Quantitative analisys of 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 movements) and by separate jumps (saltatory movements). The displacement of granules by the Brownian movements exceeded by more than an order of magnitude that mitochondria similar in 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 a nucleus to a leading edge of a cell and back, whereas the tangential movements {across the long axis of a cell) comprised below 1%. 90% of saltatory movings occured in the area starting 10-12 μm from the nucleus and ending 10-12 μm from the leading edge of a cell. The average rate of saltatory movements of the granules (2.38 μm/s) were identical in both directions. The average length of the track was 7.49 μm; the maximum track lengh reached 30 μm. An increase in the granule diameter from 0.3 to 1.4 μm resulted in an insignificant (statistically insignificant) decrease in the movement average speed. The average speed of saltatory movements of mitochondria was 1.00 μm/s, and the average track lengh was 6,04 μm. 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 suggested that thay are mediated by an unusual dynein.