chorion

For the chorion in invertebrate eggs, see Chorion (egg).

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 is one of the membranes that exists during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.

Layers

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

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.

Growth

The chorion 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 the trophoblast only, but they increase in size and ramify, whereas the mesoderm, carrying branches of the umbilical vessels, grows into them, and, in this way, 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.

Parts

The greater part of the chorion is not 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.

Monochorionic twins

Monochorionic twins are twins that share the same placenta. It occurs in 0.3% of all pregnancies,^[1] and in 75% of monozygotic (identical) twins, when the split takes place after the third day after fertilization.^[2] The remaining 25% of monozygous twins become dichorionic diamniotic.^[2] The condition may affect any type of multiple birth, resulting in monochorionic multiples.

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.

See also

• Amniotic sac
• Chorioamnionitis, an inflammation of the chorion and amnion, usually due to bacterial infection.
• Gestational trophoblastic disease, any abnormal proliferation of the trophoblasts, including choriocarcinoma, a highly invasive cancer.

References

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.