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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 ?
Pregnancy betal glycoprotein and chorionic gonadotrophin in the serum of patients with trophoblastic and non-trophoblastic tumors.
BUNDLE OF QUESTIONS As scientists move closer to developing genetic tests that match the reliability of amniocentesis and chorionic villus sampling but lessen their potential harm to a fetus, some researchers expect that more women will choose to undergo prenatal testing.
In performing CVS, a sample of chorionic villi--the tiny, fingerlike projections of the placenta through which the mother and baby exchange nutrients, gases, and fetal wastes--is extracted with a catheter that is inserted into the mother's cervix.
Correlation of human chorionic gonadotropin in early pregnancy failure with size of gestational sac and placental histology.
1] Nonstandard abbreviations: hCG, human chorionic gonadotropin; hCG[beta]cf, hCG [beta] core fragment; POC, point-of-care; OTC, over-the-counter.
The chorionic plate was not identified and there were thick ropy stem villi that have been shorn off the villous parenchyma (Figure 4).
Following the addition of Vitamin D, the FastPack menu has been expanded to eight assays, namely Prostate Specific Antigen (PSA), free PSA (FPSA), Testosterone, Thyroid Stimulating Hormone (TSH), free Thyroxine (FT4), Human Chorionic Gonadotropin (hCG), Alpha GST and Vitamin D.
The injection is to be used for weight loss as an alternative to the human chorionic gonadotropin (HCG) diet.
AT 13 WEEKS' GESTATION, a 34-year-old woman underwent chorionic villus sampling (CVS) at a maternal-fetal medicine center.
Acombination of vaginal bleeding score and human chorionic gonadotropin levels may predict the possibility of miscarriage by up to 77%, and the possibility of ongoing pregnancy by 94%.
Carbohydrate and peptide structure of the alpha- and beta-subunits of human chorionic gonadotropin from normal and aberrant pregnancy and choriocarcinoma.