CSF is formed by filtration of plasma in the choroid plexus. Which structure is primarily responsible for CSF formation?

• A. Dura mater
• B. Choroid plexus
• C. Ependymal cells
• D. Arachnoid granulations

Answer: (B)

CSF is produced mainly by the choroid plexus, a vascular network inside the brain ventricles that is covered by specialized ependymal cells. These epithelial cells actively secrete CSF by transporting ions and other solutes from the blood into the ventricular space, with water following to form the fluid. The choroid plexus’ filtration and active transport create the CSF composition and volume.

Dura mater is the outer protective layer, and arachnoid granulations are sites where CSF is absorbed back into the venous system, helping turnover rather than production. Ependymal cells line the ventricles and help circulate CSF, but the primary source of CSF formation is the choroid plexus.