Choroid plexus in brain. Light micrograph of a section through the choroid plexus of a healthy human brain. This is a network of capillary blood vessels (red/brown, for example at upper right) that is found in each of the four ventricles (fluid-filled cavities) of the brain. It produces the cerebrospinal fluid (CSF) that surrounds and protects the brain and the spinal cord. Magnification unknown.