ABSTRACT
Organelle transport has been observed, measured, and speculated about since the middle of the nineteenth century. Mitochondria, vesicles, lysosomes, lipid droplets, and ingested particles can be watched in the light microscope as they move within epithelial cells or fibroblasts flattened onto the surface of a tissue culture. These organelles typically show what is termed saltatory movement, which is rapid, though sporadic, and interspersed with long periods of quiescence. Microtubules play a vital part in positioning membrane-bounded organelles within a eucaryotic cell. The differences in microtubule polarity may contribute to the accumulation of dendrite-specific molecules. Most kinesins, including the most abundant form present in nerve cells, move toward the plus ends of microtubules. The dynactin complex also contains components that bind passively to microtubules, perhaps maintaining an attachment while the dynein heads are detached. Movement of pigment granules takes place mainly along microtubules, although actin filaments are also involved.
