Fine structural and cytochemical studies of human diploid fibroblasts arrested in an essentially nonmitotic state.

Human diploid fibroblasts can be maintained in vitro in an arrested, essentially nonmitotic state for extended periods of time by reducing the serum concentration in the medium from 10 to 0.5%. Arrested cells can be induced to re-enter the proliferative state by subcultivation in medium containing 10% serum. Fine structure, acid phosphatase, cytochrome oxidase, and extracellular carbohydrates in arrested cells were examined and compared to cultures growing in 10% serum and to cells transferred to 10% serum after 21 days in 0.5% serum. Cells in 10% serum posessed a well-developed Golgi complex, extensive rough endoplasmic reticulum, mitochondria containing transverse cristae, and many free ribosomes in the cytoplasm. In arrested cells, Golgi complexes were rarely observed, the number of both free and membrane-bound ribosomes was reduced, the number of cristae per mitochondria was decreased and the amount of demonstrable cytochrome oxidase activity was diminished. There was an accumulation of intercellular carbohydrate components. After subcultivation with medium containing 10% serum, arrested cells regained the ultrastructural characteristics of cells continuously cultured at this serum level; however, the amount of intercellular carbohydrate remained elevated. These results indicate that distinct yet reversible changes occur in the subcellular morphology and organization of cells maintained in an essentially nonmitotic state. This arrested state may be a close approximation to the situation as it occurs in vivo in expanding cell populations.