The choroid plexus is a highly vascularized tissue in the brain ventricles that forms one of the blood-brain barrier interfaces. It participates in control of the brain’s internal environment by acting as the blood-cerebrospinal fluid (CSF) barrier (BCSFB), formed by the epithelial cells, which feature tight junctions on their CSF-facing side. Mammalian choroid plexuses (ChPs) develop at four sites in the roof of the neural tube (future brain ventricles sites) shortly after its closure. They appear early in all mammalian species, at a time when the brain is still poorly vascularized. Two ChPs appear in the lateral ventricles of the telencephalon, one in the third ventricle of the diencephalon and one in the fourth ventricle of the hindbrain (hChP). hChP emerges earliest during embryogenesis and is relatively large. hChP epithelium production begins around E9.5, peaks between ~E11–12, and terminates by E14.
The choroid plexus differentiates from the ependymal cells lining the ventricular walls and is frequently considered a specialized cuboidal epithelium of ependymal lineage.
One of the main functions of the ChP is to secrete CSF, which is accomplished by active transport of small ions and water from the blood side to the CSF side. The CSF fills the four brain ventricles, the cranial subarachnoid space (SAS), the central canal, and surrounding cavities of the spinal cord. The CSF provides mechanical protection and a stable physiological environment for the central nervous system. ChPs also supply the brain with certain nutrients, hormones, and metal ions, while clearing the CSF of metabolites and xenobiotics. The basolateral membrane of the ChP monolayer is in free contact with blood vessels, whereas the apical membrane (also known as the luminal membrane) faces the CSF.