Scheme of roof of fourth ventricle. The arrow is in the median aperture.
1: Inferior medullary velum
2: Choroid plexus
3: Cisterna magna of subarachnoid space
4: Central canal
5: Corpora quadrigemina
6: Cerebral peduncle
7: Superior medullary velum
8: Ependymal lining of ventricle
9: Pontine cistern of subarachnoid space
|Anatomical terms of neuroanatomy|
The choroid plexus consists of a layer of cuboidal epithelial cells surrounding a core of capillaries and loose connective tissue. The epithelium of the choroid plexus is continuous with the ependymal cell layer that lines the ventricles. The cells of the choroid plexus are non ciliated but, 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 increases 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.
The choroid plexus mediates the production of cerebrospinal fluid (CSF). CSF acts as a medium for filtration system that facilitates the removal of metabolic waste from the brain and exchange of biomolecules and xenobiotics 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.
Blood–cerebrospinal fluid barrierEdit
The blood–cerebrospinal fluid barrier (BCSFB) is a fluid–brain barrier that is composed of a pair of membranes that separate blood from CSF and CSF from brain tissue. The blood–CSF boundary at the choroid plexus is a membrane composed of epithelial cells and tight junctions that link them. The brain–CSF boundary is the arachnoid membrane, which envelops the surface of the brain.
Similar to the blood–brain barrier, the blood–CSF barrier functions to prevent the passage of most blood-borne substances into the brain, while selectively permitting the passage specific substances into the brain and facilitating the removal of brain metabolites and metabolic waste 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 distinct structural characteristics between 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 cytokines that recruit monocyte-derived macrophages, among other cells, to the brain. This cellular trafficking has implications both in normal brain homeostasis as in neuroinflammatory processes.
Choroid plexus cystsEdit
During embryological development, some fetuses may form choroid plexus cysts. 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. 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 (trisomy 21) cases. ~75% of abnormal karyotypes associated with choroid plexus cysts are trisomy 18, while the remainder are trisomy 21.
Choroid plexus translates from the Latin plexus chorioides, which mirrors Ancient Greek χοριοειδές πλέγμα. The word chorion was used by Galen 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 (now Terminologia Anatomica) reflected this dual usage.
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