Microtubules and microfilament patterns in cultured astrocytes were revealed by using indirect immunofluorescent microscopy in conjunction with anti-tubulin immune serum and anti-actin immunoglobulins respectively. In flat epitheloid astroglial cells (either polygonal or elongated) colchicine-sensitive immunofluorescent fibres, which correspond to bundles of microtubules, extend from the perinuclear cytoplasm into the cell periphery by running for long distances through the different focal planes. These patterns of organization differ markedly from the patterns of organization of microfilaments which are arranged in fibres parallel to each other and often oriented along the cell boundary. In response to the combined treatments of serum withdrawal and administration of dBcAMP, flat epitheloid astrocytes adopt a morphology similar to that of the mature astrocytes in situ in the CNS, that is of stellate process-bearing cells. This is prevented or is reverted by the administration of colchicine at the appropriate times. There are strong suggestions indicating that during cell processes formation the microtubular network is reorganized and microtubules assembled into dense bundles which are oriented along the axis of the cell processes. In view of these results, we suggest that, in contrast to microfilaments, microtubules are not determinant for the maintenance of cellular shape in elongated or polygonal flat epitheloid astroglial cells but they are required for both the formation and maintenance of processes in stellate astrocytes.
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