View source for Chorion

{{short description|Outermost fetal membrane around the embryo in amniotes}}
{{For|the entertainment company|Chorion (company)}}
{{Infobox embryology
| Name        = Chorion
| Latin       = chorion
| Greek       = 
| Image       = Chicken egg diagram.svg
| Caption     =Diagram showing the chorion of a chicken egg
| Width       = 
| Image2      = Gray30.png
| Caption2    = Human fetus enclosed in the amnion
| Precursor   = 
| System      = 
| Artery      = 
| Vein        = 
| Nerve       = 
| Lymph       = 
}}

The '''chorion''' is the outermost [[fetal membrane]] around the [[embryo]] in [[mammal]]s, [[bird]]s and [[reptiles]] ([[amniotes]]). It is also present around the embryo of other animals, like [[insect]]s and [[mollusc]]s. 

==Structure==
In humans and other [[theria]]n mammals, the chorion is one of the [[fetal membrane]]s that exist during [[pregnancy]] between the developing [[fetus]] and mother. The chorion and the [[amnion]] together form the [[amniotic sac]].  In humans it is formed by extraembryonic [[mesoderm]] and the two layers of [[trophoblast]] that surround the embryo and other membranes;<ref>{{cite book |last1=Martin |first1=Laura |last2=Richardson |first2=Lauren |last3=Menon |first3=Ramkumar |title=Encyclopedia of Reproduction |date=1 January 2018 |publisher=Academic Press |isbn=978-0-12-815145-7 |pages=387–398 |url=https://doi.org/10.1016/B978-0-12-801238-3.64498-7 |chapter=Characteristics, Properties, and Functionality of Fetal Membranes: An Overlooked Area in the Field of Parturition |edition=Second |doi=10.1016/B978-0-12-801238-3.64498-7 }}</ref> the [[chorionic villi]] emerge from the chorion, invade the [[endometrium]], and allow the transfer of nutrients from maternal blood to fetal blood.

===Layers===
The chorion consists of two layers: an outer formed by the [[trophoblast]], and an inner formed by the extra-embryonic [[mesoderm]].

The trophoblast is made up of an internal layer of cubical or prismatic cells, the [[cytotrophoblast]] or layer of Langhans, and an external [[multinucleate]]d layer,  the [[syncytiotrophoblast]].

===Growth===
The chorion undergoes rapid proliferation and forms numerous [[process (anatomy)|process]]es, the ''[[chorionic villi]]'', which invade and destroy the [[uterine decidua]], while simultaneously absorbing nutritive materials from it for the growth of the [[embryo]].

The chorionic villi are at first small and non-vascular, and consist of the trophoblast only, but they increase in size and [[wikt:ramify|ramify]], whereas the mesoderm, carrying branches of the umbilical vessels, grows into them, and they are [[vascularized]].

Blood is carried to the villi by the paired [[umbilical arteries]], which branch into [[chorionic arteries]] and enter the [[chorionic villi]] as [[cotyledon arteries]]. After circulating through the capillaries of the villi, the blood is returned to the embryo by the [[umbilical vein]]. Until about the end of the second month of [[pregnancy]], the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.

===Parts===
[[File:Placenta with fetal membranes.jpg|thumb|[[Placenta]] with attached [[fetal membranes]] (ruptured at the margin at the left in the image), which consists of the chorion (outer layer) and amnion (inner layer).]]
The part of the chorion that is in contact with the [[Decidua#Structure|decidua capsularis]] undergoes atrophy, so that by the fourth month scarcely a trace of the villi is left. This part of the chorion becomes smooth,<ref name="Placenta">{{cite journal |last1=Genbačev |first1=O |last2=Vićovac |first2=L |last3=Larocque |first3=N |title=The role of chorionic cytotrophoblasts in the smooth chorion fusion with parietal decidua. |journal=Placenta |date=July 2015 |volume=36 |issue=7 |pages=716–22 |doi=10.1016/j.placenta.2015.05.002 |pmid=26003500|pmc=4476638 }}</ref> and is named the '''chorion laeve''' (from the Latin word ''levis'', meaning smooth). As it takes no share in the formation of the placenta, this is also named the non-placental part of the chorion. As the chorion grows, the chorion laeve comes in contact with the decidua parietalis and these layers fuse.

The villi at the embryonic pole, which is in contact with the [[decidua basalis]], increase greatly in size and complexity, and hence this part is named the '''chorion frondosum'''.<ref name="Placenta"/>

Thus the placenta develops from the chorion frondosum and the decidua basalis.

===Monochorionic twins===
{{main|Monochorionic twins}}
''Monochorionic twins'' are [[twins]] that share the same [[placenta]]. This occurs in 0.3% of all multiple-birth human pregnancies,<ref name=cordero>{{cite journal |vauthors=Cordero L, Franco A, Joy SD, O'shaughnessy RW |title=Monochorionic diamniotic infants without twin-to-twin transfusion syndrome |journal=Journal of Perinatology |volume=25 |issue=12 |pages=753–8 |date=December 2005 |pmid=16281049 |doi=10.1038/sj.jp.7211405|doi-access=free }}</ref> and in 75% of [[monozygotic]] (identical) twins, when the split takes place on or after the third day after [[fertilization]].<ref name=Shulman>{{cite book |last1=Shulman |first1=Lee S. |last2=van Vugt |first2=John M. G. |title=Prenatal medicine |url=https://archive.org/details/prenatalmedicine00john |url-access=limited |publisher=Taylor & Francis |location=Washington, DC |year=2006 |page=[https://archive.org/details/prenatalmedicine00john/page/n473 447] |isbn=0-8247-2844-0}}</ref> The remaining 25% of monozygous twins become ''dichorionic diamniotic''.<ref name=Shulman/> The condition may affect any type of [[multiple birth]], resulting in ''monochorionic multiples''.

===Infections===
Recent studies indicate that the chorion may be susceptible to [[Pathogen|pathogenic infections]].<ref>{{cite journal |vauthors=Contini C, Rotondo JC, Magagnoli F, Maritati M, Seraceni S, Graziano A | title = Investigation on silent bacterial infections in specimens from pregnant women affected by spontaneous miscarriage. | journal = J Cell Physiol | volume = 34| issue = 3 | pages = 433–440 | pmid =    30078192 | doi =10.1002/jcp.26952| year = 2019 | doi-access = free | hdl = 11392/2393176 | hdl-access = free }}</ref> Recent findings indicate that ''[[Ureaplasma parvum]]'' [[bacteria]] can infect the chorion tissue, thereby impacting pregnancy outcome.<ref>{{cite journal |vauthors=Contini C, Rotondo JC, Magagnoli F, Maritati M, Seraceni S, Graziano A, Poggi A, Capucci R, Vesce F, Tognon M, Martini F | title = Investigation on silent bacterial infections in specimens from pregnant women affected by spontaneous miscarriage. | journal = J Cell Physiol | volume = 234| issue = 1 | pages = 100–9107 | date = 2018 | pmid = 30078192 | doi =10.1002/jcp.26952| doi-access = free | hdl = 11392/2393176 | hdl-access = free }}</ref> In addition, footprints of [[JC polyomavirus]] and [[Merkel cell polyomavirus]] have been detected in chorionic villi from females affected by spontaneous abortion as well as pregnant women.<ref>{{cite journal |vauthors=Tagliapietra A, Rotondo JC, Bononi I, Mazzoni E, Magagnoli F, Maritati M | title = Footprints of BK and JC polyomaviruses in specimens from females affected by spontaneous abortion. | journal = Hum Reprod | volume = 34| issue = 3 | pages = 433–440 | date = 2019 | pmid =30590693 | doi =10.1002/jcp.27490| hdl = 11392/2397717 | s2cid = 53106591 |url=https://academic.oup.com/humrep/article-abstract/34/3/433/5259177?redirectedFrom=fulltext | hdl-access = free }}</ref><ref>{{cite journal |vauthors=Tagliapietra A, Rotondo JC, Bononi I, Mazzoni E, Magagnoli F, Maritati M | title = Droplet-digital PCR assay to detect Merkel cell polyomavirus sequences in chorionic villi from spontaneous abortion affected females | journal = J Cell Physiol | volume = 235| issue =  3| pages = 1888–1894| pmid =    31549405| doi =10.1002/jcp.29213| year = 2020 | doi-access = free| hdl = 11392/2409453| hdl-access = free}}</ref> Another [[virus]], [[BK polyomavirus]] has been detected in the same tissues, but with lesser extent.<ref>{{cite journal |vauthors=Tagliapietra A, Rotondo JC, Bononi I, Mazzoni E, Magagnoli F, Maritati M | title = Footprints of BK and JC polyomaviruses in specimens from females affected by spontaneous abortion. | journal = Hum Reprod | volume = 34| issue = 3 | pages = 433–440 | date = 2019 | pmid =30590693 | doi =10.1002/jcp.27490| hdl = 11392/2397717 | s2cid = 53106591 |url=https://academic.oup.com/humrep/article-abstract/34/3/433/5259177?redirectedFrom=fulltext | hdl-access = free }}</ref>

== Other animals==
[[File: Amniote embryo.jpg|thumb|220px|Amniotic embryo. a=embryo, b=yolk, c=allantois, d=amnion, e=chorion]]
In [[reptile]]s, [[bird]]s, and [[monotreme]]s, the chorion is one of the four [[extraembryonic membrane]]s that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival.  It is located inside the albumen, which is the white of the egg.  It encloses the embryo and the rest of the embryonic system.  The chorion is also present in [[Insect#Reproduction and development|insect]]s.  During growth and development of the embryo, there is an increased need for oxygen.  To compensate for this, the chorion and the [[allantois]] fuse together to form the [[chorioallantoic membrane]]. Together these form a double membrane, which functions to remove carbon dioxide and to replenish oxygen through the porous shell.  At the time of hatching, the fetus becomes detached from the chorion as it emerges from the shell.

In [[insect]]s, it develops by the [[follicle cell]]s while the [[Egg (biology)|egg]] is in the [[ovary]].<ref name="chapman_egg">{{cite book |last1=Chapman |first1=Reginald Frederick |title=The Insects: Structure and Function |date=1998 |publisher=Cambridge University Press |isbn=978-0-521-57890-5 |chapter=The egg and embryology |pages=325–362 |chapter-url={{GBurl|vOkIvV0MrvYC|p=325}} }}</ref>  Some [[mollusks]] also have chorions as part of their eggs.  For example, fragile [[octopus]] eggs have only a chorion as their envelope.<ref>{{cite book |last1=Boletzky |first1=S.v. |title=Biology of Early Life Stages in Cephalopod Molluscs |series=Advances in Marine Biology |date=2003 |volume=44 |pages=143–203 |doi=10.1016/S0065-2881(03)44003-0 |isbn=978-0-12-026144-4 |quote=The Octopoda are characterized by eggs that have only a chorion as an envelope }}</ref>

===Additional images===
<gallery>
 Image:Gray21.png|Section through the embryo.
 Image:Gray24.png|Diagram of the human embryo.
 Image:Gray25.png|Diagram illustrating early formation of [[allantois]] and differentiation of body-stalk.
 Image:Gray26.png|Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
 Image:Gray39.png|Scheme of placental circulation.
</gallery>

==See also==
* [[Choriogenesis]]
* [[Chorioamnionitis]], an [[inflammation]] of the chorion and amnion, usually due to bacterial infection
* [[Chorionic hematoma]]
* [[Gestational trophoblastic disease]], any abnormal proliferation of the [[trophoblasts]], including [[choriocarcinoma]], a highly invasive cancer.

==References==
{{Gray's}}
{{Reflist}}

==External links==
* {{BUHistology|19903loa}} &nbsp;— "Female Reproductive System: placenta, chorionic plate"
* [https://web.archive.org/web/20051226102224/http://sprojects.mmi.mcgill.ca/embryology/earlydev/week2/chorion%26ys.html McGill]

{{Extraembryonic and fetal membranes}}
{{Development of mammalian circulatory system}}
{{Portal bar|Anatomy}}
{{Authority control}}

[[Category:Amniotic sac]]
[[Category:Embryology]]