Classification

The classification of glial cells originally was based entirely on their morphological characteristics and dates back almost 150 years. Since then, additional techniques such as cell-specific proteins, electrophy-siology, ultrastructural features, immunocytochemis-try, and fluorescent imaging have been used to refine the classification of glia. Andreizen in 1893 first described two types of glial cells, fibrous and protoplasmic, a classification endorsed by Cajal in 1913. The initial characterization of glial cells was carried out by using light microscopy, but the advent of improved staining techniques at about the turn of the century accelerated the distinction of different cell types. It is now widely recognized that glial cells fall into the following categories: astrocytes, oligodendrocytes, and microglia in the CNS, Schwann cells in the PNS, and enteric glia in the gut. Astrocytes and oligoden-drocytes are derived from neuroectoderm and are sometimes considered together as macroglia. The main functions of these cells can be categorized as follows: myelination of axons (oligodendrocytes in the CNS, Schwann cells in the PNS), maintenance of ionic and neurotransmitter stability of the brain's extracellular microenvironment (astrocytes), nurturing neurons (astrocytes), and acting as the CNS macrophages (microglia). Each cell type will be described in detail later in this article, however, a few brief words about the common features of glial cells are needed. One feature in which glial cells differ from neurons is their intercommunication. Neurons generally communicate at chemical synapses, where individual neurons use neurotransmitter molecules released into the synaptic cleft between the neurons as mediators of communication. Glial cells, especially astrocytes and oligo-dendrocytes, directly communicate with each other via gap junctions. Gap junctions are formed from protein molecules called connexins. These aggregate into transmembrane hexamers called connexons and fuse with connexons on the membranes of adjoining cells, forming large pores that allow the passage of substances up to 1 kDa in weight. In this way, glial cells can communicate directly and allow rapid intercellular transport.

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