Brain Vessels and Blood Brain Barrier

The blood-brain barrier is a physical and physiological barrier impeding the passive diffusion of solutes from the blood into the extracellular space of the CNS. The blood-brain barrier is also a brain-blood barrier. In mammals and most vertebrates, the blood-brain barrier is a complex cellular system consisting of endothelial cells, pericytes, perivascular microglia, astrocytes, and basal laminae. Interactions of all these cells seem necessary for the induction and maintenance of the specialized functions of the blood-brain barrier which protect the brain from toxin, viruses, and macromolecules. The highly impermeable tight junctions between endothelial cells forming the capillaries in the CNS of higher vertebrates are responsible for the blood-brain barrier functions. Astrocyte perivascular processes or end feet form a virtually continuous sheath around the vascular walls and are nearly as extensive as the vascular endothelial covering. Astro-cytes end feet at the pial surface, which form the glia limitans, constitute a boundary around the entire CNS that helps to separate neural elements from mesoder-mal tissues.

During vascularization in early embryonic development, angioblasts (precurseurs of endothelial cells) invade the CNS neural tissues. When astrocytes differentiate, they extend processes on vessels and they induce blood-brain barrier differentiation in endothelial cells. Conversely, interactions of astrocyte progenitors with developing blood vessels may have a role in differentiation of astrocytes. In this context, it has been demonstrated in the retina that the vascular endothelial growth factor synthesized by astrocytes and Muller cells was an important inducer of endo-thelial cell growth and blood vessel development; therefore, astrocytes are critical in establishing the correct number of blood vessels in the brain. Recently, it has been shown that astrocytes isolated from GFAP knockout mice are unable to induce blood-brain barrier properties in endothelial cells.

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