Glial Types, Gliogenesis, and Extracellular Matrix in Mammalian CNS

Abstract

In brain and spinal cord, neurons and their processes (axons and dendrites) are almost completely ensheathed by glia with one important exception, i.e., the synaptic junctions mediating the chemical transmission of nerve impulses (Figs. 1 and 2). The central nervous system (CNS) is completely surrounded by glia not only on the surface, but also at the interface between blood vessels and the nervous tissue (Figs. 3 and 4). It is only at the site of entry of spinal and cranial nerves that the glial membrane and the overlying basal lamina are perforated (Fig. 5). In view of this anatomical arrangement, two general concepts have arisen concerning the role of glia in the CNS: Neuroglia, or, more specifically, astrocytes are the equivalent of fibroblasts in peripheral organs and provide the brain with a connective tissue; and, neurons and glia form a single tissue and this also in view of the fact that they have a common origin in the primitive neuroepithelium. From this perspective neurons and glia function as a unit.