Choroid plexus secretory cells. Coloured scanning electron micrograph SEM of a freeze_fractured choroid plexus from a brain. The fracture plane grey has revealed the vertical columns of secretory cells, whose tips make up the surface upper right. The tips are swollen due to increased secretion of the cerebrospinal fluid CSF that they produce. The choroid plexus is a network of capillary blood vessels that is found in each of the four ventricles fluid_filled cavities of the brain. It produces some of the CSF that surrounds and protects the brain and spinal cord.