Choroid plexus secretory cells. Coloured scanning electron micrograph (SEM) of a freeze-fractured choroid plexus from a brain. The fracture plane (green) 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. Magnification: x5000 at 6x7cm size.