Morphology of cerebral endothelium and astrocytes as determinants of the neuronal microenvironment.

The morphological features of the blood-brain barrier to macro-molecules under normal and perturbed conditions are reviewed in the context of some recent investigations. The electric charge on molecules of horseradish peroxidase (HRP) affect its pinocytosis and intracellular fate which pertains to problems of distinguishing endocytosis from vesicular transport across endothelium. When the barrier is opened, the number of pits, vesicles and tubules increases. Such cerebral endothelium resembles normal endothelium of certain fish where numerous membrane invaginations do not signify vesicular or tubular transport. However, such transport has not been entirely ruled out in reactive endothelium. Another route of exudation during barrier opening may be via patent endothelial junctions, especially during intravascular infusion of hyperosmotic solutions. The permeability of the tight junctions, however, is not reflected unequivocally by its intramembranous structure. Although astrocytes do not provide a barrier to the extracellular flow of solutes, their ubiquity may enable them to modify the composition of perineuronal fluid. Their orthogonal arrays of intramembranous particles may be involved. The number of assemblies increases in astrocytes reacting to trauma and to the extracellular accumulation of lactate and CO2. The assemblies might thus participate in the transport of catabolites to and from extracellular fluid.