Theplacenta (pl.:placentas orplacentae) is a temporaryembryonic and laterfetalorgan that beginsdeveloping from theblastocyst shortly afterimplantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an importantendocrine organ, producinghormones that regulate bothmaternal and fetalphysiology duringpregnancy.[1] The placenta connects to the fetus via theumbilical cord, and on the opposite aspect to the maternaluterus in aspecies-dependent manner. In humans, a thin layer of maternaldecidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth (sometimes incorrectly referred to as the 'maternal part' of the placenta). Placentas are a defining characteristic ofplacental mammals, but are also found inmarsupials and some non-mammals with varying levels of development.[2]
Mammalian placentas probably first evolved about 150 million to 200 million years ago. The proteinsyncytin, found in the outer barrier of the placenta (thesyncytiotrophoblast) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancientretrovirus: essentially avirus that helped pave the transition fromegg-laying tolive-birth.[3][4][5]
The wordplacenta comes from theLatin word fora type of cake, fromGreek πλακόεντα/πλακοῦνταplakóenta/plakoúnta, accusative of πλακόεις/πλακούςplakóeis/plakoús, "flat, slab-like",[6][7] with reference to its round, flat appearance in humans. The classical plural isplacentae, but the formplacentas is more common in modern English.
The placenta has evolved independently multiple times, probably starting infish, where it originated multiple times, including the genusPoeciliopsis.[8] Placentation has also evolved in somereptiles.[9]
Themammalian placenta evolved more than 100 million years ago and was a critical factor in the explosive diversification of placental mammals.[10] Although all mammalian placentas have the same functions, there are important differences in structure and function in different groups of mammals. For example, human, bovine, equine and canine placentas are very different at both the gross and the microscopic levels. Placentas of these species also differ in their ability to provide maternalimmunoglobulins to the fetus.[11]
Placental mammals, including humans, have achorioallantoic placenta that forms from thechorion andallantois. In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by anumbilical cord of approximately 55–60 cm (22–24 inch) in length, which contains twoumbilical arteries and oneumbilical vein.[12] The umbilical cord inserts into thechorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules calledcotyledons. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species.[13]
The placenta occasionally takes a form in which it comprises several distinct parts connected by blood vessels.[14] The parts, called lobes, may number two, three, four, or more. Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, and so on. If there is a clearly discernible main lobe and auxiliary lobe, the latter is called asuccenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation duringlabor, which is calledvasa previa.[citation needed]
About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta.[15][16] Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific. The corresponding specific proteins are mainly expressed introphoblasts and have functions related topregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues arePEG10 and thecancer testis antigen PAGE4 and expressed incytotrophoblasts,CSH1 andKISS1 expressed insyncytiotrophoblasts, andPAPPA2 andPRG2 expressed in extravillous trophoblasts.
The outer layer of the late blastocyst, is formed oftrophoblasts, cells that form the outer layer of the placenta. This outer layer is divided into two further layers: the underlyingcytotrophoblast layer and the overlyingsyncytiotrophoblast layer. The syncytiotrophoblast is amultinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblasts, a process that continues throughout placental development. The syncytiotrophoblast contributes to the barrier function of the placenta.[18]
The placenta grows throughoutpregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).[17]
Maternal blood fills theintervillous space, nutrients, water, and gases are actively and passively exchanged, then deoxygenated blood is displaced by the next maternal pulse.
In preparation for implantation of the blastocyst, the endometrium undergoesdecidualization.Spiral arteries in thedecidua are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fillsintervillous space through these spiral arteries which bathe the fetalvilli in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetalchorion, though no fluid is exchanged. As the pressure decreases betweenpulses, the deoxygenated blood flows back through the endometrial veins.[citation needed]
Maternal blood flow begins between days 5–12,[19] and is approximately 600–700 ml/min at term.
Deoxygenated fetal blood passes throughumbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to formchorionic arteries. Chorionic arteries, in turn, branch intocotyledon arteries. In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier").[20]
The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessivefree radicals. This may contribute topre-eclampsia and otherpregnancy complications.[22] It is proposed thatmelatonin plays a role as anantioxidant in the placenta.[22]
Placental expulsion begins as a physiological separation from the wall of the uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor".
Placental expulsion can be managed actively, for example by givingoxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and the risk ofpostpartum bleeding may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary.[24]
The habit is to cut the cord immediately after birth, but it may be no medical reason to do this; on the contrary, not cutting the cord could sometimes help the baby in itsadaptation to extrauterine life, for preterm infants.[25]
The placenta is traditionally thought to besterile, but recent research suggests that a resident,non-pathogenic, and diverse population ofmicroorganisms may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research.[26][27][28][29]
The placenta intermediates the transfer of nutrients between mother and fetus. The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products andcarbon dioxide back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via bothactive andpassive transport.[30] Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients.[31] Adverse pregnancy situations, such as those involving maternaldiabetes orobesity, can increase or decrease levels of nutrient transporters in the placenta potentially resulting in overgrowth or restricted growth of the fetus.[32]
The placenta functions as a selective barrier between maternal and fetal cells, preventing maternal blood, proteins andmicrobes (includingbacteria and mostviruses) from crossing the maternal-fetal barrier.[33]Deterioration in placental functioning, referred to asplacental insufficiency, may be related tomother-to-child transmission of some infectious diseases.[34]A very small number of viruses includingrubella virus,Zika virus andcytomegalovirus (CMV) can travel across the placental barrier, generally taking advantage of conditions at certain gestational periods as the placenta develops. CMV and Zika travel from the maternal bloodstream via placental cells to the fetal bloodstream.[33][35][36][37]
Beginning as early as 13 weeks of gestation, and increasing linearly, with the largest transfer occurring in the third trimester,IgG antibodies can pass through the human placenta, providing protection to the fetusin utero.[38][39]This passive immunity lingers for several months after birth, providing the newborn with a carbon copy of the mother's long-termhumoral immunity to see the infant through the crucial first months of extrauterine life.IgM antibodies, because of their larger size, cannot cross the placenta,[40] one reason why infections acquired during pregnancy can be particularly hazardous for the fetus.[41]
The first hormone released by the placenta is called thehuman chorionic gonadotropin (hCG) hormone. This is responsible for stopping the process at the end of menses when thecorpus luteum ceases activity and atrophies. If hCG did not interrupt this process, it would lead to spontaneous abortion of the fetus. The corpus luteum also produces and releasesprogesterone andestrogen, and hCG stimulates it to increase the amount that it releases. hCG is the indicator of pregnancy thatpregnancy tests look for. These tests will work when menses has not occurred or after implantation has happened on days seven to ten. hCG may also have an anti-antibody effect, protecting it from being rejected by the mother's body. hCG also assists the male fetus by stimulating the testes to produce testosterone, which is the hormone needed to allow the sex organs of the male to grow.
Progesterone helps theembryo implant by assisting passage through the fallopian tubes. It also affects thefallopian tubes and theuterus by stimulating an increase in secretions necessary for fetal nutrition. Progesterone, like hCG, is necessary to prevent spontaneous abortion because it prevents contractions of the uterus and is necessary for implantation.
Estrogen is a crucial hormone in the process of proliferation. This involves the enlargement of the breasts and uterus, allowing for growth of the fetus and production of milk. Estrogen is also responsible for increased blood supply towards the end of pregnancy throughvasodilation. The levels of estrogen during pregnancy can increase so that they are thirty times what a non-pregnant woman mid-cycles estrogen level would be.
There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternalcytotoxic T cells by inhibiting the response tointerleukin 2.[45]
However, the placental barrier is not the sole means of evading the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier.[46]
Thetrophoblast is the outer layer of cells of theblastocyst (see day 9 in Figure, above, showing the initial stages of human embryogenesis). Placental trophoblast cells have a unique genome-wideDNA methylation pattern determined by de novomethyltransferases duringembryogenesis.[47] This methylation pattern is principally required to regulate placental development and function, which in turn is critical for embryo survival.[47]
The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to acapacitor.[48]
Ultrasound image of human placenta and umbilical cord (color Doppler rendering) with central cord insertion and three umbilical vessels, at 20 weeks of pregnancy
The placenta often plays an important role in variouscultures, with many societies conductingrituals regarding its disposal. In theWestern world, the placenta is most oftenincinerated.[49]
Some culturesbury the placenta for various reasons. TheMāori ofNew Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth.[50] Likewise, theNavajo bury the placenta and umbilical cord at a specially chosen site,[51] particularly if the baby dies during birth.[52] InCambodia andCosta Rica, burial of the placenta is believed to protect and ensure the health of the baby and the mother.[53] If a mother dies in childbirth, theAymara ofBolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life.[54]
The placenta is believed by some communities to have power over the lives of the baby or its parents. TheKwakiutl ofBritish Columbia bury girls' placentas to give the girl skill in digging clams, and expose boys' placentas toravens to encourage futureprophetic visions. InTurkey, the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. InTransylvania andJapan, interaction with a disposed placenta is thought to influence the parents' future fertility.[citation needed]
Several cultures believe the placenta to be or have been alive, often a relative of the baby.Nepalese think of the placenta as a friend of the baby; theorang Asli andMalay populations inMalay Peninsula regard it as the baby's older sibling.[53][55]Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child.[49] Various cultures inIndonesia, such asJavanese and Malay, believe that the placenta has a spirit and needs to be buried outside the family house. Some Malays would bury the baby's placenta with apencil (if it is a boy) or aneedle andthread (if it is a girl).[55]
In some cultures, the placenta is eaten, a practice known ashuman placentophagy. In some eastern cultures, such asChina, the dried placenta (ziheche紫河车, literally "purple river car") is thought to be a healthful restorative and is sometimes used in preparations oftraditional Chinese medicine and various health products.[56] The practice of human placentophagy has become a more recent trend in western cultures and is not withoutcontroversy; its practice being consideredcannibalism is debated.
^Blackburn DG, Avanzati AM, Paulesu L (November 2015). "Classics revisited. History of reptile placentology: Studiati's early account of placentation in a viviparous lizard".Placenta.36 (11):1207–1211.doi:10.1016/j.placenta.2015.09.013.PMID26474917.
^Fujikura T, Benson RC, Driscoll SG (August 1970). "The bipartite placenta and its clinical features".American Journal of Obstetrics and Gynecology.107 (7):1013–1017.doi:10.1016/0002-9378(70)90621-6.PMID5429965.Bipartite placenta represented 4.2 per cent (366 of 8,505) of placentas of white women at the Boston Hospital for Women who were enrolled in the Collaborative Project.
^Uhlén M, Fagerberg L, Hallström BM, Lindskog C, Oksvold P, Mardinoglu A, et al. (January 2015). "Proteomics. Tissue-based map of the human proteome".Science.347 (6220): 1260419.doi:10.1126/science.1260419.PMID25613900.S2CID802377.
^"Placental blood circulation".Online course in embryology for medical students. Universities of Fribourg, Lausanne and Bern (Switzerland). Archived fromthe original on 2011-09-28.
^Simister NE, Story CM (December 1997). "Human placental Fc receptors and the transmission of antibodies from mother to fetus".Journal of Reproductive Immunology.37 (1):1–23.doi:10.1016/s0165-0378(97)00068-5.PMID9501287.
^Handwerger S, Freemark M (April 2000). "The roles of placental growth hormone and placental lactogen in the regulation of human fetal growth and development".Journal of Pediatric Endocrinology & Metabolism.13 (4):343–356.doi:10.1515/jpem.2000.13.4.343.PMID10776988.S2CID28778529.
The Placenta, gynob.com, with quotes fromWilliams Obstetrics, 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.
![Placenta weight by gestational age[58]](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aPlacenta_weight_by_gestational_age.svg)
