Blood–cerebrospinal fluid barrier

Choroid plexus
Scheme of roof of fourth ventricle. The arrow is in the foramen of Magendie. 1: inferior medullary velum 2: Choroid plexus 3: Cerebellomedullary cistern of subarachnoid cavity 4: Central canal 5: Corpora quadrigemina 6: Cerebral peduncle 7: Superior medullary velum 8: Ependymal lining of ventricle 9: Pontine cistern of subarachnoid cavity
Coronal section of lateral and third ventricles.
Details
Identifiers
Latin	Plexus choroideus
MeSH	A08.186.211.276.298
NeuroNames	ancil-456
NeuroLex ID	Choroid plexus
TA	A14.1.09.279 A14.1.01.307 A14.1.01.306 A14.1.01.304 A14.1.05.715
FMA	61934
Anatomical terms of neuroanatomy []

The choroid plexus is a plexus of cells that produces the cerebrospinal fluid in the ventricles of the brain. The choroid plexus consists of modified ependymal cells.

There are four choroid plexuses in the brain, one in each of the ventricles.

Choroid plexus is present in all components of the ventricular system except for the cerebral aqueduct, frontal horn of the lateral ventricle, and occipital horn of the lateral ventricle.

Choroid plexus is found in the superior part of the inferior horn of the lateral ventricles. It follows up along this boundary, continuous with the inferior of the body of the lateral ventricles. It passes into the interventricular foramen, and is present at the top of the third ventricle.

There is also choroid plexus in the fourth ventricle, in the section closest to the bottom half of the cerebellum.

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, but unlike the ependyma, the 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.