Blood-brain barrier breakdown. Confocal light micrograph of a section through the forebrain showing a breakdown in the blood-brain barrier. A fluorescent tracer (lighter areas) has been injected into the blood and is leaking from the blood vessels into the surrounding brain tissue. The barrier, which consists of tightly packed endothelial cells, protects the brain from potentially harmful molecules and micro-organisms. If it is breached, by injury or disease, it can lead to the death of the surrounding tissue.