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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



(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.


The outermost of the extraembryonic membranes of amniotes, enclosing the embryo and all of its other membranes.
References in periodicals archive ?
Diagnostic considerations in the measurement of human chorionic gonadotropin in aging women.
Evidence for the presence of human chorionic gonadotropin (hCG) and free beta-subunit of hCG in the human pituitary.
In fact, it's even been formally renamed the LH/CG-R (luteinizing hormone/ chorionic gonadotropin receptor).
Caption: These dichorionic/diamniotic twins at 10 weeks' gestation have two placental masses abutting one another, with a "twin peak" sign--visible chorionic tissue between the two sacs.
The extracellular matrix of chorionic villi can be decreased, and the villous cores, thus, as pale as the intervillous space (Figure 1, F).
The lack of a gestational sac or chorionic villi within the specimen made an ectopic pregnancy or GTN unlikely.
9) Despite the decrease in HIV transmission with antiretroviral cover, procedures that require more technical skills--such as chorionic villus sampling and cordocentesis--should still be avoided in the HIV-infected woman, as the risk of transmission to the fetus may be considerably increased.
The other component is the unique C-terminal peptide of the human chorionic gonadotropin B-subunit (hCG B-CTP).
The drug Dodgers outfielder Manny Ramirez was suspended for using is reportedly human chorionic gonadotropin, mainly used as a female fertility treatment.
Pregnancy hypertension and pre-eclampsia are associated with placental malfunction, including altered levels of hormones such as oestrogen and human chorionic gonadotropin.
amniocentesis and chorionic villus sampling, would not be morally acceptable.
Macri Technologies, which holds patents related to free beta Human Chorionic Gonadotropin (free Beta hCG), a peptide hormone recognized as a biomarker for trimester prenatal risk assessment.