Choroid plexus

The choroid plexus develops where vascular pia (the tela choroidea) invaginates the thin ependymal roof of a ventricle. It is found in:
• the body, atrium and temporal horn of each lateral ventricle;
• the roof of the third ventricle;
• the lower roof of the fourth ventricle.
Each villus has a core of fenestrated capillaries and loose connective tissue, covered by a single layer of cuboidal choroid epithelium, a modified ependyma bearing apical microvilli and some cilia. Unlike ordinary ependyma, adjacent choroid epithelial cells are sealed by tight junctions. This epithelium, not the leaky capillaries, is the blood–CSF barrier.

CSF secretion is an active process. Water follows ions through the epithelium:
• basolateral transporters (Na⁺/H⁺ and Cl⁻/HCO₃⁻ exchangers, driven by carbonic anhydrase) bring ions into the cell;
• the apical Na⁺/K⁺-ATPase and Cl⁻ and HCO₃⁻ channels move ions into the ventricle;
• water follows osmotically through aquaporin-1.
The plexus produces ≈80% of the CSF (≈0.35 mL/min, ≈500 mL/day). It also secretes transthyretin (which carries thyroxine into the CSF) and growth factors, and actively removes some metabolites and drugs from the CSF.