ABSTRACT
The blood-brain barrier governs what crosses from the blood into the brain extracellular fluid. The anatomical barrier is provided by brain capillary endothelial cells which are coupled to each other by tight junctions that are one hundred-fold tighter than is typical for other capillaries. Even small ions will not permeate between endothelial cells in brain capillaries. Moreover, brain capillary endothelial cells exhibit little pinocytosis (bulk transfer of fluid) or receptor-mediated endocytosis. Brain capillaries are entirely covered by the astrocyte endfeet which secrete growth factors (e.g., angiopoietin 1) that promote the efficacy of tight junctions (Figure 1).
