(redirected from chorionic)
Also found in: Dictionary, Thesaurus, Medical, Wikipedia.


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

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



(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 Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


The outermost of the extraembryonic membranes of amniotes, enclosing the embryo and all of its other membranes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The immunoexpressions and prognostic significance of inhibin alpha and beta human chorionic gonadotrophins (HCG) in breast carcinomas.
(A) ventral (upper) view; (B) typical ornamentation of the outer chorionic reticulum showing two types of tubercles; (C) anterior region of the egg showing micropylar apparatus, formed by a micropilar disc with a well evidenced frame; (D) anterior pole of egg, with micropyle.
The seminoma decoy: measurement of serum human chorionic gonadotropin in patients with seminoma.
Nicolaides, "Histomorphological features of chorionic villi at 10-14 weeks of gestation in trisomic and chromosomally normal pregnancies," Placenta, vol.
The predictive value of an initial serum beta human chorionic gonadotropin level for pregnancy outcome following in vitro fertilization.
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 situ hybridization using a Zika virus RNA probe demonstrated scattered, strongly positive staining cells within the villous stroma of the chorionic villi (Figure 4), which were presumably Hofbauer cells.
The control CAM is a thin membrane with the developed chorionic epithelium, allantoic epithelium, and a mesenchyme between these two epithelial layers and blood vessels (Figure 2(a)).
All About Human Chorionic Gonadotropin* (HCG) Therapy -