Cortical cytoplasm and configurational changes of RGC‐6 cells exposed to cytochalasin B

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Abstract
RGC‐6 cells, grown to confluency in monolayer cultures, are typically fusiform. In sections of these cells, fixed in situ and examined in the electron microscope, a discontinuous zone of cortical cytoplasm (5–7 nm wide), was found subjacent to the plasma membrane of the free surface. It was composed largely of 5–7 nm microfilaments distributed in an apparently irregular nework and appeared to comprise a discrete zone separating the plasma membrane from the underlying endoplasm. Following incubation with cytochalasin B (5 μg/ml), the configuration of the cells changed markedly. The perikaryal cytoplasm appeared to swell, while the cellular processes became attenuated and often terminated in flower‐like tips. Although more discontinuous and somewhat less dense, the fine structure of the cortical cytoplasm appeared little affected by incubation with cytochalasin B for one hour. By 48 hours no zone of cortical cytoplasm was found. Subplasmalemmal microfilaments were no longer identifiable and endoplasmic organelles were found in direct contact with the plasma membrane of the free surface, which appeared to have collapsed onto the underlying endoplasm. The change in cell configuration resulting from exposure to cytochalasin B occurred at 37°C but not at 4°C, suggesting a dependency on metabolic activity. It did not, however, appeared related directly to protein synthesis. Whereas incorporation of 3H‐leucine was decreased to 46.9% and 62.9% of control values after one hour or two hours exposure to cytochalasin B, respectively, no change was found after 24 or 48 hours. It is suggested that the effect of cytochalasin B on cell form, which appeared not to be a direct effect on either protein synthesis or subplasmalemmal microfilaments, may represent a primary response of the cortical cytoplasmic matrix.