Chorion

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Chorion

Diagram showing earliest observed stage of human embryo.

Human foetus, enclosed in the amnion.
Gray's	subject #12 60
MeSH	Chorion

For the entertainment company see Chorion (company).

The chorion surrounds the embryo and other membranes. It consists of two layers: an outer formed by the primitive ectoderm or trophoblast, and an inner by the somatic mesoderm; with this latter the amnion is in contact.

The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer of Langhans, and an external layer of richly nucleated protoplasm devoid of cell boundaries, the syncytiotrophoblast.

It undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo.

The chorionic villi are at first small and non-vascular, and consist of trophoblast only, but they increase in size and ramify, while the mesoderm, carrying branches of the umbilical vessels, grows into them, and in this way they are vascularized.

Blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the umbilical veins. 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.

The greater part of the chorion is in contact with the decidua capsularis, and over this portion the villi, with their contained vessels, undergo atrophy, so that by the fourth month scarcely a trace of them is left, and hence this part of the chorion becomes smooth, and is named the chorion læve; as it takes no share in the formation of the placenta, it is also named the non-placental part of the chorion.

On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the chorion frondosum.

[edit] Additional images

Section through the embryo.

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.

Scheme of placental circulation.

[edit] See also

Amniotic sac

[edit] 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.