(redirected from chorions)
Also found in: Dictionary, Thesaurus, Medical.
Related to chorions: chorion frondosum, chorion laeve


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 It might be outdated or ideologically biased.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 ?
All IGG members indicated that they had tried either mechanical (Henn and Braunbeck, 2011) or enzymatic (Mandrell et al., 2012) chorion removal.
These investigators pointed out that the literature on the utility of dechorionation is divided, with some laboratories indicating that the chorion might be more permeable than previously believed (Wiegand et al., 2000; Gustafson et al., 2012), and permeability may be affected by solvents like DMSO (Kais et al., 2013).
However, there are few systematic examinations of the utility of chorion removal in the literature.
The IGG identified two specific parameters that can potentially influence study outcomes for chemical screening using zebrafish embryos: the presence versus absence of the chorion and the use of the static versus static-renewal exposure procedure.
The input from the IGG members is currently being used to design an interlaboratory study to evaluate the effects of the chorion and exposure methods.
Structure and macromolecular composition of the zebrafish egg chorion. Zygote 4, 101-108.
Regulation of the permeability of the medaka fish embryo chorion by exogeneous sodium and calcium ions.
Microscopic studies of the teleost chorion. Trans Am Microsc Soc 77, 240-243.
The role of chorion on toxicity of silver nanoparticles in the embryonic zebrafish assay.
Automated zebrafish chorion removal and single embryo placement: Optimizing throughput of zebrafish developmental toxicity screens.