Scanning electron micrograph of capillaries or microvessels (red/orange/green 3D cylindrical structures) isolated from a mouse brain. Brain microvessels are characterized by the presence of neurovascular units, the building blocks of the blood brain barrier (a protective layer of cells that regulates entry of molecules to the brain). The neurovascular unit is made up of endothelial cells surrounded by other cell types which regulate the exchange of molecules between the blood and the brain. This together with the thin diameter of the capillary makes it difficult for circulating drugs to reach the brain. New approaches to brain drug delivery will be key to tackle many neurological disorders. Diameter of each blood vessel is approximately 5 micrometres. Supplied by Izzat Suffian, Houmam Kafa, David McCarthy & Khuloud T. Al-Jamal, courtesy of Wellcome Images.