How is microtubule-based organelle translocation regulated?

It has long been appreciated from light-microscopical studies that components move in a directed fashion within cells, and this is particularly obvious in cells showing high degrees of asymmetry. The correlation between such movements and the presence of orientated aggregations of microtubules, first observed by electron microscopy and subsequently at the light-microscopic level using immunological techniques, pointed to the involvement of these linear organelles in this intracellular translocation (see Hyams and Stebbings, 1979). Many different cellular organelles, including endosomes, lysosomes, mitochondria and various types of secretory vesicles, show directed translocation along microtubules, and their movement is clearly crucial to normal cell physiological function. The most extensively studied example has been the axonal processes of neurones, but various protozoa, fish pigment cells and insect ovaries also show exaggerated forms of intracellular translocation, and have proved valuable in studying the phenomenon.