Intermediate glial cells and reactive astrocytes revisited. A study in organotypic tissue culture

The existence of cells sharing features of oligodendrocytes and astrocytes has been repeatedly proposed. We have studied this problem ultrastructurally in organotypic tissue culture together with light-microscopic immunocytochemistry for the astrocyte marker glial fibrillary acidic protein (GFAP) and for 2 oligodendrocyte markers, myelin basic protein (MBP) and myelin-associated glycoprotein (MAG). Large numbers of GFAP+ cells (astrocytes) were seen, invariably giving rise to a wealth of tapering processes. In contrast, oligodendrocytes were found far less frequently either immunocytochemically or ultrastructurally, and showed smooth contours and scarcity of processes. Ultrastructurally, the cells corresponding to the intermediate glial cells in the literature which were far more numerous in culture than MBP-stained cells, were identified as reactive astrocytes by their numbers, location and morphological similarity with the GFAP-stained cells. Other characteristics were the presence of bundles of intermediate filaments and the covering of the plasmalemma adjacent to the collagen substrate by a basal lamina, in spite of the content of microtubules and the density of the cells. It was possible to demonstrate the difference between the wrapping of axons by astrocytic digitiform processes, and true myelination by processes identifiable as oligodendrocytes. We conclude that in this model the astrocytic and oligodendrocytic cell lines appear separate from the time of initial differentiation; in other systems such as dissociated cell culture, this may not be so. The cells with 'intermediate' features are in fact a reactive form of astrocyte.