choroid plexus A membrane rich in blood vessels that lines the ventricles of the brain. It is an extension of the pia mater and secretes cerebrospinal fluid into the ventricles; it also controls exchange of materials between the blood and cerebrospinal fluid.

choroid plexus n. a rich network of blood vessels, derived from those of the pia mater, in each of the brain's ventricles. It is responsible for the production of cerebrospinal fluid.