The chorion is the outermost fetal membrane around the embryo in , and reptiles (amniotes). It is also present around the embryo of other animals, like and .
Structure
In humans and other
mammals, the chorion is one of the
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;
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 cytotrophoblast or layer of Langhans, and an external layer, the syncytiotrophoblast.
Growth
The chorion undergoes rapid proliferation and forms numerous processes, the
chorionic villi, which invade and destroy the
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 , whereas the mesoderm, carrying branches of the umbilical vessels, grows into them, and they are vascularized.
Blood is carried to the villi by the paired umbilical arteries, which branch into chorionic arteries and enter the chorionic villi as 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
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,
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.
Thus the placenta develops from the chorion frondosum and the decidua basalis.
Monochorionic twins
Monochorionic twins are
twins that share the same
placenta. This occurs in 0.3% of all pregnancies,
and in 75% of
monozygotic (identical) twins, when the split takes place on or after the third day after
fertilization.
The remaining 25% of monozygous twins become
dichorionic diamniotic.
[ The condition may affect any type of multiple birth, resulting in monochorionic multiples.
]
Infections
Recent studies indicate that the chorion may be susceptible to Pathogen.
Other animals
In , , and , 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 , it develops by the while the egg is in the ovary.[Chapman, R.F. (1998) "The insects: structure and function", Section The egg and embryology. Previewed in Google Books [1] on 26 Sep 2009.] Some mollusks also have chorions as part of their eggs. For example, fragile
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===Additional images===
and differentiation of body-stalk.]]
See also
External links
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— "Female Reproductive System: placenta, chorionic plate"
-
McGill
