Choroid plexus

The choroid plexus is the area on the ventricles of the brain where cerebrospinal fluid (CSF) is produced by modified ependymal cells.

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

It 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 on the fourth ventricle, on the section closest to the bottom half of the cerebellum.

Structure of the choroid plexus
The choroid plexus consists of many capillaries, separated from the subarachnoid space by pia mater and choroid ependymal cells. Liquid 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.

Pathology
During embryological development, some fetuses may form choroid plexus cysts. These fluid-filled cysts are likely harmless, however if found during an ultrasound, they may be an indicator of chromosomal abnormalities in the fetus, especially trisomy 21 (Down's syndrome), and further testing may be indicated.