Blood-brain barrier. Confocal light micrograph of a section through a blood vessel in the brain, showing the arrangement of cells that form the blood-brain barrier. The lumen (interior) of the vessel runs horizontally across the upper frame. The endothelial cells that line the blood vessels of the brain are packed more tightly than elsewhere in the body. This barrier protects the brain from many potentially harmful molecules and micro-organisms, but also presents a challenge for the administration of drugs to the brain. Surrounding the blood vessel are glial cells (green), which provide structural support for neurons (nerve cells, red) and supply them with nutrients and oxygen. It is also thought that glial cells help maintain the blood-brain barrier.