Editing Placenta

{{short description|Organ that connects the fetus to the uterine wall}}
{{About|the temporary organ|the expulsion of the placenta after birth|placental expulsion|equivalent organs in other animals|Placentalia|other uses}}
{{Hatnote|For a less technical description, see the Simple English Wikipedia version at [[:simple:Placenta]]}}
{{Infobox embryology
|  Name          = Placenta
|  Latin         = placenta
|  Image        = Healthy Placenta.jpg
|  Caption      = Human placenta from [[Postpartum period|just after birth]] with the [[umbilical cord]] in place
|  Image2         = Placenta.svg
|  Caption2       = Human placenta shown in [[uterus]] connected to [[fetus]] with [[umbilical cord]]
|  System        =
|  CarnegieStage =
|  Days          =
|  Precursor     = [[Decidua basalis]], [[chorion frondosum]]
|  GivesRiseTo   =
}}
The '''placenta''' ({{plural form}}: '''placentas''' or '''placentae''') is a temporary [[embryo|embryonic]] and later [[Fetus|fetal]] [[organ (biology)|organ]] that begins [[embryonic development|developing]] from the [[blastocyst]] shortly after [[implantation (embryology)|implantation]]. It plays critical roles in facilitating nutrient, gas, and waste exchange between the physically separate maternal and fetal circulations, and is an important [[Endocrine system|endocrine organ]], producing [[hormones]] that regulate both [[Maternal physiological changes in pregnancy|maternal]] and fetal [[physiology]] during [[pregnancy]].<ref name="Jin">{{cite journal | vauthors = Jin M, Xu Q, Li J, Xu S, Tang C | title = Micro-RNAs in Human Placenta: Tiny Molecules, Immense Power | journal = Molecules | volume = 27 | issue = 18 | page = 5943 | date = September 2022 | pmid = 36144676 | pmc = 9501247 | doi = 10.3390/molecules27185943 | doi-access = free }}</ref> The placenta connects to the fetus via the [[umbilical cord]], and on the opposite aspect to the maternal [[uterus]] in a [[species]]-dependent manner. In humans, a thin layer of maternal [[decidua]]l ([[Endometrium|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 of [[placental mammal]]s, but are also found in [[marsupials]] and some non-mammals with varying levels of development.<ref>{{cite book | vauthors = Pough FH, Andrews RM, Cadle JE, Crump ML, Savitsky AH, Wells KD |year=2004 |title=Herpetology |edition=3rd |publisher=Pearson |isbn=978-0-13-100849-6 }}{{page needed|date=August 2021}}</ref>

[[Mammalian]] placentas probably first evolved about 150 million to 200 million years ago. The protein [[syncytin]], found in the outer barrier of the placenta (the [[syncytiotrophoblast]]) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient [[retrovirus]]: essentially a [[Virus#Role in evolution|virus]] that helped pave the transition from [[Oviparity|egg-laying]] to [[Viviparity|live-birth]].<ref>{{cite news | vauthors = Mitra A |title=How the placenta evolved from an ancient virus |url=https://whyy.org/segments/the-placenta-went-viral-and-protomammals-were-born/ |website=[[WHYY-FM|WHYY]] |date=31 January 2020 |access-date=9 March 2020}}</ref><ref>{{cite journal | vauthors = Chuong EB | title = The placenta goes viral: Retroviruses control gene expression in pregnancy | journal = PLOS Biology | volume = 16 | issue = 10 | pages = e3000028 | date = October 2018 | pmid = 30300353 | pmc = 6177113 | doi = 10.1371/journal.pbio.3000028 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Villarreal LP | title = Viruses and the placenta: the essential virus first view | journal = APMIS | volume = 124 | issue = 1–2 | pages = 20–30 | date = January 2016 | pmid = 26818259 | doi = 10.1111/apm.12485 | s2cid = 12042851 | doi-access = free }}</ref>

The word ''placenta''  comes from the [[Latin]] word for [[placenta cake|a type of cake]], from [[Ancient Greek|Greek]] πλακόεντα/πλακοῦντα ''plakóenta/plakoúnta'', accusative of πλακόεις/πλακούς ''plakóeis/plakoús'', "flat, slab-like",<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2383445 Henry George Liddell, Robert Scott, "A Greek-English Lexicon", at Perseus] {{webarchive|url=https://web.archive.org/web/20120405162717/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3D%2383445 |date=2012-04-05 }}.</ref><ref>[http://www.etymonline.com/index.php?term=placenta "placenta"] {{webarchive|url=https://web.archive.org/web/20160130235904/http://www.etymonline.com/index.php?term=placenta |date=2016-01-30 }}. ''[[Online Etymology Dictionary]]''.</ref> with reference to its round, flat appearance in humans. The classical plural is ''placentae'', but the form ''placentas'' is more common in modern English.

==Evolution and phylogenetic diversity==
The placenta has evolved independently multiple times, probably starting in [[fish]], where it originated multiple times, including the genus ''[[Poeciliopsis]].''<ref>{{cite journal | vauthors = Reznick DN, Mateos M, Springer MS | title = Independent origins and rapid evolution of the placenta in the fish genus Poeciliopsis | journal = Science | volume = 298 | issue = 5595 | pages = 1018–1020 | date = November 2002 | pmid = 12411703 | doi = 10.1126/science.1076018 | bibcode = 2002Sci...298.1018R | s2cid = 2372572 }}</ref> Placentation has also evolved in some [[reptile]]s.<ref>{{cite journal | vauthors = Blackburn DG, Avanzati AM, Paulesu L | title = Classics revisited. History of reptile placentology: Studiati's early account of placentation in a viviparous lizard | journal = Placenta | volume = 36 | issue = 11 | pages = 1207–1211 | date = November 2015 | pmid = 26474917 | doi = 10.1016/j.placenta.2015.09.013 }}</ref>

The '''mammalian placenta''' evolved more than 100 million years ago and was a critical factor in the explosive diversification of placental mammals.<ref>{{cite journal | vauthors = Ayala FJ | title = Darwin's greatest discovery: design without designer | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = Suppl 1 | pages = 8567–8573 | date = May 2007 | pmid = 17494753 | pmc = 1876431 | doi = 10.1073/pnas.0701072104 | doi-access = free }}</ref> 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 maternal [[Antibody|immunoglobulins]] to the fetus.<ref>{{cite web | vauthors = Bowen R | url = http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/index.html | title = Implantation and Development of the Placenta: Introduction and Index. | work = Pathophysiology of the Reproductive System | publisher = Colorado State | access-date = 7 July 2019 }}</ref>

== Structure ==
Placental mammals, including humans, have a '''chorioallantoic placenta''' that forms from the [[chorion]] and [[allantois]]. In humans, the placenta averages 22&nbsp;cm (9&nbsp;inch) in length and 2–2.5&nbsp;cm (0.8–1&nbsp;inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500&nbsp;grams (just over 1&nbsp;lb).  It has a dark reddish-blue or crimson color. It connects to the fetus by an [[umbilical cord]] of approximately 55–60&nbsp;cm (22–24&nbsp;inch) in length, which contains two [[umbilical artery|umbilical arteries]] and one [[umbilical vein]].<ref name="pmid9518951">{{cite journal | vauthors = Yetter JF | title = Examination of the placenta | journal = American Family Physician | volume = 57 | issue = 5 | pages = 1045–54 | date = March 1998 | pmid = 9518951 | doi = | url = http://www.aafp.org/afp/980301ap/yetter.html | archive-url = https://web.archive.org/web/20111016175756/http://www.aafp.org/afp/980301ap/yetter.html | archive-date=2011-10-16 }}</ref> The umbilical cord inserts into the [[Chorionic vessels|chorionic 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 called [[Cotyledon (placenta)|cotyledon]]s. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species.<ref>{{cite web | url = http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/structure.html | title = Placental Structure and Classification | archive-url = https://web.archive.org/web/20160211110831/http://www.vivo.colostate.edu/hbooks/pathphys/reprod/placenta/structure.html | archive-date=2016-02-11 | publisher = Colorado State }}</ref>

The placenta occasionally takes a form in which it comprises several distinct parts connected by blood vessels.<ref name="pmid_5429965">{{cite journal | vauthors = Fujikura T, Benson RC, Driscoll SG | title = The bipartite placenta and its clinical features | journal = American Journal of Obstetrics and Gynecology | volume = 107 | issue = 7 | pages = 1013–1017 | date = August 1970 | pmid = 5429965 | doi = 10.1016/0002-9378(70)90621-6 | quote = 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. }}</ref> 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 a ''succenturiate placenta''. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during [[childbirth|labor]], which is called ''[[vasa praevia|vasa previa]]''.{{citation needed|date=September 2023}}

=== Gene and protein expression ===
{{Further |Bioinformatics#Gene and protein expression}}
About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/placenta|title=The human proteome in placenta - The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-26|url-status=live|archive-url=https://web.archive.org/web/20170926191014/https://www.proteinatlas.org/humanproteome/placenta|archive-date=2017-09-26}}</ref><ref>{{cite journal | vauthors = Uhlén M, Fagerberg L, Hallström BM, Lindskog C, Oksvold P, Mardinoglu A, Sivertsson Å, Kampf C, Sjöstedt E, Asplund A, Olsson I, Edlund K, Lundberg E, Navani S, Szigyarto CA, Odeberg J, Djureinovic D, Takanen JO, Hober S, Alm T, Edqvist PH, Berling H, Tegel H, Mulder J, Rockberg J, Nilsson P, Schwenk JM, Hamsten M, von Feilitzen K, Forsberg M, Persson L, Johansson F, Zwahlen M, von Heijne G, Nielsen J, Pontén F | display-authors = 6 | title = Proteomics. Tissue-based map of the human proteome | journal = Science | volume = 347 | issue = 6220 | pages = 1260419 | date = January 2015 | pmid = 25613900 | doi = 10.1126/science.1260419 | s2cid = 802377 }}</ref> 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 in [[trophoblast]]s and have functions related to [[pregnancy]]. Examples of proteins with elevated expression in placenta compared to other organs and tissues are [[PEG10]] and the [[Cancer/testis antigens|cancer testis antigen]] PAGE4 and expressed in [[cytotrophoblast]]s, [[Human placental lactogen|CSH1]] and [[Kisspeptin|KISS1]] expressed in [[syncytiotrophoblast]]s, and [[PAPPA2]] and [[Major basic protein|PRG2]] expressed in extravillous trophoblasts.

==Physiology==

===Development===
[[File:Placenta.png|thumb|Placenta]]
[[File:HumanEmbryogenesis.svg|thumb|300px|The initial stages of [[human embryogenesis]]]]
{{Further|Placentation}}
The placenta begins to develop upon [[Implantation (embryology)|implantation]] of the [[blastocyst]] into the maternal [[endometrium]], very early on in pregnancy at about week 4.<ref name="Merck">{{Cite web |title=Stages of Development of the Fetus - Women's Health Issues |url=https://www.merckmanuals.com/home/women-s-health-issues/normal-pregnancy/stages-of-development-of-the-fetus |access-date=2022-06-12 |website=Merck Manuals Consumer Version |language=en-US}}</ref>

The outer layer of the late blastocyst, is formed of [[trophoblast]]s, cells that form the outer layer of the placenta. This outer layer is divided into two further layers: the underlying [[cytotrophoblast]] layer and the overlying [[syncytiotrophoblast]] layer. The syncytiotrophoblast is a [[multinucleate]]d 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.<ref>{{Cite web |title=How Your Fetus Grows During Pregnancy |url=https://www.acog.org/en/womens-health/faqs/how-your-fetus-grows-during-pregnancy |access-date=2022-06-12 |website=www.acog.org |language=en}}</ref>

The placenta grows throughout [[pregnancy]]. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).<ref name="Merck" />

===Placental circulation===
[[File:Gray39.png|thumb|Maternal blood fills the [[intervillous space]], nutrients, water, and gases are actively and passively exchanged, then deoxygenated blood is displaced by the next maternal pulse.]]

====Maternal placental circulation====
In preparation for implantation of the blastocyst, the endometrium undergoes [[decidualization]]. [[Spiral arteries]] in the [[decidua]] 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 fills [[intervillous space]] through these spiral arteries which bathe the fetal [[Chorionic villi|villi]] 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 fetal [[chorion]], though no fluid is exchanged. As the pressure decreases between [[pulse]]s, the deoxygenated blood flows back through the endometrial veins.{{citation needed|date=September 2023}}

Maternal blood flow begins between days 5–12,<ref>{{cite book | vauthors = Dashe JS, Bloom SL, Spong CY, Hoffman BL |title=Williams Obstetrics |date=2018 |publisher=McGraw Hill Professional |isbn=978-1-259-64433-7 }}{{page needed|date=August 2021}}</ref> and is approximately 600–700&nbsp;ml/min at term.

====Fetoplacental circulation====
{{Further|Fetal circulation}}
Deoxygenated fetal blood passes through [[umbilical arteries]] to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form [[chorionic arteries]]. Chorionic arteries, in turn, branch into [[cotyledon 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").<ref>{{cite web | url = http://www.embryology.ch/anglais/fplacenta/circulplac01.html#placentaire | title = Placental blood circulation | archive-url = https://web.archive.org/web/20110928163956/http://www.embryology.ch/anglais/fplacenta/circulplac01.html | archive-date=2011-09-28 | work = Online course in embryology for medical students | publisher = Universities of Fribourg, Lausanne and Bern (Switzerland) }}</ref>

[[Endothelin]] and [[prostanoid]]s cause [[vasoconstriction]] in placental arteries, while [[nitric oxide]] causes [[vasodilation]].<ref name=kiserud2004/> On the other hand, there is no neural vascular regulation, and catecholamines have only little effect.<ref name=kiserud2004>{{cite journal | vauthors = Kiserud T, Acharya G | title = The fetal circulation | journal = Prenatal Diagnosis | volume = 24 | issue = 13 | pages = 1049–1059 | date = December 2004 | pmid = 15614842 | doi = 10.1002/pd.1062 | s2cid = 25040285 }}</ref>

The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive [[free radical]]s. This may contribute to [[pre-eclampsia]] and other [[pregnancy complications]].<ref name="ReiterTan2013">{{cite journal | vauthors = Reiter RJ, Tan DX, Korkmaz A, Rosales-Corral SA | title = Melatonin and stable circadian rhythms optimize maternal, placental and fetal physiology | journal = Human Reproduction Update | volume = 20 | issue = 2 | pages = 293–307 | year = 2013 | pmid = 24132226 | doi = 10.1093/humupd/dmt054 | doi-access = free }}</ref> It is proposed that [[melatonin]] plays a role as an [[antioxidant]] in the placenta.<ref name="ReiterTan2013"/>

This begins at day 17–22.<ref>{{Cite book|title=Williams book of obsteritcis}}</ref>

===Birth===
{{Main|Placental expulsion}}
[[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 giving [[oxytocin]] 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 of [[postpartum 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.<ref>{{cite journal | vauthors = Begley CM, Gyte GM, Devane D, McGuire W, Weeks A, Biesty LM | title = Active versus expectant management for women in the third stage of labour | journal = The Cochrane Database of Systematic Reviews | volume = 2019 | issue = 2 | pages = CD007412 | date = February 2019 | pmid = 30754073 | pmc = 6372362 | doi = 10.1002/14651858.CD007412.pub5 }}</ref>

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 its [[adaptation to extrauterine life]], for preterm infants.<ref>{{cite journal | vauthors = Mercer JS, Vohr BR, Erickson-Owens DA, Padbury JF, Oh W | title = Seven-month developmental outcomes of very low birth weight infants enrolled in a randomized controlled trial of delayed versus immediate cord clamping | journal = Journal of Perinatology | volume = 30 | issue = 1 | pages = 11–16 | date = January 2010 | pmid = 19847185 | pmc = 2799542 | doi = 10.1038/jp.2009.170 }}</ref>

===Microbiome===
{{main|Placental microbiome}}
The placenta is traditionally thought to be [[Asepsis|sterile]], but recent research suggests that a resident, [[Nonpathogenic organisms|non-pathogenic]], and diverse population of [[microorganisms]] 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.<ref name="Per2017">{{cite journal | vauthors = Perez-Muñoz ME, Arrieta MC, Ramer-Tait AE, Walter J | title = A critical assessment of the "sterile womb" and "in utero colonization" hypotheses: implications for research on the pioneer infant microbiome | journal = Microbiome | volume = 5 | issue = 1 | pages = 48 | date = April 2017 | pmid = 28454555 | pmc = 5410102 | doi = 10.1186/s40168-017-0268-4 | doi-access = free }}</ref><ref name="MorKwon2015">{{cite journal | vauthors = Mor G, Kwon JY | title = Trophoblast-microbiome interaction: a new paradigm on immune regulation | journal = American Journal of Obstetrics and Gynecology | volume = 213 | issue = 4 Suppl | pages = S131–S137 | date = October 2015 | pmid = 26428492 | pmc = 6800181 | doi = 10.1016/j.ajog.2015.06.039 }}</ref><ref name="PrinceAntony2014">{{cite journal | vauthors = Prince AL, Antony KM, Chu DM, Aagaard KM | title = The microbiome, parturition, and timing of birth: more questions than answers | journal = Journal of Reproductive Immunology | volume = 104-105 | pages = 12–19 | date = October 2014 | pmid = 24793619 | pmc = 4157949 | doi = 10.1016/j.jri.2014.03.006 }}</ref><ref>{{cite journal | vauthors = Hornef M, Penders J | title = Does a prenatal bacterial microbiota exist? | language = En | journal = Mucosal Immunology | volume = 10 | issue = 3 | pages = 598–601 | date = May 2017 | pmid = 28120852 | doi = 10.1038/mi.2016.141 | doi-access = free }}</ref>

==Physiology of placenta==
===Nutrition and gas exchange===
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 and [[Carbon dioxide#Human physiology|carbon dioxide]] back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via both [[Active transport|active]] and [[passive transport]].<ref name="Kay2011">{{cite book|title=The Placenta: From Development to Disease|url=https://archive.org/details/placentafromdeve00kayh|url-access=limited|vauthors = Wright C, Sibley CP|publisher=John Wiley and Sons|year=2011|isbn=978-1-4443-3366-4|veditors = Kay H, Nelson M, Yuping W|pages=[https://archive.org/details/placentafromdeve00kayh/page/n79 66]|chapter=Placental Transfer in Health and Disease}}</ref> Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients.<ref name="pmid27913585">{{cite journal | vauthors = Perazzolo S, Hirschmugl B, Wadsack C, Desoye G, Lewis RM, Sengers BG | title = The influence of placental metabolism on fatty acid transfer to the fetus | journal = Journal of Lipid Research | volume = 58 | issue = 2 | pages = 443–454 | date = February 2017 | pmid = 27913585 | pmc = 5282960 | doi = 10.1194/jlr.P072355  |doi-access=free }}</ref> Adverse pregnancy situations, such as those involving maternal [[diabetes]] or [[obesity]], can increase or decrease levels of nutrient transporters in the placenta potentially resulting in overgrowth or restricted growth of the fetus.<ref name="kappen2012">{{cite journal | vauthors = Kappen C, Kruger C, MacGowan J, Salbaum JM | title = Maternal diet modulates placenta growth and gene expression in a mouse model of diabetic pregnancy | journal = PLOS ONE | volume = 7 | issue = 6 | pages = e38445 | date = 2012 | pmid = 22701643 | pmc = 3372526 | doi = 10.1371/journal.pone.0038445 | doi-access = free | bibcode = 2012PLoSO...738445K }}</ref>

===Excretion===
Waste products excreted from the fetus such as [[urea]], [[uric acid]], and [[creatinine]] are transferred to the maternal blood by [[diffusion]] across the placenta.{{citation needed|date=September 2023}}

===Immunity===
The placenta functions as a selective barrier between maternal and fetal cells, preventing maternal blood, proteins and [[microbe]]s (including [[bacteria]] and most [[viruses]]) from crossing the maternal-fetal barrier.<ref name="Madhusoodanan"/>
Deterioration in placental functioning, referred to as [[placental insufficiency]], may be related to [[mother-to-child transmission]] of some infectious diseases.<ref name="Erlebacher">{{cite journal | vauthors = Erlebacher A | title = Immunology of the maternal-fetal interface | journal = Annual Review of Immunology | volume = 31 | issue = 1 | pages = 387–411 | date = 2013-03-21 | pmid = 23298207 | doi = 10.1146/annurev-immunol-032712-100003 }}</ref>
A very small number of viruses including ''[[rubella virus]]'', ''[[Zika virus]]'' and ''[[cytomegalovirus]]'' (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.<ref name="Madhusoodanan">{{Cite journal | vauthors = Madhusoodanan J |title=Break on through: How some viruses infect the placenta |url=https://knowablemagazine.org/article/health-disease/2018/break-through-how-some-viruses-infect-placenta |journal=Knowable Magazine |doi=10.1146/knowable-101018-1 |date=October 10, 2018 |doi-access=free}}</ref><ref name="Pereira">{{cite journal | vauthors = Pereira L | title = Congenital Viral Infection: Traversing the Uterine-Placental Interface | journal = Annual Review of Virology | volume = 5 | issue = 1 | pages = 273–299 | date = September 2018 | pmid = 30048217 | doi = 10.1146/annurev-virology-092917-043236 | s2cid = 51724379 }}</ref><ref name="Arora">{{cite journal | vauthors = Arora N, Sadovsky Y, Dermody TS, Coyne CB | title = Microbial Vertical Transmission during Human Pregnancy | journal = Cell Host & Microbe | volume = 21 | issue = 5 | pages = 561–567 | date = May 2017 | pmid = 28494237 | pmc = 6148370 | doi = 10.1016/j.chom.2017.04.007 }}</ref><ref name="Robbins">{{cite journal | vauthors = Robbins JR, Bakardjiev AI | title = Pathogens and the placental fortress | journal = Current Opinion in Microbiology | volume = 15 | issue = 1 | pages = 36–43 | date = February 2012 | pmid = 22169833 | pmc = 3265690 | doi = 10.1016/j.mib.2011.11.006 }}</ref>

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 fetus ''in utero''.<ref name="Palmeira">{{cite journal | vauthors = Palmeira P, Quinello C, Silveira-Lessa AL, Zago CA, Carneiro-Sampaio M | title = IgG placental transfer in healthy and pathological pregnancies | journal = Clinical & Developmental Immunology | volume = 2012 | pages = 985646 | date = 2012 | pmid = 22235228 | pmc = 3251916 | doi = 10.1155/2012/985646 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Simister NE, Story CM | title = Human placental Fc receptors and the transmission of antibodies from mother to fetus | journal = Journal of Reproductive Immunology | volume = 37 | issue = 1 | pages = 1–23 | date = December 1997 | pmid = 9501287 | doi = 10.1016/s0165-0378(97)00068-5 }}</ref>
This passive immunity lingers for several months after birth, providing the newborn with a carbon copy of the mother's long-term [[humoral immunity]] to see the infant through the crucial first months of extrauterine life. [[IgM]] antibodies, because of their larger size, cannot cross the placenta,<ref>{{cite book | vauthors = Pillitteri A |title=Maternal and Child Health Nursing: Care of the Childbearing and Childrearing Family |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year=2009 |isbn=978-1-58255-999-5 |url=https://books.google.com/books?id=apeLf0mPx1QC&pg=PA202|page=202}}</ref> one reason why infections acquired during pregnancy can be particularly hazardous for the fetus.<ref name="NICHD">{{cite web |title=What infections can affect pregnancy? |url=https://www.nichd.nih.gov/health/topics/pregnancy/conditioninfo/infections |website=National Institute of Child Health and Human Development |date=27 April 2021 |access-date=25 June 2021}}</ref>

===Hormonal regulation===
* The first hormone released by the placenta is called the [[human chorionic gonadotropin]] (hCG) hormone. This is responsible for stopping the process at the end of menses when the [[corpus 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 releases [[progesterone]] and [[estrogen]], and hCG stimulates it to increase the amount that it releases. hCG is the indicator of pregnancy that [[pregnancy test]]s 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 the [[embryo]] implant by assisting passage through the fallopian tubes. It also affects the [[Fallopian tube|fallopian]] tubes and the [[uterus]] 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 through [[vasodilation]]. 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.
* [[Human placental lactogen]] (hPL) is a hormone used in pregnancy to develop fetal metabolism and general growth and development. Human placental lactogen works with [[growth hormone]] to stimulate [[insulin-like growth factor]] production and regulating intermediary metabolism. In the fetus, hPL acts on lactogenic receptors to modulate embryonic development, metabolism and stimulate production of IGF, [[insulin]], surfactant and [[Adrenocortical hormone|adrenocortical]] hormones. hPL values increase with multiple pregnancies, intact molar pregnancy, [[Diabetes mellitus|diabetes]] and [[Rh incompatibility]]. They are decreased with [[Toxemia of pregnancy|toxemia]], [[choriocarcinoma]], and [[Placental insufficiency]].<ref>{{cite journal | vauthors = Handwerger S, Freemark M | title = The roles of placental growth hormone and placental lactogen in the regulation of human fetal growth and development | journal = Journal of Pediatric Endocrinology & Metabolism | volume = 13 | issue = 4 | pages = 343–356 | date = April 2000 | pmid = 10776988 | doi = 10.1515/jpem.2000.13.4.343 | s2cid = 28778529 }}</ref><ref name="ucsfhealth, 2009">{{cite web|title=Human Placental Lactogen|url=https://www.ucsfhealth.org/tests/003915.html|website=www.ucsfhealth.org|access-date=July 21, 2017|language=en|date=May 17, 2009|url-status=dead|archive-url=https://web.archive.org/web/20170429050115/https://www.ucsfhealth.org/tests/003915.html|archive-date=April 29, 2017}}</ref>

===Immunological barrier===
{{Further|Immune tolerance in pregnancy}}
The placenta and fetus may be regarded as a [[foreign body]] inside the mother and must be protected from the normal [[immune response]] of the mother that would cause it to be [[Transplant rejection|rejected]]. The placenta and fetus are thus treated as sites of [[immune privilege]], with [[immune tolerance]].

For this purpose, the placenta uses several mechanisms :
* It secretes [[neurokinin&nbsp;B]]-containing [[phosphocholine]] molecules. This is the same mechanism used by [[parasitic]] [[nematode]]s to avoid detection by the immune system of their [[host (biology)|host]].<ref>{{cite news|title=Placenta 'fools body's defences'|url=http://news.bbc.co.uk/1/hi/health/7081298.stm|date=10 November 2007|work=BBC News|url-status=live|archive-url=https://web.archive.org/web/20120429002452/http://news.bbc.co.uk/2/hi/health/7081298.stm|archive-date=29 April 2012}}</ref>
* There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal [[cytotoxic T&nbsp;cell]]s by inhibiting the response to [[interleukin&nbsp;2]].<ref name=clark>{{cite journal | vauthors = Clark DA, Chaput A, Tutton D | title = Active suppression of host-vs-graft reaction in pregnant mice. VII. Spontaneous abortion of allogeneic CBA/J x DBA/2 fetuses in the uterus of CBA/J mice correlates with deficient non-T suppressor cell activity | journal = Journal of Immunology | volume = 136 | issue = 5 | pages = 1668–1675 | date = March 1986 | pmid = 2936806 | doi = 10.4049/jimmunol.136.5.1668 | s2cid = 22815679 | doi-access = free }}</ref>

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.<ref>{{cite journal | vauthors = Williams Z, Zepf D, Longtine J, Anchan R, Broadman B, Missmer SA, Hornstein MD | title = Foreign fetal cells persist in the maternal circulation | journal = Fertility and Sterility | volume = 91 | issue = 6 | pages = 2593–2595 | date = June 2009 | pmid = 18384774 | doi = 10.1016/j.fertnstert.2008.02.008 | doi-access = free }}</ref>

===DNA methylation===

The [[trophoblast]] is the outer layer of cells of the [[blastocyst]] (see day 9 in Figure, above, showing the initial stages of human embryogenesis).  Placental trophoblast cells have a unique genome-wide [[DNA methylation]] pattern determined by de novo [[methyltransferase]]s during [[Human embryonic development|embryogenesis]].<ref name = Andrews2023>{{cite journal |vauthors=Andrews S, Krueger C, Mellado-Lopez M, Hemberger M, Dean W, Perez-Garcia V, Hanna CW |title=Mechanisms and function of de novo DNA methylation in placental development reveals an essential role for DNMT3B |journal=Nat Commun |volume=14 |issue=1 |pages=371 |date=January 2023 |pmid=36690623 |pmc=9870994 |doi=10.1038/s41467-023-36019-9 |bibcode=2023NatCo..14..371A }}</ref>  This methylation pattern is principally required to regulate placental development and function, which in turn is critical for embryo survival.<ref name = Andrews2023/>

===Other===
The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a [[capacitor]].<ref>{{cite journal | vauthors = Assad RS, Lee FY, Hanley FL | title = Placental compliance during fetal extracorporeal circulation | journal = Journal of Applied Physiology | volume = 90 | issue = 5 | pages = 1882–1886 | date = May 2001 | pmid = 11299282 | doi = 10.1152/jappl.2001.90.5.1882 | s2cid = 8785947 }}</ref>
[[File:Human placenta umbilical cord Ultrasound by Dr. W. Moroder.jpg|thumb|Ultrasound image of human placenta and umbilical cord (color Doppler rendering) with central cord insertion and three umbilical vessels, at 20 weeks of pregnancy]]

==Clinical significance==
{{Main|Placental disease}}
[[Image:CMV placentitis1 mini.jpg|thumb|right|160px|[[Micrograph]] of a [[cytomegalovirus]] (CMV) infection of the placenta (CMV [[placentitis]]). The characteristic large [[cell nucleus|nucleus]] of a CMV-infected [[cell (biology)|cell]] is seen off-centre at the bottom-right of the image. [[H&E&nbsp;stain]].]]
Numerous pathologies can affect the placenta.{{citation needed|date=September 2023}}
* [[Placenta accreta]], when the placenta implants too deeply, all the way to the actual muscle of uterine wall (without penetrating it)
* [[Placenta praevia]], when the placement of the placenta is too close to or blocks the [[cervix]]
* [[Placental abruption]], premature detachment of the placenta
* [[Placentitis]], inflammation of the placenta, such as by [[TORCH infections]].

==Society and culture==
The placenta often plays an important role in various [[culture]]s, with many societies conducting [[rituals]] regarding its disposal. In the [[Western world]], the placenta is most often [[incinerate]]d.<ref name="bbc">{{cite news |title=Why eat a placenta? |url=http://news.bbc.co.uk/2/hi/uk_news/magazine/4918290.stm |work=BBC News |date=18 April 2006 }}</ref>

Some cultures [[Burial|bury]] the placenta for various reasons. The [[Māori people|Māori]] of [[New Zealand]] traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth.<ref>{{cite journal | vauthors= Metge J |title=Working In/Playing With Three Languages |journal=Sites: A Journal of Social Anthropology and Cultural Studies |date=2005 |volume=2 |issue=2 |pages=83–90 |doi=10.11157/sites-vol2iss2id65 |doi-access=free }}</ref> Likewise, the [[Navajo people|Navajo]] bury the placenta and umbilical cord at a specially chosen site,<ref>{{cite news | vauthors = Francisco E |title=Bridging the Cultural Divide in Medicine |url=https://www.science.org/content/article/bridging-cultural-divide-medicine |work=Science |date=3 December 2004 }}</ref> particularly if the baby dies during birth.<ref>{{cite journal | vauthors = Shepardson M | title = Changes in Navajo mortuary practices and beliefs | journal = American Indian Quarterly | volume = 4 | issue = 4 | pages = 383–396 | date = 1978 | pmid = 11614175 | doi = 10.2307/1184564 | jstor = 1184564 }}</ref> In [[Cambodia]] and [[Costa Rica]], burial of the placenta is believed to protect and ensure the health of the baby and the mother.<ref name="mothering">{{cite web |url=http://www.mothering.com/articles/pregnancy_birth/birth_preparation/amazing_placenta_side.html |title=Placenta Rituals and Folklore from around the World | vauthors = Buckley SJ |work=Mothering |access-date=7 January 2008 |archive-url=https://web.archive.org/web/20080106075807/http://www.mothering.com/articles/pregnancy_birth/birth_preparation/amazing_placenta_side.html |archive-date=6 January 2008 |url-status=dead }}</ref> If a mother dies in childbirth, the [[Aymara people|Aymara]] of [[Bolivia]] bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life.<ref>{{cite journal | vauthors = Davenport A |title=The Love Offer |journal=Johns Hopkins Magazine |date=June 2005 |volume=57 |issue=3 |url=https://pages.jh.edu/jhumag/0605web/ruminate.html }}</ref>

The placenta is believed by some communities to have power over the lives of the baby or its parents. The [[Kwakiutl]] of [[British Columbia]] bury girls' placentas to give the girl skill in digging clams, and expose boys' placentas to [[raven]]s to encourage future [[prophet]]ic visions. In [[Turkey]], the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In [[Transylvania]] and [[Japan]], interaction with a disposed placenta is thought to influence the parents' future fertility.{{citation needed|date=January 2021}}

Several cultures believe the placenta to be or have been alive, often a relative of the baby. [[Nepal]]ese think of the placenta as a friend of the baby; the [[orang Asli]] and [[Malays (ethnic group)|Malay]] populations in [[Malay Peninsula]] regard it as the baby's older sibling.<ref name="mothering"/><ref name="EPM">{{Cite book | vauthors = Barakbah A |title=Ensiklopedia Perbidanan Melayu |publisher=Universiti Islam Malaysia Press |year=2017 |isbn=978-967-13305-9-3 |pages=236–237}}</ref> [[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.<ref name="bbc"/>  Various cultures in [[Indonesia]], such as [[Javanese people|Javanese]] 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 a [[pencil]] (if it is a boy) or a [[Sewing needle|needle]] and [[Thread (yarn)|thread]] (if it is a girl).<ref name="EPM" />

In some cultures, the placenta is eaten, a practice known as [[human placentophagy]]. In some eastern cultures, such as [[China]], the dried placenta (''ziheche'' {{linktext|紫|河|车}}, literally "purple river car") is thought to be a healthful restorative and is sometimes used in preparations of [[traditional Chinese medicine]] and various health products.<ref>{{cite web|url=http://www.gentlebirth.org/archives/eatplcnt.html|title=Medicinal Uses of the Placenta| vauthors = Falcao R |access-date=25 November 2008|url-status=live|archive-url=https://web.archive.org/web/20081205030123/http://www.gentlebirth.org/archives/eatplcnt.html|archive-date=5 December 2008}}</ref>  The practice of human placentophagy has become a more recent trend in western cultures and is not without [[Human placentophagy#Controversy|controversy]]; its practice being considered [[Human cannibalism|cannibalism]] is debated.

Some cultures have [[alternative uses for placenta]] that include the manufacturing of cosmetics, pharmaceuticals and food.<ref>{{Cite journal| vauthors = Kroløkke C, Dickinson E, Foss KA |date=May 2018|title=The placenta economy: From trashed to treasured bio-products|url=http://journals.sagepub.com/doi/10.1177/1350506816679004|journal=European Journal of Women's Studies|language=en|volume=25|issue=2|pages=138–153|doi=10.1177/1350506816679004|s2cid=151874106 |issn=1350-5068|url-access=subscription}}</ref>

==Additional images==
<gallery>
File:Human placenta baby side.jpg |Human placenta immediately post birth.
File:Gray30.png|Fetus of about 8&nbsp;weeks, enclosed in the amnion. Magnified a little over two diameters.
File:Placenta with fetal membranes.jpg|Placenta with attached [[fetal membranes]], ruptured at the margin at the left in the image
File:CMV placentitis1.jpg|[[Micrograph]] of a placental [[infection]] ([[cytomegalovirus|CMV]] placentitis)
File:CMV placentitis2.jpg|Micrograph of CMV placentitis
File:Placenta vasculature 3D power doppler 00001.gif|A 3D Power Doppler image of vasculature in 20-week placenta
File:2910 The Placenta-02.jpg|Schematic view of the placenta
File:Human placenta 01.JPG|Maternal side of a whole human placenta, just after birth
File:Human placenta 02.JPG|Fetal side of same placenta
File:Fetal side close-up of freshly delivered placenta.jpeg|Close-up of umbilical attachment to fetal side of freshly delivered placenta
File:Placenta weight by gestational age.svg|Placenta weight by gestational age<ref>{{cite book |doi=10.1017/9781316848616.039 |chapter=Placental Weights: Means, Standard Deviations, and Percentiles by Gestational Age |title=Placental and Gestational Pathology |year=2017 |page=336 |isbn=978-1-316-84861-6 }}</ref>
File:Ziheche.jpg|''Ziheche'' (紫河车), dried human placenta used in traditional Chinese medicine
</gallery>

== See also ==
* [[Caul]]
* [[Choriovitelline placenta]]
* [[Pregnancy in fish]]
* [[Zygote]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Placenta}}
{{Wiktionary|placenta}}
* [https://www.proteinatlas.org/humanproteome/placenta The placenta-specific proteome] at [https://www.proteinatlas.org the Human Protein Atlas]
* [http://www.gynob.com/placenta.htm The Placenta], gynob.com, with quotes from [[Williams Obstetrics]], 18th Edition, F. Gary Cunningham, M.D., Paul C. MacDonald, M.D., Norman F. Grant, M.D., Appleton & Lange, Publishers.

{{Extraembryonic and fetal membranes}}
{{Development of circulatory system}}

{{Authority control}}

[[Category:Placenta| ]]
[[Category:Vertebrate developmental biology]]
[[Category:Embryology of cardiovascular system]]
[[Category:Organs (anatomy)]]
[[Category:Reproductive system]]
[[Category:Microbiomes]]
[[Category:Human female endocrine system]]