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The most numerous cellular constituents of the central nervous system are the non-neuronal, neuroglial ("nerve glue") cells that occupy the space between neurons. It has been estimated that there are roughly 360 billion glial cells, which comprise 80-90% of the cells in the CNS. This section will cover the general classifications of the neuroglial cells and describe some of the general properties that distinguish neuroglia from neurons.

Neuroglia differ from neurons in several general ways in that they

do not form synapses,
have essentially only one type of process,
retain the ability to divide, and
are less electrically excitable than neurons.

Neuroglia are classified based on size and shape fo their neucleus and distinguished from neurons, at the light microscopic level. Alkaline (basic) dyes are used to show nuclear morphology. In addition, several metal stains are used show the shape of the cell and cytoplasmic architecture. Characteristics of nuclei, including size, shape, staining intensity, and distribution of chromatin, are used to distinguish cell types in pathological material. Cell body characteristics, including size, shape, location, branching pattern, and density of processes, are also used.

Neuroglia are divided into two major categories based on size, the macroglia and the microglia. The macroglia are of ectodermal origin and consist of astrocytes, oligodendrocytes and ependymal cells. Microglia cells are probably of mesodermal origin. A comparison of the various neuroglial types is shown in Figure 8.11.

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Glial Cells and Function

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