 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Amnion" – news ·newspapers ·books ·scholar ·JSTOR(June 2021) (Learn how and when to remove this message) |
| Amnion | |
|---|---|
 Chicken egg with amnion surrounding the embryo | |
 Human fetus, enclosed in the amnion. | |
| Details | |
| Identifiers | |
| Latin | amniosinas |
| MeSH | D000650 |
| TE | E6.0.1.2.0.0.9 |
| FMA | 80223 |
| Anatomical terminology | |
Theamnion (pl.:amnions oramnia) is a membrane that closely covers human and various otherembryos when they first form. It fills withamniotic fluid, which causes the amnion to expand and become theamniotic sac that provides a protective environment for the developing embryo. The amnion, along with thechorion, theyolk sac and theallantois protect the embryo. Inbirds,reptiles andmonotremes, the protective sac is enclosed in a shell. Inmarsupials andplacental mammals, it is enclosed in auterus.
The amnion is a feature of thevertebrate cladeAmniota, which includes reptiles, birds, andmammals.Amphibians andfish lack the amnion and thus areanamniotes (non-amniotes). The amnion stems from the extra-embryonic somaticmesoderm on the outer side and the extra-embryonicectoderm ortrophoblast on the inner side.[1]
Etymologists have traditionally assumed that the Greek term ἀμνίον (amnion) relates toAncient Greek ἀμνίον :amníon, "little lamb", a diminutive of ἀμνός :amnós, "lamb", and cognate with the English verbyean, "to bring forth young (usually lambs)". However, an alternative etymology references an ancient Greek goddess of childbirth,Eileithyia, worshipped inAmnisos (on the island of Crete) and nicknamed Ἀμνιάς (Amnias).[2][3]
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, theamniotic ectoderm, and its floor consists of the prismatic ectoderm of theembryonic disk. Outside the amniotic ectoderm is a thin layer ofmesoderm, which is continuous with that of thesomatopleure and is connected by the body-stalk with the mesodermal lining of thechorion.
When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth weekamniotic fluid (also calledliquor 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 thecoelom 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]
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 ofprotein, and frequently a trace ofsugar. 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.
Extra-amniotic pregnancy is a rare condition that results from a rupture of the amnion, leading to development of the fetus within theextraembryonic coelom.[4]
Inreptiles,birds, and manymammals the amnion develops in the following manner:
At the point of constriction where the primitive digestive tube of the embryo joins theyolk 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 thecaudal 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, theamniotic 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 orserosa.
The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.
Cats anddogs are born inside of the amnion; the mother cuts it open and eats it.
Inelephants, "The amnion is continued from the base of the umbilical cord upon theallantois, which is of considerable size, and is so interposed between thechorion and amnios, as to prevent any part of the amnios attaining the inner surface of theplacenta. 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."[5]: 348
The amniotic membrane is used as a biological dressing to heal incurable wounds.[6] For this purpose, theplacenta incesarean 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 asHIV andhepatitis, 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.
This article incorporates text in thepublic domain frompage 56 of the 20th edition ofGray's Anatomy(1918)
