The amnion is a membrane that closely covers the human and various other

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s when first formed. It fills with

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

, 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 The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...

protect the embryo. In birds,

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s and

monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...

s, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus. The term is from Ancient Greek ἀμνίον 'little lamb', diminutive of ἀμνός 'lamb'. it is cognate with the English verb 'yean', bring forth young (usually lambs). The amnion is a feature of the vertebrate clade ''Amniota'', which includes reptiles, birds, and

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

Amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s and fish are not amniotes and thus lack the amnion. The amnion stems from the extra-embryonic somatic

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...

on the outer side and the extra-embryonic

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

or trophoblast on the inner side.

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

, 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

(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 The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...

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 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 Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

. 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.TheFetus.net > Amniotic band syndrome
By Luís Flávio Gonçalves, MD, Philippe Jeanty, MD, PhD. 1999-09-26-18

Other animals

s, birds, and many

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

caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...

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

Cats The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of t ...

and

dogs The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

are born inside of the amnion; the mother cuts it open and eats it. In elephants, "The amnios is continued from the base of the umbilical cord upon the

, 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."

Application

The amniotic membrane is used as a biological dressing to heal incurable wounds. For this purpose, the placenta in

cesarean delivery Caesarean section, also known as C-section or caesarean delivery, is the surgical procedure by which one or more babies are delivered through an incision in the mother's abdomen, often performed because vaginal delivery would put the baby or mo ...

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 The human immunodeficiency viruses (HIV) are two species of '' Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immu ...

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

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 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 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 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 Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s in

meconium Meconium is the earliest stool of a mammalian infant resulting from defecation. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic ...

stained fetal membranes. H&E stain. File:Catfetus1.jpg, Opened uterus with

cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...

fetus in midgestation: 1 umbilicus, 2 amniotic sac ( chorion and amnion), 3

, 4 yolk sac, 5 developing marginal

hematoma A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillary, capillaries. A he ...

, 6 maternal part of placenta (

endometrium The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional laye ...

)

References

External links

* - "Female Reproductive System: placenta, chorionic plate"
McGill

The Foeto-Amnio-Placental complex
{{Authority control Embryology Membrane biology Biological matter