The Blood-Brain Barrier The Blood-Brain Barrier

The blood-brain barrier (BBB) principally consists of the interface between the blood and the brain's extracellular fluid. Evidence for such a barrier was first observed over 100 years ago. As with other physiologic phenomena, the BBB's structure and function are intimately related. The different anatomic features of the barrier give rise to its selective permeability. In the normal state, this "modified tight epithelium" provides an element of control over the brain's ambient environment. Furthermore, blood-brain barrier dysfunction may be an important etiological factor in a number of neurologic diseases.

Perhaps the first scientific researcher to document the occurrence of a blood-brain barrier was Paul Ehrlich (1885, 1906) (10:1). He observed that intravenous injection of water-soluble vital dyes colored all of the body's tissues except for those of the central nervous system (CNS). The physiologic mechanisms underlying this "hindrance to diffusion," however, remained unclear for many years (5:1-40). It wasn't until the early years of this century, that Stern and Gautier (1921, 1922) studied the movement of different substances into the brain. From their observations, the two researchers concluded that the cerebrospinal fluid (CSF) acts as an intermediary between the blood and the CNS (6:28). Eventually, their work led to the identification of two separate barriers: the blood-CSF barrier and the BBB.

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tems permit for the directional movement of solutes (9:87-88). By allowing the CNS endothelium to act as both a passive and an active barrier, the tight junctions ultimately serve to regulate the composition of the interstitial fluid bathing the brain's neurons and glia. Since these cells are highly responsive to the extracellular fluid's ionic composition, the endothelial cell tight junctions play a significant role in CNS function. In fact, tight junctions are only found in the capillaries and postcapillary venules of the cerebral cortex (3:107). Other important cellular components of the BBB include astrocytes, pericytes, and microglial cells. Astrocytes and pericytes generally occur at the abluminal surface of the endothelium associated with its basal lamina. For example, astrocyte cell bodies may extend endfoot processes to the endothelial cells' basement membrane. In contrast, pericytes are contractile cells have been found to extend long processes around the capillary endothelium. Thirdly, the microglia occur primarily in the perivascular region. These cells are thought to be involved in the immunopathology of several CNS diseases (9:87-88). Clearly, the blood-brain barrier does more than function merely as a

Category: Medical - T