Blood–brain barrier

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Blood–brain barrier_table_infobox_0

Blood-brain barrierBlood–brain barrier_header_cell_0_0_0
DetailsBlood–brain barrier_header_cell_0_1_0
SystemBlood–brain barrier_header_cell_0_2_0 Neuroimmune systemBlood–brain barrier_cell_0_2_1
IdentifiersBlood–brain barrier_header_cell_0_3_0
Acronym(s)Blood–brain barrier_header_cell_0_4_0 BBBBlood–brain barrier_cell_0_4_1
MeSHBlood–brain barrier_header_cell_0_5_0 Blood–brain barrier_cell_0_5_1

The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system where neurons reside. Blood–brain barrier_sentence_0

The blood-brain barrier is formed by endothelial cells of the capillary wall, astrocyte end-feet ensheathing the capillary, and pericytes embedded in the capillary basement membrane. Blood–brain barrier_sentence_1

This system allows the passage of some molecules by passive diffusion, as well as the selective and active transport of various nutrients, ions, organic anions, and macromolecules such as glucose, water and amino acids that are crucial to neural function. Blood–brain barrier_sentence_2

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. Blood–brain barrier_sentence_3

Cells of the barrier actively transport metabolic products such as glucose across the barrier using specific transport proteins. Blood–brain barrier_sentence_4

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. Blood–brain barrier_sentence_5

Specialized brain structures participating in sensory and secretory integration within brain neural circuits—the circumventricular organs and choroid plexus—have highly permeable capillaries. Blood–brain barrier_sentence_6

Structure Blood–brain barrier_section_0

The blood–brain barrier results from the selectivity of the tight junctions between the endothelial cells of brain capillaries, restricting the passage of solutes. Blood–brain barrier_sentence_7

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. Blood–brain barrier_sentence_8

Each of these transmembrane proteins is anchored into the endothelial cells by another protein complex that includes tight junction protein 1 and associated proteins. Blood–brain barrier_sentence_9

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. Blood–brain barrier_sentence_10

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–brain barrier_sentence_11

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. Blood–brain barrier_sentence_12

Several areas of the human brain are not on the brain side of the BBB. Blood–brain barrier_sentence_13

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. Blood–brain barrier_sentence_14

The pineal gland secretes the hormone melatonin "directly into the systemic circulation", thus melatonin is not affected by the blood-brain barrier. Blood–brain barrier_sentence_15

Development Blood–brain barrier_section_1

The blood-brain barrier appears to be functional by the time of birth. Blood–brain barrier_sentence_16

P-glycoprotein, a transporter, exists already in the embryonal endothelium. Blood–brain barrier_sentence_17

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. Blood–brain barrier_sentence_18

Function Blood–brain barrier_section_2

See also: Neuroimmune system Blood–brain barrier_sentence_19

The blood-brain barrier acts effectively to protect the brain from circulating pathogens. Blood–brain barrier_sentence_20

Accordingly, blood-borne infections of the brain are rare. Blood–brain barrier_sentence_21

Infections of the brain that do occur are often difficult to treat. Blood–brain barrier_sentence_22

Antibodies are too large to cross the blood-brain barrier, and only certain antibiotics are able to pass. Blood–brain barrier_sentence_23

In some cases, a drug has to be administered directly into the cerebrospinal fluid where it can enter the brain by crossing the blood-cerebrospinal fluid barrier. Blood–brain barrier_sentence_24

The blood-brain barrier may become leaky in select neurological diseases, such as amyotrophic lateral sclerosis, epilepsy, brain trauma and edema, and in systemic diseases, such as liver failure. Blood–brain barrier_sentence_25

The blood-brain barrier becomes more permeable during inflammation, potentially allowing antibiotics and phagocytes to move across the BBB. Blood–brain barrier_sentence_26

Circumventricular organs Blood–brain barrier_section_3

Main article: Circumventricular organs Blood–brain barrier_sentence_27

Circumventricular organs (CVOs) are individual structures located adjacent to the fourth ventricle or third ventricle in the brain, and are characterized by dense capillary beds with permeable endothelial cells unlike those of the blood-brain barrier. Blood–brain barrier_sentence_28

Included among CVOs having highly permeable capillaries are the area postrema, subfornical organ, vascular organ of the lamina terminalis, median eminence, pineal gland, and three lobes of the pituitary gland. Blood–brain barrier_sentence_29

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. Blood–brain barrier_sentence_30

Consequently, the CVO permeable capillaries are the point of bidirectional blood-brain communication for neuroendocrine function. Blood–brain barrier_sentence_31

Specialized permeable zones Blood–brain barrier_section_4

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. Blood–brain barrier_sentence_32

Such zones exist at the border of the area postrema—nucleus tractus solitarii (NTS), and median eminence—hypothalamic arcuate nucleus. Blood–brain barrier_sentence_33

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. Blood–brain barrier_sentence_34

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. Blood–brain barrier_sentence_35

Therapeutic research Blood–brain barrier_section_5

As a drug target Blood–brain barrier_section_6

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. Blood–brain barrier_sentence_36

Overcoming the difficulty of delivering therapeutic agents to specific regions of the brain presents a major challenge to treatment of most brain disorders. Blood–brain barrier_sentence_37

In its neuroprotective role, the blood-brain barrier functions to hinder the delivery of many potentially important diagnostic and therapeutic agents to the brain. Blood–brain barrier_sentence_38

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. Blood–brain barrier_sentence_39

Mechanisms for drug targeting in the brain involve going either "through" or "behind" the BBB. Blood–brain barrier_sentence_40

Modalities for drug delivery to the brain in unit doses through the BBB entail its disruption by osmotic means, or biochemically by the use of vasoactive substances, such as bradykinin, or even by localized exposure to high-intensity focused ultrasound (HIFU). Blood–brain barrier_sentence_41

Other methods used to get through the BBB may entail the use of endogenous transport systems, including carrier-mediated transporters, such as glucose and amino acid carriers, receptor-mediated transcytosis for insulin or transferrin, and the blocking of active efflux transporters such as p-glycoprotein. Blood–brain barrier_sentence_42

Some studies have shown that vectors targeting BBB transporters, such as the transferrin receptor, have been found to remain entrapped in brain endothelial cells of capillaries, instead of being ferried across the BBB into the targeted area. Blood–brain barrier_sentence_43

Nanoparticles Blood–brain barrier_section_7

Main article: Nanoparticles for drug delivery to the brain Blood–brain barrier_sentence_44

Nanotechnology is under preliminary research for its potential to facilitate the transfer of drugs across the BBB. Blood–brain barrier_sentence_45

Capillary endothelial cells and associated pericytes may be abnormal in tumors and the blood-brain barrier may not always be intact in brain tumors. Blood–brain barrier_sentence_46

Other factors, such as astrocytes, may contribute to the resistance of brain tumors to therapy using nanoparticles. Blood–brain barrier_sentence_47

Fat soluble molecules less than 400 Daltons in weight can freely diffuse past the BBB through lipid mediated passive diffusion. Blood–brain barrier_sentence_48

History Blood–brain barrier_section_8

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. Blood–brain barrier_sentence_49

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. Blood–brain barrier_sentence_50

At that time, Ehrlich attributed this lack of staining to the brain simply not picking up as much of the dye. Blood–brain barrier_sentence_51

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. Blood–brain barrier_sentence_52

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. Blood–brain barrier_sentence_53

At that time, it was thought that the blood vessels themselves were responsible for the barrier, since no obvious membrane could be found. Blood–brain barrier_sentence_54

The concept of the blood–brain barrier (then termed hematoencephalic barrier) was proposed by a Berlin physician, Lewandowsky, in 1900. Blood–brain barrier_sentence_55

See also Blood–brain barrier_section_9

Blood–brain barrier_unordered_list_0

  • Blood–air barrier – Membrane separating alveolar air from blood in lung capillariesBlood–brain barrier_item_0_0
  • Blood–ocular barrier – A physical barrier between the local blood vessels and most parts of the eye itselfBlood–brain barrier_item_0_1
  • Blood–retinal barrier – Part of the blood–ocular barrier that prevents certain substances from entering the retinaBlood–brain barrier_item_0_2
  • Blood–testis barrier – A physical barrier between the blood vessels and the seminiferous tubules of the animal testesBlood–brain barrier_item_0_3
  • Blood–thymus barrier – A barrier formed by the continuous blood capillaries in the thymic cortexBlood–brain barrier_item_0_4

Credits to the contents of this page go to the authors of the corresponding Wikipedia page:–brain barrier.