Chromosomes attach to the bundle, which then helps to guide them to the daughter cells. In 1988, Marc Kirschner, who was then at the University of California, San Francisco, and his colleagues found strong evidence that chromosomes move toward the poles as the microtubules slowly dissolve. Between cell divisions, microtubules act as miniature highways along which vesicles carrying such materials as hormones, neurotransmitters, and nutrients move. Kinesin also moves vesicles filled with neurotransmitters along the microtubules within nerve cell axons. A second, more recently discovered motor protein, called dynein, moves vesicles in the opposite direction, toward the cell's interior.
Microtubules are also involved in the movement of cilia and flagella.
These whiplike filaments project from certain cells and perform a variety of tasks. Large numbers of cilia are found on cells that line the respiratory tract, for instance, where they help to sweep out dust and debris. Both cilia and flagella play an important role in human reproduction. The inherent ability of microtubules and microfilaments to assemble and disassemble rapidly allows for the construction and destruction of these cytoskeletal components to suit the needs of a moving cell. In contrast, intermediate filaments are the most stable of the cytoskeletal fibers.