Structure and Function of the Choroid Plexus and Other Sites of Cerebrospinal Fluid Formation

Abstract

Publisher Summary This chapter discusses the structure and function of the choroid plexus and other sites of cerebrospinal fluid (CSF) formation. A considerable volume of the CSF is formed continuously within the cerebral ventricles. The choroid plexuses contribute to this formation, but a significant fraction of the CSF is formed extrachoroidally. The exact contributions of fluid from choroidal and extrachoroidal sites remain to be determined. The elaboration of choroidal fluid probably involves the following steps: filtration of the .blood plasma across fenestrated choroidal capillaries, formation of a protein-rich interstitial fluid within the choroidal stroma, and movement of constituents of the interstitial fluid across the choroidal epithelium by the combined processes of ultrafiltration and active transport. Contributing to the choroid plexus blood-CSF barrier are at least three specialized features: a system of circumferential tight functions joining adjacent epithelial cells; a heterolytic system of pinocytotic vesicles and lysosomes within epithelial cells; and a system of epithelial cell enzymes concerned with the active bidirectional transport of substances between the plasma and CSF. It is likely that the choroid plexus epithelium elaborates a carefully regulated fluid and, at the same time, participates in the homeostasis of the CSF. Evidence exists that the extracellular fluid of the brain is continuously formed across the cerebral endothelium, that it drains in bulk to the adjacent CSF cavities, and that it serves as a vehicle for removing intracerebral metabolites. Overall, there is probably a steady, net addition of new fluid at all points along the pathways of CSF circulation until the major sites of absorption (arachnoid villi) are reached.