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Semipermeable capillary border that allows selective passage of blood constituents into the brain
The blood-brain barrier restricts the passage of pathogens, the diffusion of solutes in the blood, and large or hydrophilic molecules into the cerebrospinal fluid, while allowing the diffusion of hydrophobic molecules (O2, CO2, hormones) and small polar molecules. Cells of the barrier actively transport metabolic products such as glucose across the barrier using specific transport proteins. The barrier also restricts the passage of peripheral immune factors, like signaling molecules, antibodies, and immune cells, into the CNS, thus insulating the brain from damage due to peripheral immune events.
The astrocytes type 1 surrounding capillaries in the brain
Sketch showing constitution of blood vessels inside the brain
The blood-brain barrier results from the selectivity of the tight junctions between the endothelial cells of brain capillaries, restricting the passage of solutes. At the interface between blood and the brain, endothelial cells are adjoined continuously by these tight junctions, which are composed of smaller subunits of transmembrane proteins, such as occludin, claudins, junctional adhesion molecule. Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes tight junction protein 1 and associated proteins.
The blood-brain barrier is composed of endothelial cells restricting passage of substances from the blood more selectively than endothelial cells of 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 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.
Several areas of the human brain are not on the brain side of the BBB. Some examples of this include the circumventricular organs, the roof of the third and fourth ventricles, capillaries in the pineal gland on the roof of the diencephalon and the pineal gland. The pineal gland secretes the hormone melatonin "directly into the systemic circulation", thus melatonin is not affected by the blood-brain barrier.
The blood-brain barrier appears to be functional by the time of birth. P-glycoprotein, a transporter, exists already in the embryonal endothelium.
Measurement of brain uptake of various blood-borne solutes showed that newborn endothelial cells were functionally similar to those in adults, indicating that a selective BBB is operative at birth.
Permeable capillaries of the sensory CVOs (area postrema, subfornical organ, vascular organ of the lamina terminalis) enable rapid detection of circulating signals in systemic blood, while those of the secretory CVOs (median eminence, pineal gland, pituitary lobes) facilitate transport of brain-derived signals into the circulating blood. Consequently, the CVO permeable capillaries are the point of bidirectional blood-brain communication for neuroendocrine function.
Specialized permeable zones
The border zones between brain tissue "behind" the blood-brain barrier and zones "open" to blood signals in certain CVOs contain specialized hybrid capillaries that are leakier than typical brain capillaries, but not as permeable as CVO capillaries. Such zones exist at the border of the area postrema--nucleus tractus solitarii (NTS), and median eminence--hypothalamicarcuate nucleus. These zones appear to function as rapid transit regions for brain structures involved in diverse neural circuits--like the NTS and arcuate nucleus--to receive blood signals which are then transmitted into neural output. The permeable capillary zone shared between the median eminence and hypothalamic arcuate nucleus is augmented by wide pericapillary spaces, facilitating bidirectional flow of solutes between the two structures, and indicating that the median eminence is not only a secretory organ, but may also be a sensory organ.
As a drug target
The blood-brain barrier is formed by the brain capillary endothelium and excludes from the brain 100% of large-molecule neurotherapeutics and more than 98% of all small-molecule drugs. Overcoming the difficulty of delivering therapeutic agents to specific regions of the brain presents a major challenge to treatment of most brain disorders. In its neuroprotective role, the blood-brain barrier functions to hinder the delivery of many potentially important diagnostic and therapeutic agents to the brain. Therapeutic molecules and antibodies that might otherwise be effective in diagnosis and therapy do not cross the BBB in adequate amounts to be clinically effective.
Nanotechnology is under preliminary research for its potential to facilitate the transfer of drugs across the BBB. Capillary endothelial cells and associated pericytes may be abnormal in tumors and the blood-brain barrier may not always be intact in brain tumors. Other factors, such as astrocytes, may contribute to the resistance of brain tumors to therapy using nanoparticles. Fat soluble molecules less than 400 Daltons in weight can freely diffuse past the BBB through lipid mediated passive diffusion.
Paul Ehrlich was a bacteriologist studying staining, a procedure that is used in many microscopy studies to make fine biological structures visible using chemical dyes. As Ehrlich injected some of these dyes (notably the aniline dyes that were then widely used), the dye stained 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 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 directly into the cerebrospinal fluids of animal brains. He found then the brains did become dyed, but the rest of the body did not, demonstrating the existence of a 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.
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