Blood–brain barrier (BBB) – separation of circulating blood from BECF in CNS

The blood–brain barrier (BBB) is a separation of circulating blood from the brain extracellular fluid (BECF) 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 cerebrospinal fluid (CSF), while allowing the diffusion of small hydrophobic molecules (O2, CO2, hormones). Cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins. This barrier also includes a thick basement membrane and astrocytic endfeet

Existence 

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 a Berlin physician, Lewandowsky, in 1900. 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 proved to exist.

Barrier , endothelial cells  and Trans-membrane Proteins

English: Schematic sketch showing the blood-br...

This “barrier” results from the selectivity of the tight junctions between endothelial cells in CNS vessels that restricts the passage of solutes. At the interface between blood and the brain, endothelial cells are stitched together by these tight junctions, which are composed of smaller subunits, frequently biochemical dimers, that are trans-membrane proteins such as occludin, claudins, junctional adhesion molecule (JAM), or ESAM, for example. Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes zo-1 and associated proteins.

Astrocyte cell projections called astrocytic feet

English: Schematic sketch showing the transpor...

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

English: Schematic sketch showing endothelial-...

 

Astrocyte cell projections called astrocytic feet (also known as “glia limitans“) surround the endothelial cells of the BBB, providing biochemical support to those cells

 

 

 

 

Blood–cerebrospinal-fluid barrier and  Blood–retinal barrier

The blood–brain barrier (BBB) is a separation of circulating blood from the brain extracellular fluid (BECF) in the central nervous system (CNS) . The BBB is distinct from the quite similar blood–cerebrospinal-fluid barrier, which is 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 realm of such barriers.

Defect of the blood-brain barrier after stroke...

 

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