Quantification of mitochondria during the cell cycle of Chinese hamster cells

Abstract

Quantitative electron microscopy was used to study mitochondria during the cell cycle of Chinese hamster cells synchronized in metaphase following a 2 h Colcemid treatment. As the cells entered G1, the ratio of total mitochondrial volume to cytoplasmic volume increased by a factor of two because of the large decrease in cytoplasmic volume. The ratio decreased by about 20% as the cells entered early S phase at 3–5 h, and then increased again by 20% as the cells entered late S phase and G2 at 7–9 h. Most of the 2-fold increase in total mitochondrial volume during the cell cycle occurred between 5 and 9.5 h as the cells traversed S and G2. The number of mitochondria relative to cytoplasmic volume decreased by 30–40% during G1 and again during late S or G2, followed immediately during early S and G2 by a 100 and 60% increase, respectively, in the total number of mitochondria; these changes were associated with fusion and fission of mitochondria, as evidenced by a high frequency of dumbbell-shaped mitochondria as well as by changes in size and shape. During S phase between 3 and 7 h, the number of mitochondria per cytoplasmic volume also increased by about 50% (about a 2-fold increase in the total number of mitochondria) in association with an increase in the frequency of budding or branching mitochondria. Synchronous cells collected in mitosis without Colcemid treatment also were studied during the first 4 h when the cells were in G1 and early S phase, and fusion and fission of mitochondria were not observed. Thus, the fusion and fission during G1, mentioned above, apparently was induced by the Colcemid treatment and not associated with the normal G1. Also, there were no mitochondrial changes during G1 when the mitotic cells were X-irradiated (600 rads) or plated into medium containing actinomycin D (5 μg/ml). However, when the mitotic cells were plated into medium containing cycloheximide (5 μg/ml), the mitochondria appeared to fission during G1, as evidenced by both a 50% decrease in volume per mitochondrion and a 50% increase in number of mitochondria per cytoplasmic volume. Thus, there can be fundamental changes in mitochondrial number and size associated with changes in cellular physiology, such as those related to Colcemid treatment, cycloheximide treatment, and position in the cell cycle.