Transmission electron micrograph of a neurovascular unit in a mouse brain capillary. The neurovascular unit is a building block of the blood brain barrier, which is a protective layer of cells that regulates entry of molecules to the brain. A red blood cell (center, solid red color) can be seen in the lumen. The lumen is lined with an endothelial cell layer (bluish-gray), which is in turn surrounded by other cell types (different colors). The endothelial cells regulate the exchange of molecules between the blood and the brain, and this, together with the thin diameter of the capillary, make 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 the neurovascular unit is approximately 9 micrometres. Supplied by Khuloud T. Al-Jamal & Houmam Kafa, courtesy of Wellcome Images.