Adult zebrafish brain, light micrograph. Zebrafish (Danio rerio) are popular model organisms in biological research, and can be used for studying brain function. Dense networks of blood vessels (red) line the outer surface of the brain. There are fluorescent granular perithelial cells (FGPs, yellow/green) next to these blood vessels, which haven't previously been studied extensively. Researchers believe these cells continuously take in and store waste products to keep the brain functioning well.