The amnion (: amnions or amnia) is a membrane that closely covers human and various other

embryo An embryo ( ) is the initial stage of development for 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 sp ...

s when they first form. 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 The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of biological membrane, membranes that hold a developing embryo (and l ...

that provides a protective environment for the developing embryo. The amnion, along with the

chorion The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs. Structure In humans and other therian mammals, the cho ...

, the

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

and the

allantois The allantois ( ; : allantoides or allantoises) is one of the extraembryonic membranes arising from the yolk sac. It is a hollow sac-like structure filled with clear fluid that forms part of the developing conceptus in an amniote that helps the ...

protect the embryo. In

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s and

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s, the protective sac is enclosed in a shell. In

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s and

placental mammal Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguish ...

s, it is enclosed in a

uterus The uterus (from Latin ''uterus'', : uteri or uteruses) or womb () is the hollow organ, organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic development, embryonic and prenatal development, f ...

. The amnion is a feature of the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

clade ''Amniota'', which includes reptiles, birds, and

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

Amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s and

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

lack the amnion and thus are

anamniotes The anamniotes are an informal group of craniates comprising all fish and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either ...

(non-amniotes). 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 ...

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 the o ...

trophoblast The trophoblast (from Greek language, Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after Human fertilization, fertilization in humans. They provide nutrients to the embryo an ...

on the inner side.

Etymology

Etymologists have traditionally assumed that the Greek term ἀμνίον (''amnion'') relates to

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

ἀμνίον : , "little lamb", a diminutive of ἀμνός : , "lamb", and cognate with the English verb ', "to bring forth young (usually lambs)". However, an alternative etymology references an ancient Greek goddess of childbirth,

Eileithyia Eileithyia or Ilithyia (; ; (''Eleuthyia'') in Crete, also (''Eleuthia'') or (''Elysia'') in Laconia and Messene, and (''Eleuthō'') in literature)Nilsson Vol I, p. 313 was the Greek goddess of childbirth and midwifery, and the daughter o ...

, worshipped in

Amnisos Amnisos, also Amnissos and Amnisus (Greek language, Greek: or ; Linear B: 𐀀𐀖𐀛𐀰 ''A-mi-ni-so''), is the current but unattested name given to a Bronze Age settlement on the north shore of Crete that was used as a port to the palace ci ...

(on the island of Crete) and nicknamed Ἀμνιάς (Amnias).

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 The bilaminar embryonic disc, bilaminar blastoderm or embryonic disc is the distinct two-layered structure of cells formed in an embryo. In the development of the human embryo this takes place by day eight. It is formed when the inner cell mass, a ...

. Outside the amniotic ectoderm is a thin layer of

, which is continuous with that of the

somatopleure In the anatomy of an embryo, the somatopleure is a structure created during embryogenesis when the lateral plate mesoderm splits into two layers. The outer (or somatic) layer becomes applied to the inner surface of the ectoderm, and with it (parti ...

and is connected by the body-stalk with the mesodermal lining of the

. 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 many 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, i ...

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. 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 Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

, 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 Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

. 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 The gestational sac is the large cavity of fluid surrounding the embryo. During early embryogenesis, it consists of the extraembryonic coelom, also called the chorionic cavity. The gestational sac is normally contained within the uterus. It is the ...

Other animals

s, 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

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 th ...

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 The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until sh ...

. 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 serous membrane (or serosa) is a smooth epithelial membrane of mesothelium lining the contents and inner walls of body cavities, which secrete serous fluid to allow lubricated sliding movements between opposing surfaces. The serous mem ...

. 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''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

and

dogs The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers ...

are born inside of the amnion; the mother cuts it open and eats it. In

elephant Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...

s, "The amnion is continued from the base of the umbilical cord upon the

, which is of considerable size, and is so interposed between the

and amnios, as to prevent any part of the amnios attaining the inner surface of the

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

. 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.Frech, T. M., et al. (2019). "Amniotic membrane dressings: an effective therapy for SSc-related wounds." Rheumatology (Oxford) 58(4): 734-736. For this purpose, the

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

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 im ...

and

hepatitis Hepatitis is inflammation of the liver parenchyma, liver tissue. Some people or animals with hepatitis have no symptoms, whereas others develop yellow discoloration of the skin and whites of the eyes (jaundice), Anorexia (symptom), poor appetite ...

, 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 The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic ...

(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 MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

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, am ...

stained

H&E stain Hematoxylin and eosin stain ( or haematoxylin and eosin stain or hematoxylin–eosin stain; often abbreviated as H&E stain or HE stain) is one of the principal tissue stains used in histology. It is the most widely used stain in medical diag ...

. File:Catfetus1.jpg, Opened uterus with

cat The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

fetus in midgestation: 1

umbilicus Umbilicus may refer to: *The navel or belly button *Umbilicus (mollusc), a feature of gastropod, Nautilus and Ammonite shell anatomy *Umbilicus (plant), ''Umbilicus'' (plant), a genus of over ninety species of perennial flowering plants *Umbilicus ...

, 2

(

and amnion), 3

, 4

, 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 capillaries. A hematoma is ...

, 6 maternal part of

(

endometrium The endometrium is the inner epithelium, 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 funct ...

)

References

External links

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

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

Etymology

In humans

Clinical significance

Other animals

Application

Additional images

See also

References

External links