The glioarchitectonics of the chicken brain were studied by electron microscopy, following light microscopic studies (Inoue 1970.1971. Inoue et al.1971.1973) and light and electron microscopic study on the chicken optic nerve (Inoue et al.1976a). The results obtained were as follows.
1. Three types of glial elements, that is, astrocytes, oligodendrocytes and microglia, could be identified, as was done by light microscopy. Atypical glial cells with fine, multiple processes and stained only by the Golgi method (Inoue etal.1973) were not clearly identified from the electron microscopic structure, but were inferred to be a subtype of the oligodendrocytes.
2. The three elements of glial cells were almost identical in terms of fine structure with those in the experimental mammalian central nervous system, described previously by many investigators. Astrocytes in the gray matter containing few myelinated nerve fibers, however, hardly processed those glial filaments that were characteristic in this type of cell in the mammalian brain.
3. All three types of glial elements could function as perineuronal satellite glial cells, whose cell bodies were closely apposed to the neuronal perikaryon. Between the plasma membranes of the neuronal soma and glial one, patches filled with electron dense material in the intercellular cleft occurred in various places, and, except for microglia, desmosome-like structures were often formed between the neuronal and glial elements.
4. Pericytes were sometimes in direct contact with the neuropil through the opened portion of the vascular basal lamina, and similarity between the cytoplasmic expansions of some microglia and pericytes could be recognized. Thus, the idea that microglia might be derived from the vascular wall cells or from mesenchymal cells could not be denied on the basis of the present study.