta delivers in 2-10 minutes after delivery of the newborn, but it is considered normal as long as the placenta delivers within 30 minutes. If the placenta fails to deliver in 30 minutes, manual extraction may be required, and in some cases a Curettage is necessary to ensure that no remnants of the placenta remain. Risks of retained placenta include hemorrhage and infection. Non-humans In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of Prostaglandin on the uterus after birth. This is known as Placentophagy . top right. The white fringe surrounding the bottom is the remnants of the wall.]] SPECIES VARIATION Although a predominant feature in Eutheria n mammals, placentae have Evolved Separately in snakes, as well as some invertebrates. In mammals The major group mammals referred to as "placental mammals" belong to Eutheria . In actuality Marsupial s ( Metatheria ) all have placentae as well. Egg laying Monotreme s do not. Aside from the Bandicoot , About Marsupials by Molly Kalafut most of the placenta in marsupials are not considered true-placenta, as they do not invasively implant into the uterine wall and tend to function for only a fraction of the time (see below). The shape and exchanging surfaces of eutherian mammals varies according to family:
Marsupials are often assumed to not possess placentae, although some may RenfreeMB (2006) " Life in the pouch: womb with a view " ''Reproduction, Fertility and Development'' placentae of the eutheria. However, MossmanMossman HW (1937) Comparative morphogenesis of the fetal membranes and accessory uterine structures. Carnegie Inst. Contrib. Embryol 26, 129–246. describes the eutherian placenta as "an apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange". This process has been shown to occur in the Tammar Wallaby ,Renfree MB (1972) Influence of the embryo on the marsupial uterus. Nature 240, 475–477. and is likely evident in other Macropod s (kangaroos). As mentioned, the bandicoot is considered to be a "placental marsupial" because it has a chorio-allantoic structure similar to eutherian species as well as the chorio vitelline structure associated with marsupials. PATHOLOGY ADDITIONAL IMAGES |
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