The amnion (PL: amnions or amnia) is a membrane that closely covers the human and various other embryos when first formed. 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 birds, reptiles and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in a uterus.
The amnion is a feature of the vertebrate clade Amniota, which includes reptiles, birds, and mammals. Amphibians and fish lack the amnion and thus are not 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.[1]
Traditionally, the term amnion has been assumed to derive from Ancient Greek ἀμνίον : amníon, 'little lamb', diminutive of ἀμνός : amnós, 'lamb'. It is cognate with the English verb 'yean', bring forth young (usually lambs). However, this etymology is perhaps incorrect as the term may actually refer to an ancient Greek goddess of childbirth worshipped in Amnisos on the island of Crete [2]
In humansEdit
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 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.
Clinical significanceEdit
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.[3]
Other animalsEdit
In reptiles, birds, and many mammals 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 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 and dogs are born inside of the amnion; the mother cuts it open and eats it.
In elephants, "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."[4]: 348
ApplicationEdit
The amniotic membrane is used as a biological dressing to heal incurable wounds.[5] 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 imagesEdit
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)
Surface view of embryo of Hylobates concolor.
Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30.
Section through the embryo.
Human embryo of 2.6 mm.
Diagram of a transverse section, showing the mode of formation of the amnion in the chick.
Model of human embryo 1.3 mm. long.
Sectional plan of the gravid uterus in the third and fourth month.
Scheme of placental circulation.
Human embryo of about fourteen days, with yolk-sac.
^Pigeon, J. (1960). "Treatment of second-degree burns with amniotic membranes". Can Med Assoc J. 83 (16): 844–845. PMC1938392. PMID 13735672.
^Singer, C. (1959). "The strange histories of some anatomical terms". Med. Hist. 3 (1): 1–7. PMC1034442. PMID 13632203.
^TheFetus.net > Amniotic band syndrome 2017-11-28 at the Wayback Machine By Luís Flávio Gonçalves, MD, Philippe Jeanty, MD, PhD. 1999-09-26-18
^Owen, R. (1857). "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". Philosophical Transactions of the Royal Society of London. 147: 347–353. Bibcode:1857RSPT..147..347O. doi:10.1098/rstl.1857.0017. JSTOR 108622.
^Frech, T. M., et al. (2019). "Amniotic membrane dressings: an effective therapy for SSc-related wounds." Rheumatology (Oxford) 58(4): 734-736.
External linksEdit
Histology image: 19903loa – Histology Learning System at Boston University - "Female Reproductive System: placenta, chorionic plate"
amnion, fictional, alien, species, cycle, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, newspapers, books, s. For the fictional alien species see Amnion Gap Cycle This article needs additional citations for verification Please help improve this article by adding 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 template message The amnion PL amnions or amnia is a membrane that closely covers the human and various other embryos when first formed 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 birds reptiles and monotremes the protective sac is enclosed in a shell In marsupials and placental mammals it is enclosed in a uterus AmnionChicken egg with amnion surrounding the embryoHuman fetus enclosed in the amnion DetailsIdentifiersLatinAmniosinasMeSHD000650TEE6 0 1 2 0 0 9FMA80223Anatomical terminology edit on Wikidata The amnion is a feature of the vertebrate clade Amniota which includes reptiles birds and mammals Amphibians and fish lack the amnion and thus are not 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 1 Contents 1 Etymology 2 In humans 3 Clinical significance 4 Other animals 5 Application 6 Additional images 7 See also 8 References 9 External linksEtymology EditTraditionally the term amnion has been assumed to derive from Ancient Greek ἀmnion amnion little lamb diminutive of ἀmnos amnos lamb It is cognate with the English verb yean bring forth young usually lambs However this etymology is perhaps incorrect as the term may actually refer to an ancient Greek goddess of childbirth worshipped in Amnisos on the island of Crete 2 In humans EditIn 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 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 Clinical significance EditExtra amniotic pregnancy is a rare condition that results from a rupture of the amnion leading to development of the fetus within the extraembryonic coelom 3 Other animals EditIn reptiles birds and many mammals 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 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 and dogs are born inside of the amnion the mother cuts it open and eats it In elephants 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 4 348 Application EditThe amniotic membrane is used as a biological dressing to heal incurable wounds 5 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 Edit nbsp 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 nbsp Surface view of embryo of Hylobates concolor nbsp Human embryo length 2 mm Dorsal view with the amnion laid open X 30 nbsp Section through the embryo nbsp Human embryo of 2 6 mm nbsp Diagram of a transverse section showing the mode of formation of the amnion in the chick nbsp Model of human embryo 1 3 mm long nbsp Sectional plan of the gravid uterus in the third and fourth month nbsp Scheme of placental circulation nbsp Human embryo of about fourteen days with yolk sac nbsp Meconium laden macrophages in meconium stained fetal membranes H amp E stain nbsp Opened uterus with cat fetus in midgestation 1 umbilicus 2 amniotic sac chorion and amnion 3 allantois 4 yolk sac 5 developing marginal hematoma 6 maternal part of placenta endometrium See also EditAmniotic sacReferences Edit nbsp This article incorporates text in the public domain from page 56 of the 20th edition of Gray s Anatomy 1918 Pigeon J 1960 Treatment of second degree burns with amniotic membranes Can Med Assoc J 83 16 844 845 PMC 1938392 PMID 13735672 Singer C 1959 The strange histories of some anatomical terms Med Hist 3 1 1 7 PMC 1034442 PMID 13632203 TheFetus net gt Amniotic band syndrome Archived 2017 11 28 at the Wayback Machine By Luis Flavio Goncalves MD Philippe Jeanty MD PhD 1999 09 26 18 Owen R 1857 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 Philosophical Transactions of the Royal Society of London 147 347 353 Bibcode 1857RSPT 147 347O doi 10 1098 rstl 1857 0017 JSTOR 108622 Frech T M et al 2019 Amniotic membrane dressings an effective therapy for SSc related wounds Rheumatology Oxford 58 4 734 736 External links EditHistology image 19903loa Histology Learning System at Boston University Female Reproductive System placenta chorionic plate McGill The Foeto Amnio Placental complex Portal nbsp Anatomy Retrieved from https en wikipedia org w index php title Amnion amp oldid 1174543963, wikipedia, wiki, book, books, library,
