Chorion

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Chorion

The outermost of the several extraembryonic membranes in amniotes (reptiles, birds, and mammals) enclosing the embryo and all of its other membranes. The chorion, or serosa, is composed of an outer layer of ectodermal cells and an inner layer of mesodermal cells, collectively the somatopleure. Both layers are continuous with the corresponding tissue of the embryo. The chorion arises in conjunction with the amnion, another membrane that forms the outer limb of the somatopleure which folds up over the embryo in reptiles, birds, and some mammals. The chorion is separated from the amnion and yolk sac by a fluid-filled space, the extraembryonic coelom, or body cavity. In those mammals in which the amnion forms by a process of cavitation in a mass of cells, instead of by folding, the chorion forms directly from the trophoblastic capsule, the extraembryonic ectoderm, which becomes gradually underlain by extraembryonic mesoderm.

In reptiles and birds the chorion fuses with another extraembryonic membrane, the allantois, to form the chorioallantois, which lies directly below the shell membranes. An extensive system of blood vessels develops in the mesoderm of this compound membrane which serves as the primary respiratory and excretory organ for gaseous interchanges. In all mammals above the marsupials, the chorion develops special fingerlike processes (chorionic villi) extending outward from its surface. To a varying degree in different species of mammals, the villous regions of the chorion come into more or less intimate contact with the uterine mucosa, or uterine lining, of the mother, thereby forming the various placental types. See Allantois, Amniota, Fetal membrane, Germ layers

Chorion

 

(1) The outermost embryonic membrane of birds, reptiles, and mammals (including human beings) that is formed in the early stages of development; also called serosa. The embryo receives oxygen from the environment through the chorion, and the embryo of mammals also receives food and eliminates metabolites and carbon dioxide through the membrane.

In birds and reptiles, the chorion fuses with the allantois to form a common chorioallantoic membrane next to the eggshell. The chorion of mammals, including humans, is covered with villi, into which the blood vessels of the embryo grow. The villi penetrate into the uterine wall, where they form the placenta.

(2) A secondary eggshell of invertebrates and some lower vertebrates.

chorion

[′kȯr·ē·än]
(embryology)
The outermost of the extraembryonic membranes of amniotes, enclosing the embryo and all of its other membranes.
References in periodicals archive ?
Conclusion: The superiority of antibacterial effects of the chorionic membrane compared with the amniotic membrane can represent the key role of maternal part in placenta in protecting the fetus against possible infections.
In summary, the more vascular the chorionic villi, the better off the pregnancy.
DISCUSSION: Chorionic villus sampling has emerged as the only safe invasive prenatal diagnostic procedure in the first trimester.
First-service pregnancy rate in beef heifers as influenced by human chorionic gonadotropin administration before and/or after breeding, Theriogenol.
Intermediate mature and immature chorionic villi constitute 40-45% of the placental area.
False-negative results in point-of-care qualitative human chorionic gonadotropin (hCG) devices due to hCG[beta]core fragment.
Further evidence against the reliability of the human chorionic discriminatory level.
Background" human chorionic gonadotropin in healthy, nonpregnant women.
Then Handschuh K et al17 compared hCG secretion by primary cultures of VCT and EVCT isolated from the same first trimester human chorionic villi.
The only definitive test involves chorionic villus sampling and amniocentesis but that carries a risk of miscarriage.
Home pregnancy tests are designed to detect the hormone human chorionic gonadotropin, or hCG, which is produced by the developing placenta that will help nourish a fetus after conception.