The Neuroglial Compartments at Light Microscopic and Electron Microscopic Levels

Abstract

From a neurohistological viewpoint three major interdigitating cellular compartments are visible in brain sections studied under the light microscope: the vascular, the neuronal, and the glial compartment, which can further be subdivided into an astrocytic, an oligodendrocytic and a microglial compartment. The extracellular space or compartment observable only after fixation of the brain appears relatively narrow and can be seen only in electron microscopy. The astrocytic compartment has aroused special interest for the last fifteen years as this intervenes between capillaries and neurons. The ensheathing of capillaries by glial endfeet prevents direct contact of neurons with the vascular system. Furthermore, light microscopy shows that glial processes appear to seal off not only the internal, for example mesenchymal, areas of the blood vessels but also the external surfaces of the brain from the pia-arachnoid. In a way, the neurons and their processes are suspended within an enormous sponge of neuroglia. In this sponge, the major pillars are the blood vessels, while the neuroglia, in particular the astrocytes, interconnect the vascular tree by means of processes (Fig. 15.1). This arrangement suggests a supporting function of glia and, on account of glial envelopment of capillaries, a metabolic transport function.