Chorion

Source: Wikipedia, the free encyclopedia.
For the entertainment company, see
Chorion (company)
.
Chorion
Diagram showing the chorion of a chicken egg
Human fetus enclosed in the amnion
Details
Identifiers
Latinchorion
TEE5.11.3.1.1.0.3
Anatomical terminology

The chorion is the outermost

molluscs
.

Structure

In humans and other

fetal membranes that exist during pregnancy between the developing fetus and mother. The chorion and the amnion together form the amniotic sac. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes;[1] the chorionic villi emerge from the chorion, invade the endometrium
, and allow the transfer of nutrients from maternal blood to fetal blood.

Layers

The chorion consists of two layers: an outer formed by the trophoblast, and an inner formed by the extra-embryonic mesoderm.

The trophoblast is made up of an internal layer of cubical or prismatic cells, the

multinucleated layer, the syncytiotrophoblast
.

Growth

The chorion undergoes rapid proliferation and forms numerous

uterine decidua, while simultaneously absorbing nutritive materials from it for the growth of the embryo
.

The chorionic villi are at first small and non-vascular, and consist of the trophoblast only, but they increase in size and

vascularized
.

Blood is carried to the villi by the paired

cotyledon arteries. After circulating through the capillaries of the villi, the blood is returned to the embryo by the umbilical vein. Until about the end of the second month of pregnancy
, the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.

Parts

Placenta with attached fetal membranes (ruptured at the margin at the left in the image), which consists of the chorion (outer layer) and amnion (inner layer).

The part of the chorion that is in contact with the decidua capsularis undergoes atrophy, so that by the fourth month scarcely a trace of the villi is left. This part of the chorion becomes smooth,[2] and is named the chorion laeve (from the Latin word levis, meaning smooth). As it takes no share in the formation of the placenta, this is also named the non-placental part of the chorion. As the chorion grows, the chorion laeve comes in contact with the decidua parietalis and these layers fuse.

The villi at the embryonic pole, which is in contact with the

decidua basalis, increase greatly in size and complexity, and hence this part is named the chorion frondosum.[2]

Thus the placenta develops from the chorion frondosum and the decidua basalis.

Monochorionic twins

Main article: Monochorionic twins

Monochorionic twins are

fertilization.[4] The remaining 25% of monozygous twins become dichorionic diamniotic.[4] The condition may affect any type of multiple birth
, resulting in monochorionic multiples.

Infections

Recent studies indicate that the chorion may be susceptible to

BK polyomavirus has been detected in the same tissues, but with lesser extent.[9]

Other animals

Amniotic embryo. a=embryo, b=yolk, c=allantois, d=amnion, e=chorion

In reptiles, birds, and monotremes, the chorion is one of the four extraembryonic membranes that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival. It is located inside the albumen, which is the white of the egg. It encloses the embryo and the rest of the embryonic system. The chorion is also present in insects. During growth and development of the embryo, there is an increased need for oxygen. To compensate for this, the chorion and the allantois fuse together to form the chorioallantoic membrane. Together these form a double membrane, which functions to remove carbon dioxide and to replenish oxygen through the porous shell. At the time of hatching, the fetus becomes detached from the chorion as it emerges from the shell.

In

mollusks also have chorions as part of their eggs. For example, fragile octopus eggs have only a chorion as their envelope.[11]

Additional images

  • Section through the embryo.
    Section through the embryo.
  • Diagram of the human embryo.
    Diagram of the human embryo.
  • Diagram illustrating early formation of allantois and differentiation of body-stalk.
    Diagram illustrating early formation of allantois and differentiation of body-stalk.
  • Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
    Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
  • Scheme of placental circulation.
    Scheme of placental circulation.

See also

References

Public domain This article incorporates text in the public domain from page 60 of the 20th edition of Gray's Anatomy (1918)

  1. ISBN 978-0-12-815145-7
    .
  2. ^
    PMID 26003500
    .
  3. PMID 16281049
    .
  4. ^
    ISBN 0-8247-2844-0
    .
  5. PMID 30078192
    .
  6. PMID 30078192
    .
  7. S2CID 53106591
    .
  8. PMID 31549405
    .
  9. S2CID 53106591
    .
  10. ^ Chapman, R.F. (1998) "The insects: structure and function", Section The egg and embryology. Previewed in Google Books [1] on 26 Sep 2009.
  11. ^ “The Octopoda are characterized by eggs that have only a chorion as an envelope”https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/octopoda
Retrieved from "https://en.wikipedia.org/w/index.php?title=Chorion&oldid=1261021679"