The chorion is the outermost
fetal membrane
The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic m ...
around the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
in
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s,
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 lightweigh ...
s and
reptiles
Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
(
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 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 far ...
, which lies outside the
zona pellucida (in mammals), known as the
vitelline membrane 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 pairs ...
s 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
monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s), the chorion is one of the
fetal membrane
The fetal membranes are the four extraembryonic membranes, associated with the developing embryo, and fetus in humans and other mammals.. They are the amnion, chorion, allantois, and yolk sac. The amnion and the chorion are the chorioamniotic m ...
s that exist during
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ca ...
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 deve ...
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 shor ...
. In humans it is formed by extraembryonic
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
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 pl ...
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
The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional laye ...
, 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
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 pl ...
, and an inner formed by the somatic
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical E ...
.
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 embryo ...
or layer of Langhans, and an external
multinucleated layer, the
syncytiotrophoblast
Syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nut ...
.
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
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 ...
'', which invade and destroy the
uterine decidua
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 ...
, while simultaneously absorbing nutritive materials from it for the growth of the
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
.
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 splitting ...
.
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 bl ...
, 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
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 ...
as
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. Until about the end of the second month of
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ca ...
, the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.
Parts
![Placenta with fetal membranes](https://upload.wikimedia.org/wikipedia/commons/b/b0/Placenta_with_fetal_membranes.jpg)
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 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 mater ...
. 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 former ...
'' bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
can infect the chorion tissue, thereby impacting pregnancy outcome. In addition, footprints of JC polyomavirus 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 Ivanovsky's 1 ...
, BK polyomavirus has been detected in the same tissues, but with lesser extent.
Other animals
In reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s, 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 lightweigh ...
s, and monotreme
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s, the chorion is one of the four extraembryonic membranes 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 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 pairs ...
s. 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. 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
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 ...
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.
See also
* Choriogenesis
* Chorioamnionitis
Chorioamnionitis, also known as intra-amniotic infection (IAI), is inflammation of the fetal membranes ( amnion and chorion), usually due to bacterial infection. In 2015, a National Institute of Child Health and Human Development Workshop expert ...
, an inflammation
Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
of the chorion and amnion, usually due to bacterial infection
* Chorionic hematoma
* Gestational trophoblastic disease
Gestational trophoblastic disease (GTD) 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 proliferate uncontrollably. The cells that form gestational trophoblastic tum ...
, any abnormal proliferation of the trophoblasts
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 ...
, including choriocarcinoma
Choriocarcinoma is a malignant, trophoblastic cancer, usually of the placenta. It is characterized by early hematogenous spread to the lungs. It belongs to the malignant end of the spectrum in gestational trophoblastic disease (GTD). It is also cl ...
, a highly invasive cancer.
References
External links
* — "Female Reproductive System: placenta, chorionic plate"
McGill
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Embryology