page_title: Blood–brain barrier
section_title: Structure
hierarchical_section_title: Blood–brain barrier / Structure
caption_attribution_description: English: A network of capillaries supply brain cells with nutrients. Tight seals in their walls keep blood toxins—and many beneficial drugs—out of the brain. From: Bridging the Blood-Brain Barrier: New Methods Improve the Odds of Getting Drugs to the Brain Cells That Need Them Ferber D PLoS Biology Vol. 5, No. 6, e169 doi:10.1371/journal.pbio.0050169 Français&#160;: Un réseau de capillaires fournit les cellules du cerveau avec des nutriments. Des jonctions serrées dans leurs parois empêchent les toxines du sang - et beaucoup de médicaments bénéfiques - de pénétrer dans le cerveau. Ελληνικά: Ένα δίκτυο τριχοειδών αγγείων παρέχει θρεπτικά συστατικά στα εγκεφαλικά κύτταρα. Τα σφιχτά συνδεδεμένα κύτταρα του τοίχους των τριχοειδών κρατούν τις τοξίνες του αίματος - αλλά και πολλά ωφέλιμα φάρμακα - έξω από τον εγκέφαλο.
context_page_description: The blood–brain barrier 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. 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. This system allows the passage of some molecules by passive diffusion, as well as the selective transport of various nutrients, ions, organic anions, and macromolecules such as glucose, water and amino acids that are crucial to neural function.
  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 and small polar molecules. Cells of the barrier actively transport metabolic products such as glucose across the barrier using specific transport proteins.
context_section_description: 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.
caption: Part of a network of capillaries supplying brain cells
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