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Chorion
Diagram showing the chorion of a chicken egg
Human fetus enclosed in the amnion
Details
Identifiers
Latinchorion
TEE5.11.3.1.1.0.3
Anatomical terminology

The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It is also present around the embryo of other animals, like insects and molluscs.

Structure

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In humans and other therian mammals, the chorion is one of the fetal membranes that exist during pregnancy between the developing fetus and mother. The chorion and the amnion together form the amniotic sac. In humans it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes;[1] the chorionic villi emerge from the chorion, invade the endometrium, and allow the transfer of nutrients from maternal blood to fetal blood.

Layers

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The chorion consists of two layers: an outer formed by the trophoblast, and an inner formed by the extra-embryonic mesoderm.

The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer of Langhans, and an external multinucleated layer, the syncytiotrophoblast.

Growth

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The chorion undergoes rapid proliferation and forms numerous processes, 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.

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

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Placenta with attached fetal membranes (ruptured at the margin at the left in the image), which consists of the chorion (outer layer) and amnion (inner layer).

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,[2] 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, increase greatly in size and complexity, and hence this part is named the chorion frondosum.[2]

Thus the placenta develops from the chorion frondosum and the decidua basalis.

Monochorionic twins

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Main article: Monochorionic twins

Monochorionic twins are twins that share the same placenta. This occurs in 0.3% of all multiple-birth human pregnancies,[3] and in 75% of monozygotic (identical) twins, when the split takes place on or after the third day after fertilization.[4] The remaining 25% of monozygous twins become dichorionic diamniotic.[4] The condition may affect any type of multiple birth, resulting in monochorionic multiples.

Infections

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Recent studies indicate that the chorion may be susceptible to pathogenic infections.[5] Recent findings indicate that Ureaplasma parvum bacteria can infect the chorion tissue, thereby impacting pregnancy outcome.[6] In addition, footprints of JC polyomavirus and Merkel cell polyomavirus have been detected in chorionic villi from females affected by spontaneous abortion as well as pregnant women.[7][8] Another virus, BK polyomavirus has been detected in the same tissues, but with lesser extent.[9]

Other animals

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Amniotic embryo. a=embryo, b=yolk, c=allantois, d=amnion, e=chorion

In reptiles, birds, and monotremes, 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 insects. 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. 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.

In insects, it develops by the follicle cells while the egg is in the ovary.[10] Some mollusks also have chorions as part of their eggs. For example, fragile octopus eggs have only a chorion as their envelope.[11]

Additional images

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  • Section through the embryo.
    Section through the embryo.
  • Diagram of the human embryo.
    Diagram of the human embryo.
  • Diagram illustrating early formation of allantois and differentiation of body-stalk.
    Diagram illustrating early formation of allantois and differentiation of body-stalk.
  • Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
    Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.
  • Scheme of placental circulation.
    Scheme of placental circulation.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Chorion

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from Grokipedia
The chorion is the outermost extraembryonic membrane in amniotes, including reptiles, birds, and mammals, forming from the somatopleure ( and somatic ) to enclose the developing , , and other membranes. In reptiles and birds, the chorion adheres to the and is highly vascularized to facilitate (oxygen and ) between the and the external environment. In mammals, it originates from trophoblastic tissue and extraembryonic , evolving into the fetal component of the to enable nutrient uptake, waste removal, and material exchange with the maternal bloodstream via . Structurally, the mammalian chorion consists of a reticular layer containing mesenchymal cells, a pseudo-basement , and a multilayer of cells that fuse with the maternal at the feto-maternal interface. Beyond its roles in exchange and protection, the chorion modulates immune during by secreting anti-inflammatory factors, cytokines, and to promote maternal-fetal tolerance and prevent immune rejection. Toward term, it contributes to parturition through , inflammatory signaling, and production, facilitating membrane weakening and rupture.

Mammalian Chorion

Anatomical Structure

The chorion is the outermost extraembryonic membrane in mammals, enveloping the and while contributing to the formation of the . It originates from the cells of the , specifically the trophectoderm layer that surrounds the during early embryonic development. Structurally, the chorion consists of two primary layers derived from the : the inner , a single layer of mononucleated cuboidal cells that provides and cells, and the outer , a multinucleated formed by the fusion of cells, which lacks cell boundaries and facilitates direct interaction with maternal tissues. These layers are underlain by extraembryonic , adding vascular and components to the membrane. The chorion forms initially during blastocyst implantation, when the trophectoderm differentiates into cells that proliferate rapidly to expand the membrane around the developing . This growth involves ongoing cellular proliferation in the layer and fusion events that renew the , allowing the chorion to enlarge and adapt to the uterine environment over the course of . The chorion is regionally differentiated into the villous chorion, which features finger-like that project into the maternal to maximize surface area for placental development, and the smooth chorion, an avascular region that covers the abembryonic pole of the without villi. In later , the villi in the smooth region regress, resulting in the chorion laeve, a thin, smooth membrane that fuses with the to form the chorioamniotic membrane enclosing the amniotic cavity.

Embryonic Development

The chorion begins to form during the stage of human embryogenesis, approximately 4 to 5 days after fertilization, when the outer layer of the blastocyst differentiates into the chorionic membrane that will surround the developing . This initial structure consists of a single layer of cells that adhere to the uterine wall, marking the onset of implantation around days 6 to 7 post-fertilization. Implantation progresses as the trophoblast invades the uterine , completing by days 9 to 10, during which the chorion establishes its role in anchoring the . By the end of the first trimester, around 12 weeks of , the chorion achieves full maturation, with its vascularization and regional differentiation largely established to support subsequent fetal growth. Central to chorion development are the cellular processes involving differentiation and . The divides into two layers: the inner , composed of proliferative mononuclear cells, and the outer , a multinucleated layer formed by fusion of cells that secretes enzymes to facilitate endometrial . This differentiation enables the to erode maternal tissue, creating lacunae filled with maternal blood by day 9, which nourishes the early . Extraembryonic migrates into the chorion around week 3, vascularizing the structure and forming secondary and tertiary villi. Key events include the formation of the chorionic cavity by day 13, which expands to enclose the and , and the attachment of the chorion to the via connecting stalk . Further maturation involves differentiation into villous regions, where persist for nutrient exchange, and smooth regions, where villi regress to form the avascular chorion of the , a process completing by the end of the first trimester. In monochorionic diamniotic twins, the chorion is shared due to splitting of the between days 4 and 8 post-fertilization, after chorion formation but before development, resulting in a single l structure with separate amniotic sacs. Genetic and molecular drivers orchestrate these processes, while GCM1 regulates formation and development by overriding FGF signaling pathways. These factors ensure precise spatiotemporal control, leading to the chorion's integration with the and formation of structural layers like the trophoblastic shell.

Physiological Functions

The mammalian chorion plays a central role in embryonic support by enabling the exchange of nutrients, waste products, and gases between maternal and fetal circulations, primarily through the diffusion across lined by . In the hemochorial placenta characteristic of humans and many , the chorion directly interfaces with maternal blood in intervillous spaces, allowing efficient transfer without direct mixing of blood streams; this arrangement supports the uptake of oxygen and essential nutrients like glucose and while facilitating the removal of fetal and metabolic wastes. The expansive surface area of the , reaching approximately 10 m² in humans by term, optimizes this diffusive exchange to meet the growing fetus's demands. Beyond transport, the chorion provides protective functions by acting as an immunological barrier, expressing on cells to suppress maternal immune responses and prevent rejection of the semiallogeneic . This non-classical MHC molecule inhibits natural killer cells, T lymphocytes, and antigen-presenting cells, ensuring at the maternal-fetal interface. The chorion is selectively permeable, allowing transfer of protective IgG antibodies while impermeable to larger IgM antibodies and immune cells, further safeguarding the from harmful maternal immune components. The chorion also exhibits endocrine capabilities, with its synthesizing (hCG) to maintain the and sustain early progesterone production for continuation. As progresses, the same layer produces progesterone directly and (hPL), which promotes maternal metabolic adaptations like to prioritize fetal nutrient availability. These hormonal outputs ensure uterine quiescence and nutritional provisioning throughout gestation.

Clinical and Pathological Aspects

Role in Pregnancy Complications

Monochorionic twin pregnancies, in which twins share a single chorion, occur in approximately 0.3-0.5% of all pregnancies and are associated with heightened risks due to vascular anastomoses connecting the fetal circulations within the shared . These anastomoses can lead to (TTTS), a serious complication affecting 10-15% of , where unbalanced blood flow causes volume depletion in one twin and overload in the other, potentially resulting in fetal demise or preterm delivery. Abnormalities in the chorion can also manifest as , also known as subchorionic hemorrhage, which involves bleeding between the chorion and uterine wall and increases the risk of , particularly when the hematoma is large or detected early in . Another related issue is premature rupture of the chorioamniotic membranes, where the chorion and separate or tear before term, often leading to and associated neonatal complications such as respiratory distress. In cases of complete hydatidiform mole, a type of , there is abnormal proliferation of without a viable , resulting in swollen, grape-like structures that fill the . is typically confirmed via , which reveals a characteristic "snowstorm" appearance due to the diffuse echogenic pattern of the hydropic villi. Confined placental mosaicism represents a genetic complication where chromosomal abnormalities, such as trisomies, are present in chorionic cells but absent in the , potentially impairing placental function and leading to or other developmental issues. The mechanisms underlying monozygotic twinning, including those resulting in monochorionic placentation, were refined through 20th-century embryological studies that elucidated post-fertilization embryo splitting and its timing relative to chorion formation.

Infections and Immune Interactions

The chorion contributes to at the maternal-fetal interface by expressing (FasL), which induces in activated maternal T cells that express Fas receptors, thereby preventing immune rejection of the . Syncytiotrophoblasts, cytotrophoblasts, and chorionic extravillous trophoblasts in the chorion produce FasL, promoting local suppression of maternal lymphocyte responses. Additionally, the chorion expresses (IDO), an enzyme that catabolizes into metabolites, depleting local tryptophan levels and inhibiting maternal T-cell proliferation and activation. This IDO-mediated mechanism is particularly active in syncytiotrophoblasts and chorionic macrophages, safeguarding the semi-allogeneic from inflammatory T-cell responses. Infections can breach the chorion via ascending routes, where pathogens from the lower genital tract migrate through the and ruptured membranes to reach the chorioamniotic space, often causing chorioamnionitis. Group B (GBS), a common vaginal colonizer, exemplifies this pathway, leading to bacterial ascension and inflammation of the chorion. Transplacental infections, such as those caused by (CMV), occur via hematogenous spread from maternal blood, allowing viral particles to cross the chorionic barrier directly into . These routes compromise the chorion's protective role, triggering acute inflammatory responses. Chorioamnionitis, a key pathological outcome of chorionic infections, is histologically identified by infiltration into the chorion and layers, indicating acute at the maternal-fetal interface. This condition is associated with 30-40% of spontaneous preterm labors, where microbial invasion prompts preterm premature and . At the molecular level, chorionic trophoblasts express Toll-like receptors (TLRs), such as TLR2 and TLR4, which detect pathogen-associated molecular patterns and initiate innate immune signaling. Upon activation, these receptors induce the release of pro-inflammatory cytokines, including IL-6 and IL-8, from trophoblasts, recruiting and amplifying the inflammatory cascade to combat but potentially exacerbating preterm labor. Post-2015 (ZIKV) outbreaks highlighted vulnerabilities in the chorion barrier, with the virus infecting placental trophoblasts and causing direct fetal transmission. In affected pregnancies, ZIKV replicates extensively in chorionic tissues, leading to barrier disruption, vascular damage, and fetal outcomes like and . This transplacental route was confirmed in human cohorts from the 2015 Brazilian epidemic, where high viral loads in placentas facilitated fetal infection despite maternal .

Comparative Biology

In Birds and Reptiles

In birds and reptiles, the chorion is a thin extraembryonic membrane derived from the of the somatopleure, forming an avascular epithelial layer that envelops the developing within the shelled . This structure arises early in embryogenesis, equivalent to the trophectoderm in mammalian development, and lines the chorionic cavity during around embryonic day 2-3 in avian models like the . Composed primarily of squamous or cuboidal epithelial cells supported by a , the chorion in chickens features two distinct layers of al cells, with an overall thickness typically ranging from 5 to 10 μm in mature stages, minimizing diffusion barriers for subsequent physiological roles. In reptiles, such as oviparous squamates and crocodilians, the chorion similarly originates from extraembryonic but exhibits variations in epithelial adapted to environmental demands, remaining avascular until fusion with underlying . Developmentally, the chorion expands rapidly as the grows, establishing the outer boundary of the egg's extraembryonic space by embryonic day 4 in chickens. It subsequently fuses with the —an outpouching of the —beginning around embryonic day 5 in birds, where mesodermal layers from both membranes adhere via mesothelial , forming the chorioallantoic (CAM). This fusion process, mediated by cellular interdigitation and vascular ingrowth from the allantois, vascularizes the chorion's mesodermal component while preserving its ectodermal as the outer barrier, completing CAM maturation by embryonic day 8-11 in chickens. In reptiles, chorion-allantois fusion follows a parallel timeline, occurring post-gastrulation in species like Chalcides, where the allantois contacts the chorion to create a diffuse chorioallantois that adheres to the shell . Unlike in mammals, this CAM lacks direct contact with maternal circulation, relying instead on the for environmental interface. The primary functions of the chorion and CAM in these oviparous amniotes center on respiratory and within the closed eggshell system. The CAM serves as the embryo's principal "lung," facilitating oxygen influx and efflux through the shell's porous structure, with avian CAM capillaries positioned just beneath a sub-micrometer-thick ectodermal covering (<1 μm in chickens) to optimize gradients. In birds, this exchange peaks between embryonic days 7 and 15, supported by the CAM's dense capillary network, which covers nearly the entire egg's inner surface by embryonic day 10–11. Reptilian chorioallantois performs analogous roles, with enhanced vascular density in arid-adapted species like crocodiles, and reduced shell aiding water retention while permitting gas via cuticle-limited pores. These adaptations ensure embryonic viability in terrestrial environments without maternal provisioning, contrasting sharply with the nutrient-exchanging placental role in viviparous mammals.

In Fish and Amphibians

In fish, the chorion is an acellular envelope, also known as the zona radiata, that surrounds the and embryo, providing a protective barrier composed primarily of (ZP) proteins synthesized by cells. This structure consists of multiple layers, including an inner layer rich in ZP proteins and an outer filamentous layer, forming a semi-permeable matrix that regulates and osmotic balance in aquatic environments. In amphibians, the equivalent structure is the vitelline envelope, often embedded within multiple jelly coat layers secreted by the oviduct, serving as the primary membrane with components that contribute to the egg's overall . These jelly coats, varying in thickness and composition across species like Xenopus laevis, add hydration and species-specific adhesion properties to the vitelline envelope. The chorion and vitelline envelope fulfill critical protective functions in these non-amniote vertebrates, shielding the from mechanical damage, osmotic stress, and environmental pathogens in aquatic settings. In , the chorion's semi-permeability allows selective , such as water and small solutes, to maintain internal while preventing excessive swelling or . Similarly, in amphibians, the jelly coats and vitelline envelope buffer against hypotonic freshwater conditions and mechanical abrasion during . Both structures act as primary sites for binding during fertilization; in teleost , adhere to the chorion surface via specific glycoproteins, facilitating entry through a specialized micropyle—a narrow that ensures monospermy. In amphibians, the vitelline envelope provides initial recognition, with jelly coats enhancing chemotactic guidance. Development of these envelopes occurs primarily pre-fertilization through secretion by cells in , where ZP proteins are assembled into the zona radiata during , forming a multilayered . Post-fertilization, the envelope hardens via enzymatic cross-linking, often mediated by reactions involving ZP proteins, which increases rigidity and impermeability to block . In amphibians, the vitelline envelope forms during as a thin layer, with jelly coats added sequentially in the ; upon fertilization, cortical granule triggers cross-linking and transformation into a tougher fertilization , elevating the structure and preventing additional penetration. This hardening process, driven by release from cortical granules, is essential for blocking in both groups, with chorions expanding the perivitelline space via osmotic influx to further isolate the . Specific adaptations highlight the chorion's role in aquatic reproduction: in teleost fish, the micropyle not only permits single-sperm entry but also influences permeability, allowing localized influx for hydration without compromising overall barrier integrity. In amphibians, the vitelline envelope's transformation actively prevents by altering surface charge and structure, as seen in species like Rana pipiens, where it combines with a rapid electrical block at the plasma membrane. Recent studies since 2020 have explored chorion biodegradation through hatching enzymes—proteases like high choriolytic enzyme (HCE) in teleosts—to optimize in , reducing manual interventions and promoting sustainable practices by minimizing waste accumulation and enhancing larval survival rates in controlled systems.

Evolutionary Perspectives

Origins and Homology

The chorion first appeared as a defining feature of early amniotes approximately 356 million years ago during the late to early period, coinciding with the terrestrialization of vertebrate reproduction. Recent track evidence from supports this refined timeline for the origin of crown-group Amniota. This extraembryonic membrane evolved as part of the amniotic complex, enabling internal development independent of aquatic environments by facilitating and protection from . Phylogenetic analyses place its origin within the crown group Amniota, distinguishing them from anamniotic tetrapods like amphibians. Precursors to the chorion are evident in the vitelline envelopes surrounding anamniote eggs, such as the jelly coats in amphibians and , which provided basic protection but lacked the specialized vascular integration seen in amniotes. These structures represent an evolutionary continuum, with the chorion emerging as an adaptation to land-based egg-laying during the , when rising oxygen levels and forested habitats supported the shift from aquatic to terrestrial reproduction. Fossil records of early eggs are scarce, but indirect evidence from reptiliomorph lineages suggests transitional membranes that prefigured the fully formed chorion in basal amniotes. In terms of homology, the chorion in mammals is directly homologous to that in birds and reptiles, as all derive from the embryonic and contribute to the chorioallantoic membrane for respiratory functions in oviparous species or placental interfaces in viviparous ones. This shared developmental origin underscores its status as a core trait. By contrast, the chorion in is analogous rather than homologous, originating from ovarian granulosa cells rather than embryonic tissues and serving primarily as a protective envelope without involvement in extraembryonic gas exchange. Molecular phylogeny reveals conservation of key s underlying chorion formation, particularly the (ZP) , which encodes glycoproteins essential for egg coat assembly across vertebrates. In amniotes, ZP1 and ZP4 subfamilies arose through Tetrapoda-specific duplications, maintaining structural roles in the chorion, while exhibit expanded ZP gene repertoires (up to 33 members) adapted to their distinct chorion composition. These genetic linkages highlight the chorion's evolutionary continuity within Amniota. Updated cladistic analyses since 2015, incorporating morphological and molecular data from extant and extinct taxa, confirm the chorion as a synapomorphy uniquely diagnosing Amniota, resolving prior ambiguities in stem-group relationships and reinforcing its role in the clade's diversification.

Adaptive Significance

The chorion, as an extraembryonic membrane, has evolved diverse structural and functional adaptations across lineages to optimize embryonic survival under varying environmental pressures, particularly in response to challenges like nutrient acquisition, , and . In mammals, the chorion's invasive integration into the placental interface facilitates direct nutrient and gas exchange with maternal blood, a key innovation enabling that emerged around 200 million years ago during the diversification of therian mammals. This adaptation allowed for prolonged internal gestation, reducing exposure to external threats while supporting metabolic demands through hemochorial , where chorionic cells penetrate uterine tissue for efficient resource transfer. In oviparous amniotes such as birds and reptiles, the chorion contributes to a gas-permeable barrier that supports respiratory exchange during incubation, with the chorioallantoic membrane (formed by fusion of chorion and ) enabling oxygen diffusion across the . In birds, this permeability is crucial for prolonged embryonic development in a stable nest environment, where the vascularized chorioallantoic membrane maintains high diffusive capacity for O₂ and CO₂, balancing the needs of growing embryos over weeks of incubation. Selective pressures from terrestrial habitats have further shaped chorion properties in reptiles, where its impermeability to water prevents in arid conditions, allowing eggs to be laid on land without lethal fluid loss while still permitting through shell pores. Aquatic environments impose distinct osmoregulatory demands on the chorion, particularly in teleost fish, where the acellular chorionic envelope acts as a selective barrier regulating and influx to maintain embryonic in hypotonic or hypertonic waters. In freshwater species, the chorion's low permeability to ions helps counter osmotic swelling, while in marine teleosts, it facilitates controlled hydration during oocyte swelling, preventing rupture under high . Evolutionary trade-offs in chorion thickness are evident in fish eggs, where thicker envelopes enhance mechanical protection against predation but delay , potentially reducing larval survival if environmental cues for emergence are missed; for instance, embryos accelerate hatching via enzymatic weakening of the chorion in response to predation risks, illustrating a balance between defense and timely development. Variations in chorion-related structures also reflect altitude-specific adaptations, as seen in high-altitude birds where enhanced vascularization of the chorioallantoic improves oxygen conductance to compensate for low ambient , ensuring adequate despite thinner effective barriers in some . In reptiles, transitions to —such as in certain squamates—adapt the chorion for internal without external laying, retaining yolk-based while the chorioallantois supports respiration in a protected oviductal environment, a strategy that mitigates desiccation risks in variable climates. Recent ecological studies from the highlight emerging pressures on egg permeability in amphibians, where climate-driven droughts increase hydric stress on permeable egg envelopes (analogous to chorion functions in basal vertebrates), potentially elevating rates and disrupting osmotic balance during early development.

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