What is Amnion?

Amnion
Surface view of embryo of Hylobates concolor.
Human fetus, enclosed in the amnion.
	subject #12 56
MeSH	Amnion

For the alien race in Stephen Donaldson's The Gap Cycle, see Amnion (Gap Cycle).

The amnion is a membranous sac which surrounds and protects the embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibia and fish, which are consequently termed “Anamnia”. The primary function of this is the protection of the embryo for its future development into a fetus and eventually an animal.

In humans

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

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.	Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amnion, 3 allantois, 4 Yolk sac, 5 developing marginal hematoma, 6 maternal part of placenta (endometrium)

See also

• Amniotic sac

External links

• Histology at BU 19903loa - "Female Reproductive System: placenta, chorionic plate"
• McGill

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.

•  • [ e] Human development of embryo and development of fetus (some dates are approximate - see Carnegie stages and a timeline)
Week 1	Zygote - Morula - Blastomere - Blastula/Blastosphere/Blastocyst/Blastocoele - Inner cell mass
Week 2	Bilaminar disc (Hypoblast, Epiblast) - Archenteron/Primitive streak
Week 3 (Trilaminar embryo)	Primitive knot/Blastopore - Primitive groove - Gastrula/Gastrulation Germ layer: Ectoderm (Neuroectoderm) - Endoderm - Mesoderm (Chorda-, Paraxial, Intermediate, Lateral plate) - Splanchnopleure/Somatopleure Somite/Somitomere (Sclerotome, Myotome, Dermatome)
Uterine support	Allantois - Trophoblast (Cytotrophoblast, Syncytiotrophoblast, Gestational sac) - Yolk sac - Vitelline duct - Placenta - Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Amniotic sac (Amnion, Chorion) - Decidua - Decidual cells - Intervillous space - Chorionic villi
Branchial region	Branchial arch (1st/Meckel's cartilage, 2nd)/Branchial pouch - Cervical sinus - Lateral lingual swelling - Tuberculum impar - Frontonasal prominence - Olfactory pit - Nasal processes - Globular process - Primitive palate
Histogenesis and organogenesis	Glands: Thyroglossal duct Limb development: Limb bud - Apical ectodermal ridge/AER