Share on Facebook Share on Twitter Email
Answers.com

amnion

 
Dictionary: am·ni·on   (ăm'nē-ən, -ŏn') pronunciation
n., pl., -ni·ons, or -ni·a (-nē-ə).
A thin, tough, membranous sac that encloses the embryo or fetus of a mammal, bird, or reptile. It is filled with a serous fluid in which the embryo is suspended.

[Greek amniōn.]

amniotic am'ni·ot'ic (-ŏt'ĭk) or am'ni·on'ic (-ŏn'ĭk) adj.

Search unanswered questions...
Enter a question here...
Search: All sources Community Q&A Reference topics

A thin, cellular, extraembryonic membrane forming a closed sac surrounding the embryo in all reptiles, birds, and mammals. It is present only in these forms; the collective term amniotes is applied to these animals. The amnion contains a serous fluid in which the embryo is immersed. See also Amniota.

Typically, the amnion wall is a tough, transparent, nerve-free, and nonvascular membrane consisting of two layers of cells: an inner, single-cell-thick layer of ectodermal epithelium and an outer covering of mesodermal, connective, and specialized smooth muscular tissue. Early after the formation of the amnion, waves of contraction of the muscles pass over the amniotic sac and produce a characteristic rocking of the embryo. See also Germ layers.

The major function of the amnion and its fluid is to protect the delicate embryo. Thus, developmental stages of terrestrial animals are provided with the same type of cushioning against mechanical shock as is provided by the water environment of aquatic forms. See also Fetal membrane.


Wikipedia: Amnion
Top
Amnion
Gray14.png
Surface view of embryo of Hylobates concolor.
Gray30.png
Human fetus, enclosed in the amnion.
Gray's subject #12 56
MeSH Amnion
Dorlands/Elsevier Amnion
For the alien race in Stephen R. Donaldson's The Gap Cycle, see Amnion (Gap Cycle).

The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish (Ichthyopsida), which are consequently termed “Anamniota”. The primary function of this is the protection of the embryo for its development. It stems from parts of the mesoderm on the outer side and the ectoderm on the inner side.

Contents

In humans

In the human embryo the earliest stages of the formation of the amnion have not been observed; in the youngest embryo which 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—the continuity between the roof and floor being established at the margin 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 fluid (liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the inner surface of the chorion, so that the extra-embryonic part of the coelom is obliterated. The liquor amnii 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 1 liter.

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.

In reptiles, birds, and many mammals

In reptiles, birds, and many mammals the amnion is developed 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 caudal end and sides of the embryo, and gradually rising more and more, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the amniotic cavity.

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 serosa.

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

Additional images

See also


External links

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.


Translations: Amnion
Top

Dansk (Danish)
n. - inderste fosterhinde

Nederlands (Dutch)
vruchtvlies

Français (French)
n. - (Anat) amnios

Deutsch (German)
n. - (biol.) Amnion, Embryonalhülle

Ελληνική (Greek)
n. - (ανατ.) άμνιο

Italiano (Italian)
amnio

Português (Portuguese)
n. - âmnio (m) (Anat.)

Русский (Russian)
амнион, водная оболочка

Español (Spanish)
n. - amnios

Svenska (Swedish)
n. - inre fosterhinna

中文(简体)(Chinese (Simplified))
胞衣, 羊膜

中文(繁體)(Chinese (Traditional))
n. - 胞衣, 羊膜

한국어 (Korean)
n. - 양막(모래집)

日本語 (Japanese)
n. - 羊膜

العربيه (Arabic)
‏(الاسم) الغشا الأميني : الغشا الداخلي الذي يحيط بالجنين مباشره‏

עברית (Hebrew)
n. - ‮שפיר, קרום השליה‬


 
 

 

Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Amnion" Read more
Translations. Copyright © 2007, WizCom Technologies Ltd. All rights reserved.  Read more