The choroid plexus, or plica choroidea, is a plexus of cells that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central nervous system. The choroid plexus consists of modified ependymal cells surrounding a core of capillaries and loose connective tissue. Multiple cilia on the ependymal cells move to circulate the cerebrospinal fluid.

The third ventricle is one of the four connected ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ventricles, and is filled with cerebrospinal fluid (CSF).