View source for Amnion

{{Short description|Innermost membranous sac that surrounds and protects a developing embryo}}
{{For|the fictional alien species|Amnion (Gap Cycle)}}
{{More citations needed|date=June 2021}}
{{Infobox anatomy
| Name        = Amnion
| Latin       = amniosinas
| Image       = Chicken egg diagram.svg
| Caption     = Chicken egg with amnion surrounding the embryo
| Image2      = Gray30.png
| Caption2    = Human fetus, enclosed in the amnion.
| System      =
}}
{{wikt | amnion}}
The '''amnion''' ({{plural form}}: '''amnions''' or '''amnia''') is a membrane that closely covers human and various other [[embryo]]s when they first form. It fills with [[amniotic fluid]], which causes the amnion to expand and become the [[amniotic sac]] that provides a protective environment for the developing embryo. The amnion, along with the [[chorion]], the [[yolk sac]] and the [[allantois]] protect the embryo. In [[bird]]s, [[reptile]]s and [[monotreme]]s, the protective sac is enclosed in a shell. In [[marsupial]]s and [[Placentalia|placental mammal]]s, it is enclosed in a [[uterus]].

The amnion is a feature of the [[vertebrate]] clade [[amniote|''Amniota'']], which includes reptiles, birds, and [[mammal]]s. [[Amphibian]]s and [[fish]] lack the amnion and thus are [[anamniotes]] (non-amniotes). The amnion stems from the extra-embryonic somatic [[mesoderm]] on the outer side and the extra-embryonic [[ectoderm]] or [[trophoblast]] on the inner side.<ref name="Pigeon60">{{cite journal | last1= Pigeon | first1= J. | year= 1960 | journal= Can Med Assoc J | volume= 83 | issue= 16 | pages= 844–845 | pmc= 1938392 | pmid= 13735672 | title= Treatment of second-degree burns with amniotic membranes}}
</ref>

== Etymology ==
Etymologists have traditionally assumed that the Greek term ἀμνίον (''amnion'') relates to [[Ancient Greek]] ἀμνίον : {{lang | grc | amníon}}, "little lamb", a diminutive of ἀμνός : {{lang | grc | amnós}}, "lamb", and cognate with the English verb ''{{linktext|yean}}'', "to bring forth young (usually lambs)". However, an alternative etymology references an ancient Greek goddess of childbirth, [[Eileithyia]], worshipped in [[Amnisos]] (on the island of Crete) and nicknamed Ἀμνιάς (Amnias).<ref name="Singer">{{cite journal | last1 =Singer| first1 =C. | year=1959 | journal =Med. Hist. | volume =3 | issue =1 | pages =1–7 | pmc =1034442 | pmid =13632203| title =The strange histories of some anatomical terms| doi =10.1017/S0025727300024200 }}</ref><ref>
{{oed | amnios}} - "[...] it has been suggested that ἄμνιος derives from Ἀμνιάς, epithet of Eileithyia, a goddess of childbirth, which may reflect her supposed birthplace, Ἀμνισός, the port of Knossos in Crete."
</ref>

==In humans==
In the human embryo, the earliest stages of the formation of the amnion have not been observed; in the youngest embryo that has been studied the amnion was already present as a closed sac, and appears in the inner cell-mass as a cavity. This cavity is roofed in by a single stratum of flattened, ectodermal cells, the '''amniotic ectoderm''', and its floor consists of the prismatic ectoderm of the [[embryonic disk]]. Outside the amniotic ectoderm is a thin layer of [[mesoderm]], which is continuous with that of the [[somatopleure]] and is connected by the body-stalk with the mesodermal lining of the [[chorion]].

When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth week [[amniotic fluid]] (also called ''liquor amnii'') begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the chorion's inner surface, so that the extra-embryonic part of the [[coelom]] is obliterated. The amniotic fluid increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amounts to about one liter.{{citation needed|date=June 2024}}

The amniotic fluid allows the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without. It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of [[protein]], and frequently a trace of [[sugar]]. That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.

==Clinical significance==
Extra-amniotic pregnancy is a rare condition that results from a rupture of the amnion, leading to development of the fetus within the [[extraembryonic coelom]].<ref>[http://www.sonoworld.com/Fetus/page.aspx?id=405 TheFetus.net > Amniotic band syndrome] {{Webarchive|url=https://web.archive.org/web/20171128141000/https://sonoworld.com/Fetus/page.aspx?id=405 |date=2017-11-28 }} By Luís Flávio Gonçalves, MD, Philippe Jeanty, MD, PhD. 1999-09-26-18</ref>

==Other animals==
In [[reptile]]s, [[bird]]s, and many [[mammal]]s the amnion develops in the following manner:

At the point of constriction where the primitive digestive tube of the embryo joins the [[yolk sac]] a reflection or folding upward of the somatopleure takes place.

This, the amniotic fold, first makes its appearance at the cephalic extremity, and subsequently at the [[Anatomical terms of location|caudal]] end and sides of the embryo, and gradually rising, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the [[amniotic cavity]]. This kind of amnion is known as pleuroamnion (formed by folding), as opposed to schyzoamnion (formed by delamination).

After the fusion of the edges of the amniotic fold, the two layers of the fold become completely separated, the inner forming the amnion, the outer the false amnion or [[Serous membrane|serosa]].

The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.

[[Kitten|Cats]] and [[puppy|dogs]] are born inside of the amnion; the mother cuts it open and eats it.

In [[elephant]]s, "The amnion is continued from the base of the umbilical cord upon the [[allantois]], which is of considerable size, and is so interposed between the [[chorion]] and amnios, as to prevent any part of the amnios attaining the inner surface of the [[placenta]]. The amnios consists of two layers:one is the granular layer, continued upon the inner or foetal surface of the allantois, and thence upon the umbilical cord; the other is the smooth outer layer, continued upon the outer or chorional surface of the allantois, and thence upon the inner surface of the chorion."<ref name="Owen57">{{cite journal | last1=Owen | first1=R. | title=Description of the foetal membranes and placenta of the elephant (''Elephas Indicus'', Cuv.), with remarks on the value of placentary characters in the classification of the mammalia | journal=Philosophical Transactions of the Royal Society of London | volume=147 | year=1857 | pages=347–353 | jstor=108622 | doi=10.1098/rstl.1857.0017| bibcode=1857RSPT..147..347O | doi-access=free }}</ref>{{rp|348}}

== Application ==
The amniotic membrane is used as a biological dressing to heal incurable wounds.<ref>Frech, T. M., et al. (2019). "Amniotic membrane dressings: an effective therapy for SSc-related wounds." Rheumatology (Oxford) 58(4): 734-736.</ref> For this purpose, the [[placenta]] in [[cesarean delivery]] is collected and under aseptic conditions, the amniotic membrane is separated and packaged and sold commercially. In valid commercial products to prevent transmission of viral infections such as [[HIV]] and [[hepatitis]], the donor's blood (mother) is tested. Products usually pass the sterility and endotoxin test in accordance with the rules of the Food and Drug Administration of the country of manufacture.

==Additional images==
<gallery>
File:Placenta with fetal membranes.jpg|Placenta with attached [[fetal membranes]] (ruptured at the margin at the left in the image), which consists of the amnion (inner layer) and chorion (outer layer)
File:Gray14.png|Surface view of embryo of ''Hylobates concolor''.
File:Gray17.png|Human embryo—length, 2&nbsp;mm. Dorsal view, with the amnion laid open. X 30.
File:Gray21.png|Section through the embryo.
File:Gray22.png|Human embryo of 2.6&nbsp;mm.
File:Gray29.png|Diagram of a transverse section, showing the mode of formation of the amnion in the chick.
File:Gray31.png|Model of human embryo 1.3&nbsp;mm. long.
File:Gray34.png|Sectional plan of the gravid uterus in the third and fourth month.
File:Gray39.png|Scheme of placental circulation.
File:Gray459.png|Human embryo of about fourteen days, with yolk-sac.
File:Meconium-laden macrophages high mag.jpg|Meconium-laden [[macrophage]]s in [[meconium]] stained [[fetal membranes]]. [[H&E stain]].
File:Catfetus1.jpg|Opened uterus with [[cat]] fetus in midgestation: 1 [[Navel|umbilicus]], 2 [[amniotic sac]] ([[chorion]] and amnion), 3 [[allantois]], 4 [[yolk sac]], 5 developing marginal [[hematoma]], 6 maternal part of [[placenta]] ([[endometrium]])
</gallery>

==See also==
* [[Amniotic sac]]

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

==External links==
* {{BUHistology|19903loa}} - "Female Reproductive System: placenta, chorionic plate"
* [https://web.archive.org/web/20051226102224/http://sprojects.mmi.mcgill.ca/embryology/earlydev/week2/chorion%26ys.html McGill]
* [http://casereports.bmj.com/content/2009/bcr.06.2008.0081.full The Foeto-Amnio-Placental complex]
{{Extraembryonic and fetal membranes}}

{{Portal bar|Anatomy}}
{{Authority control}}

[[Category:Amniotic sac]]
[[Category:Embryology]]
[[Category:Membrane biology]]
[[Category:Biological matter]]