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blood brain barrier...
This "barrier" results from the selectivity of the tight junctions between endothelial [1] cells in CNS vessels that restricts the passage of solutes. At the interface between blood and brain, endothelial cells and associated astrocytes are stitched together by structures called "tight junctions." The tight junction is composed of smaller subunits, frequently dimers, that are transmembrane proteins such as occludin, claudins, junctional adhesion molecule (JAM), ESAM and others. Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes zo-1 and associated proteins.
The blood-brain barrier is composed of high density cells restricting passage of substances from the bloodstream much more than endothelial cells in capillaries elsewhere in the body. Astrocyte cell projections called astrocytic feet (also known as "glia limitans") surround the endothelial cells of the BBB, providing biochemical support to those cells. The BBB is distinct from the similar blood-cerebrospinal fluid barrier, a function of the choroidal cells of the choroid plexus, and from the Blood-retinal barrier, which can be considered a part of the whole. Several areas of the brain are not "behind" the BBB. One example is the pineal gland which secretes the hormone melatonin "directly into the systemic circulation."
[1] The endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. These cells reduce turbulence of the flow of blood allowing the fluid to be pumped farther.
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editor: H.m.Hanse copyright in accordance with the GNU licence