The chorion is the outermost fetal membrane around the embryo in mammals,

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s and reptiles ( amniotes). It develops from an outer fold on the surface of the

yolk sac The yolk sac is a membranous 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'' is ...

, which lies outside the

zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first ap ...

(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

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

s it is developed by the follicle cells 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.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 Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occur ...

between the developing

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...

and mother. The chorion and the amnion 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 s ...

. In humans it is formed by extraembryonic mesoderm and the two layers of

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

that surround the embryo and other membranes; the chorionic villi 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

, and an inner formed by the somatic mesoderm. The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer of Langhans, and an external

multinucleate Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordina ...

d layer, the syncytiotrophoblast.

Growth

The chorion undergoes rapid proliferation and forms numerous

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

es, the '' chorionic villi'', 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 splitt ...

. Blood is carried to the villi by the paired

umbilical arteries The umbilical artery is a paired artery (with one for each half of the body) that is found in the abdominal and pelvic regions. In the fetus, it extends into the umbilical cord. Structure Development The umbilical arteries supply deoxygenated ...

, which branch into chorionic arteries and enter the chorionic villi as cotyledon arteries. 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 v ...

. Until about the end of the second month of

, 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 influen ...

, 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 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 ...

that share the same

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

. 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 ...

(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 A multiple birth is the culmination of one multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such bir ...

, resulting in ''monochorionic multiples''.

Infections

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

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

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 an ...

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 d ...

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

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

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,

s, 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 typi ...

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

s. During growth and development of the embryo, there is an increased need for oxygen. To compensate for this, the chorion and the allantois 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 memb ...

. 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 allantois 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