Quantitative analysis of the movements of cytoplasmic granules in polarized fibroblasts

Movements of cytoplasmic organelles were analyzed in Vero fibroblasts. In the cells polarized at the edge of an experimental wound, cytoplasmic granules moved randomly (Brownian motions) and by separate jumps (saltatory movements). The displacement of granules by the Brownian motions exceeded by more than an order of magnitude that of the mitochondria similar by weight. Lipid droplets moved predominantly by saltations, whereas mitochondria and lysosomes moved much less often. In a front part of the polarized cells, the main directions of saltatory movements were from the nucleus to the leading edge of a cell and back, whereas the tangential movements (across the long axis of a cell) were less than 1%. 90% of saltatory movements occurred in the area starting 10-12 microm from the nucleus and ending 10-12 microm from the leading edge of a cell. The average rate of saltatory movements of the granules (2.38 microm/s) was identical in both directions. The average length of the track was 7.49 microm; the maximum track length reached 30 microm. An increase in the granule diameter from 0.3 to 1.4 microm resulted in a minor (statistically insignificant) decrease in the average rate of the movements. The average rate of saltatory movements of mitochondria was 1.00 microm/s, and the average track length was 6.04 microm. Therefore, mitochondria, in contrast to lipid droplets, are rigidly fixed in the cytoplasm, and the force holding mitochondria is equal to the force produced by the microtubule-associated motors. Taking into account the characteristic of the centrifugal saltations, we suggest that they are mediated by an unusual dynein.