Blood bathes every organ to provide oxygen and nutrients. In the brain, however, the blood vessels are specialized to allow delivery of oxygen, water and certain molecules like glucose, but to prevent exposure of the delicate brain tissue to many larger molecules and toxins. This barrier also makes it difficult to deliver medicines to the brain when treating neurodegenerative disorders such as Parkinson's. This picture is a thin section of a zebrafish brain capillary (grey) surrounded by brain tissue (blue). This photo was taken on an electron microscope, which can resolve cellular structures at very high magnification. These grey capillary endothelial cells are surrounded by a dark line, called the basement membrane, which reinforces the blood-brain barrier. The dark center is a red blood cell that fills the lumen of the capillary and makes this picture look a bit like an ancient cave drawing. The blood-brain barrier is indeed a primitive structure, shared by all vertebrates, that probably evolved around 500 million years ago.
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