Blood-brain barrier

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The blood-brain barrier (BBB) is a separation of circulating blood and cerebrospinal fluid (CSF) in the central nervous system (CNS). It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion of microscopic objects (e.g. bacteria) and large or hydrophilic molecules into the CSF, while allowing the diffusion of small hydrophobic molecules (O2, hormones, CO2). Cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins.



Paul Ehrlich was a bacteriologist studying staining, a procedure that is used in many microscopic studies to make fine biological structures visible using chemical dyes. When Ehrlich injected some of these dyes (notably the aniline dyes that were then widely-used), the dye would stain all of the organs of some kinds of animals except for their brains. At that time, Ehrlich attributed this lack of staining to the brain's simply not picking up as much of the dye.

However, in a later experiment in 1913, Edwin Goldmann (one of Ehrlich's students) injected the dye into the cerebro-spinal fluids of animals' brains directly. He found that in this case the brains did become dyed, but the rest of the body did not. This clearly demonstrated the existence of some sort of compartmentalization between the two. At that time, it was thought that the blood vessels themselves were responsible for the barrier, since no obvious membrane could be found. The concept of the blood-brain barrier (then termed hematoencephalic barrier) was proposed by Lina Stern in 1921.[1] It was not until the introduction of the scanning electron microscope to the medical research fields in the 1960s that the actual membrane could be observed and proven to exist.

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