For the entertainment company see Chorion (company)
Chorion (company)
Chorion Limited is a major international media production company with offices in London, New York and Sydney. The company produces TV shows and feature films, and is best known for the heritage properties included in its portfolio. These include children's characters such as Paddington Bear, Peter...


The chorion is one of the membranes that exist during pregnancy between the developing fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

 and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes. The chorionic villi
Chorionic villi
Chorionic villi are villi that sprout from the chorion in order to give a maximum area of contact with the maternal blood.Embryonic blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the...

 emerge from the chorion, invade the endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

, and allow transfer of nutrients from maternal blood to fetal blood.


The chorion consists of two layers: an outer formed by the trophoblast
Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...

, and an inner formed by the somatic mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

; the amnion
The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the...

 is in contact with the latter.

The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast
The cytotrophoblast is the inner layer of the trophoblast, interior to the syncytiotrophoblast in an embryo.Cytotrophoblast is considered to be the trophoblastic stem cell; it differentiates into the other forms of trophoblastic tissue .-External links: - "Female Reproductive System: placental...

 or layer of Langhans, and an external layer of richly nucleated protoplasm devoid of cell boundaries, the syncytiotrophoblast
Syncytiotrophoblast is the epithelial covering of the placenta villous tree. It is a unique tissue in that it is a multi-nucleated, terminally differentiated syncytium, extending to 13m^2...



The chorion undergoes rapid proliferation and forms numerous process
Process (anatomy)
In anatomy, a process is a projection or outgrowth of tissue from a larger body. The vertebra has several kinds of processes,such as: transverse process, prezygapophysis, postzygapophysis.-Examples:Examples of processes include:...

es, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...


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, in this way, are vascularized.

Blood is carried to the villi by the paired umbilical arteries, which branch into chorionic arteries and enter the chorionic villi
Chorionic villi
Chorionic villi are villi that sprout from the chorion in order to give a maximum area of contact with the maternal blood.Embryonic blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the...

 as cotyledon arteries. After circulating through the capillaries of the villi, the blood is returned to the embryo by the umbilical veins. Until about the end of the second month of pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

, the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.


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.

On the other hand, the villi at the embryonic pole, which is in contact with the decidua basalis, 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
Two Wide-Angle Imaging Neutral-Atom Spectrometers are a pair of NASA instruments aboard two United States National Reconnaissance Office satellites in Molniya orbits. TWINS was designed to provide stereo images of the Earth's ring current. The first instrument, TWINS-1, was launched aboard USA-184...

 that share the same placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...

. It occurs in 0.3% of all pregnancies, and in 75% of monozygotic (identical) twins, when the split takes place after the third day after fertilization. The remaining 25% of monozygous twins become dichorionic diamniotic. The condition may affect any type of multiple birth
Multiple birth
A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets...

, resulting in monochorionic multiples.

See also

  • Amniotic sac
    Amniotic sac
    The amniotic sac is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the Chorion,...

  • Choriogenesis
    In developmental biology, choriogenesis is the formation of the chorion, an outer membrane of the placenta that eventually forms chorionic villi that allow the transfer of blood and nutrients from mother to fetus.- Influence on monozygotic twins :...

  • Chorioamnionitis
    Chorioamnionitis is an inflammation of the fetal membranes due to a bacterial infection. It typically results from bacteria ascending into the uterus from the vagina and is most often associated with prolonged labour...

    , an inflammation
    Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

     of the chorion and amnion, usually due to bacterial infection.
  • Gestational trophoblastic disease
    Gestational trophoblastic disease
    Gestational trophoblastic disease is a term used for a group of pregnancy-related tumours. These tumours are rare, and they appear when cells in the womb start to grow out of control. The cells that form gestational trophoblastic tumours are called trophoblasts and come from tissue that grows to...

    , any abnormal proliferation of the trophoblasts, including choriocarcinoma
    Choriocarcinoma is a malignant, trophoblastic and aggressive cancer, usually of the placenta. It is characterized by early hematogenous spread to the lungs...

    , a highly invasive cancer.

External links

- "Female Reproductive System: placenta, chorionic plate"
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