The choroid plexus, or plica choroidea, is a

plexus In neuroanatomy, a plexus (from the Latin term for "braid") is a branching network of vessels or nerves. The vessels may be blood vessels (veins, capillaries) or lymphatic vessels. The nerves are typically axons outside the central nervous system ...

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the

cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates. CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the ...

(CSF) of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

. The choroid plexus consists of modified ependymal cells surrounding a core of capillaries and loose connective tissue. Multiple

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

on the ependymal cells move to circulate the cerebrospinal fluid.

Structure

Location

There is a choroid plexus in each of the four ventricles. In the lateral ventricles it is found in the body, and continued in an enlarged amount in the atrium. There is no choroid plexus in the anterior horn. In the

third ventricle The third ventricle is one of the four connected ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ...

there is a small amount in the roof that is continuous with that in the body, via the interventricular foramina, the channels that connect the lateral ventricles with the third ventricle. A choroid plexus is in part of the roof of the fourth ventricle.

Microanatomy

The choroid plexus consists of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue. The

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

of the choroid plexus is continuous with the ependymal cell layer (ventricular layer) that lines the ventricular system. Progenitor ependymal cells are monociliated but they differentiate into multiciliated ependymal cells. Unlike the ependyma, the choroid plexus epithelial layer has tight junctions between the cells on the side facing the ventricle (apical surface). These tight junctions prevent the majority of substances from crossing the cell layer into the cerebrospinal fluid (CSF); thus the choroid plexus acts as a blood–CSF barrier. The choroid plexus folds into many villi around each capillary, creating frond-like processes that project into the ventricles. The villi, along with a brush border of microvilli, greatly increase the surface area of the choroid plexus. CSF is formed as plasma is filtered from the blood through the epithelial cells. Choroid plexus epithelial cells actively transport sodium ions into the ventricles and water follows the resulting osmotic gradient. The choroid plexus consists of many capillaries, separated from the ventricles by choroid epithelial cells. Fluid filters through these cells from blood to become cerebrospinal fluid. There is also much active transport of substances into, and out of, the CSF as it is made.

Function

The choroid plexus regulates the production and composition of

(CSF), that provides the protective buoyancy for the brain. CSF acts as a medium for the glymphatic filtration system that facilitates the removal of metabolic waste from the brain, and the exchange of biomolecules and

xenobiotics A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural comp ...

into and out of the brain. In this way the choroid plexus has a very important role in helping to maintain the delicate extracellular environment required by the brain to function optimally. The choroid plexus is also a major source of

transferrin Transferrins are glycoproteins found in vertebrates which bind to and consequently mediate the transport of iron (Fe) through blood plasma. They are produced in the liver and contain binding sites for two Fe3+ ions. Human transferrin is encod ...

secretion that plays a part in

iron homeostasis Human iron metabolism is the set of chemical reactions that maintain human homeostasis of iron at the systemic and cellular level. Iron is both necessary to the body and potentially toxic. Controlling iron levels in the body is a critically imp ...

in the brain.

Blood–cerebrospinal fluid barrier

The blood–cerebrospinal fluid barrier (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF at the capillary level and CSF from brain tissue. The blood–CSF boundary at the choroid plexus is a membrane composed of

epithelial cells Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...

and tight junctions that link them. There is a CSF-brain barrier at the level of the pia mater, but only in the embryo. Similar to the

blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of the central nervous system where ne ...

, the blood–CSF barrier functions to prevent the passage of most blood-borne substances into the brain, while selectively permitting the passage of specific substances into the brain and facilitating the removal of brain metabolites and metabolic products into the blood. Despite the similar function between the BBB and BCSFB, each facilitates the transport of different substances into the brain due to the distinctive structural characteristics of each of the two barrier systems. For a number of substances, the BCSFB is the primary site of entry into brain tissue. The blood–cerebrospinal fluid barrier has also been shown to modulate the entry of leukocytes from the blood to the central nervous system. The choroid plexus cells secrete

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...

s that recruit monocyte-derived macrophages, among other cells, to the brain. This cellular trafficking has implications both in normal brain homeostasis and in neuroinflammatory processes.

Clinical significance

Choroid plexus cysts

During fetal development, some

choroid plexus cysts Choroid plexus cysts (CPCs) are cysts that occur within choroid plexus of the brain. They are the most common type of intraventricular cyst, occurring in 1% of all pregnancies. It is believed that many adults have one or more tiny CPCs. The fetal ...

may form. These fluid-filled cysts can be detected by a detailed second trimester ultrasound. The finding is relatively common, with a prevalence of ~1%. Choroid plexus cysts are usually an isolated finding. The cysts typically disappear later during pregnancy, and are usually harmless. They have no effect on infant and early childhood development. Cysts confers a 1% risk of fetal

aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with an ...

. The risk of aneuploidy increases to 10.5-12% if other risk factors or ultrasound findings are noted. Size, location, disappearance or progression, and whether the cysts are found on both sides or not do not affect the risk of aneuploidy. 44-50% of Edwards syndrome (trisomy 18) cases will present with choroid plexus cysts, as well 1.4% of

Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual d ...

(trisomy 21) cases. ~75% of abnormal karyotypes associated with choroid plexus cysts are trisomy 18, while the remainder are trisomy 21.

Other

There are three graded types of choroid plexus tumor that mainly affect young children. These types of

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

are rare.

Etymology

Choroid plexus translates from the Latin plexus chorioides,Suzuki, S., Katsumata, T., Ura, R. Fujita, T., Niizima, M. & Suzuki, H. (1936). Über die Nomina Anatomica Nova. ''Folia Anatomica Japonica, 14'', 507-536. which mirrors Ancient Greek χοριοειδές πλέγμα. The word ''chorion'' was used by

Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be o ...

to refer to the outer membrane enclosing the fetus. Both meanings of the word plexus are given as pleating, or braiding. As often happens language changes and the use of both ''choroid'' or ''chorioid'' is both accepted.

Nomina Anatomica ''Nomina Anatomica'' (''NA'') was the international standard on human anatomic terminology from 1895 until it was replaced by '' Terminologia Anatomica'' in 1998. In the late nineteenth century some 30,000 terms for various body parts were in us ...

(now

Terminologia Anatomica ''Terminologia Anatomica'' is the international standard for human anatomical terminology. It is developed by the Federative International Programme on Anatomical Terminology, a program of the International Federation of Associations of Anatomi ...

) reflected this dual usage.

Additional images

File:Gray749.png, Coronal section of inferior horn of lateral ventricle. File:Choroid Plexus Histology 40x.png, Choroid Plexus Histology 40x File:Slide2ff.JPG, Choroid plexus File:Slide3oo.JPG, Choroid plexus File:Choroid plexus.jpg, Choroid plexus

References

Sources

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External links

3-Dimensional images of choroid plexus (marked red)
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Images of Choroid Plexus * More info a
BrainInfo
{{DEFAULTSORT:Choroid Plexus Meninges Ventricular system