oligodendrocytes are highly variable, differing in location, morphology and function, but definable by some morphological criteria. The cell soma ranges from 10 to 20 μm and is roughly globular and more dense than that of an astrocyte. The margin of the cell is irregular and compressed against the adjacent neuropil. Few cell processes are seen, in contrast to the astrocyte. Within the cytoplasm, many organelles are found. Parallel cisternae of the rough ER and a widely dispersed Golgi apparatus are common. Free ribosomes occur, scattered amid occasional multivesicular bodies, mitochondria and coated vesicles. Distinguishing the oligodendrocyte from the astrocyte are the apparent absence of glial filaments and the constant presence of 24-nm microtubules (Fig. 1-13).