The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (

amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...

). It develops from an outer fold on the surface of the

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

, which lies outside the zona pellucida (in mammals), known as the

vitelline membrane The vitelline membrane or vitelline envelope is a structure surrounding the outer surface of the plasma membrane of an ovum (the oolemma) or, in some animals (e.g., birds), the extracellular yolk and the oolemma. It is composed mostly of protein f ...

in other animals. In insects it is developed by the

follicle cell An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with th ...

s while the

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

is in the

ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...

.Chapman, R.F. (1998) "The insects: structure and function", Section ''The egg and embryology''. Previewed in Google Book

on 26 Sep 2009.

Structure

In humans and other mammals (excluding monotremes), the chorion is one of the fetal membranes that exist during pregnancy between the developing fetus and mother. The chorion and the

amnion The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the develo ...

together form the

amniotic sac The amniotic sac, also called the bag of waters or the membranes, is the sac in which the embryo and later fetus develops in amniotes. It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until sho ...

. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes; the

chorionic villi Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histomorphologic perspective, and are, by definition, a product of conception. Branche ...

emerge from the chorion, invade the endometrium, and allow the transfer of nutrients from maternal blood to fetal blood.

Layers

The chorion consists of two layers: an outer formed by the trophoblast, and an inner formed by the somatic mesoderm. The trophoblast is made up of an internal layer of cubical or prismatic cells, the

cytotrophoblast "Cytotrophoblast" is the name given to both the inner layer of the trophoblast (also called layer of Langhans) or the cells that live there. It is interior to the syncytiotrophoblast and external to the wall of the blastocyst in a developing embry ...

or layer of Langhans, and an external multinucleated layer, the syncytiotrophoblast.

Growth

The chorion undergoes rapid proliferation and forms numerous processes, the ''

'', which invade and destroy the uterine decidua, while simultaneously absorbing nutritive materials from it for the growth of the embryo. The chorionic villi are at first small and non-vascular, and consist of the trophoblast only, but they increase in size and ramify, whereas the mesoderm, carrying branches of the umbilical vessels, grows into them, and they are

vascularized Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels, formed in the earlier stage of vasculogenesis. Angiogenesis continues the growth of the vasculature by processes of sprouting and splittin ...

. Blood is carried to the villi by the paired umbilical arteries, which branch into

chorionic arteries Chorionic (plate) vessels, also fetal surface vessels are blood vessels, including both arteries and veins, that carry blood through the chorion in the fetoplacental circulation. Chorionic arteries branch off the umbilical artery, and supply the ...

and enter the

cotyledon arteries The placenta of humans, and certain other mammals contains structures known as cotyledons, which transmit fetal blood and allow exchange of oxygen and nutrients with the maternal blood. Ruminants The Artiodactyla have a cotyledonary placenta. I ...

. After circulating through the capillaries of the villi, the blood is returned to the embryo by the

umbilical vein The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood vo ...

. Until about the end of the second month of pregnancy, the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.

Parts

The part of the chorion that is in contact with the decidua capsularis undergoes atrophy, so that by the fourth month scarcely a trace of the villi is left. This part of the chorion becomes smooth, and is named the chorion laeve (from the Latin word ''levis'', meaning smooth). As it takes no share in the formation of the placenta, this is also named the non-placental part of the chorion. As the chorion grows, the chorion laeve comes in contact with the decidua parietalis and these layers fuse. The villi at the embryonic pole, which is in contact with the

decidua basalis The decidua is the modified mucosal lining of the uterus (that is, modified endometrium) that forms every month, in preparation for pregnancy. It is shed off each month when there is no fertilised egg to support. The decidua is under the influe ...

, increase greatly in size and complexity, and hence this part is named the chorion frondosum. Thus the placenta develops from the chorion frondosum and the decidua basalis.

Monochorionic twins

''Monochorionic twins'' are twins that share the same placenta. This occurs in 0.3% of all pregnancies, and in 75% of

monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...

(identical) twins, when the split takes place on or after the third day after

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. The remaining 25% of monozygous twins become ''dichorionic diamniotic''. The condition may affect any type of multiple birth, resulting in ''monochorionic multiples''.

Infections

Recent studies indicate that the chorion may be susceptible to pathogenic infections. Recent findings indicate that ''

Ureaplasma parvum ''Ureaplasma parvum'' is a species of ''Ureaplasma'', a genus of bacteria belonging to the family Mycoplasmataceae. In Indonesia, ureaplasma parvum is most commonly contracted through contact with public toilets. ''Ureaplasma parvum'' was forme ...

'' bacteria can infect the chorion tissue, thereby impacting pregnancy outcome. In addition, footprints of

JC polyomavirus ''Human polyomavirus 2'', commonly referred to as the JC virus or John Cunningham virus, is a type of human polyomavirus (formerly known as papovavirus). It was identified by electron microscopy in 1965 by ZuRhein and Chou, and by Silverman and ...

and

Merkel cell polyomavirus Merkel cell polyomavirus (MCV or MCPyV) was first described in January 2008 in Pittsburgh, Pennsylvania. It was the first example of a human viral pathogen discovered using unbiased metagenomic next-generation sequencing with a technique called ...

have been detected in chorionic villi from females affected by spontaneous abortion as well as pregnant women. Another virus,

BK polyomavirus The BK virus is a member of the polyomavirus family. Past infection with the BK virus is widespread, but significant consequences of infection are uncommon, with the exception of the immunocompromised and the immunosuppressed. BK virus is an ab ...

has been detected in the same tissues, but with lesser extent.

Other animals

In reptiles, birds, and monotremes, the chorion is one of the four

extraembryonic membrane The extraembryonic membranes are four membranes which assist in the development of an animal's embryo. Such membranes occur in a range of animals from humans to insects. They originate from the embryo, but are not considered part of it. They typ ...

s that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival. It is located inside the albumen, which is the white of the egg. It encloses the embryo and the rest of the embryonic system. The chorion is also present in insects. During growth and development of the embryo, there is an increased need for oxygen. To compensate for this, the chorion and the

allantois The allantois (plural ''allantoides'' or ''allantoises'') is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus (which consists of all embryonic and extraembryonic tissues). It helps the embryo ...

fuse together to form the

chorioallantoic membrane The Chorioallantoic Membrane (CAM), also known as the chorioallantois, is a highly vascularized membrane found in the eggs of certain amniotes like birds and reptiles. It is formed by the fusion of the mesodermal layers of two extra-embryonic mem ...

. Together these form a double membrane, which functions to remove carbon dioxide and to replenish oxygen through the porous shell. At the time of hatching, the fetus becomes detached from the chorion as it emerges from the shell.

Additional images

Image:Gray21.png, Section through the embryo. Image:Gray24.png, Diagram of the human embryo. Image:Gray25.png, Diagram illustrating early formation of

and differentiation of body-stalk. Image:Gray26.png, Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac. Image:Gray39.png, Scheme of placental circulation.

References

External links

* — "Female Reproductive System: placenta, chorionic plate"
McGill
{{Authority control Embryology