Microtubules are composed of repeating heterodimers of alpha and beta tubulin. The heterodimers, under the proper conditions, will spontaeously assemble into tubules in vitro or in vivo . Growth of existing microtubules is generally directional with one end growing faster than the other. Various microtubular organizing centers, such as basal bodies, centrioles and centromeres, direct the assembly and disassembly of microtubules. Microtubular life span is variable with some, as in those of the mitotic spindle, being transient, while others, such as those in cilia, being very stable.
Some antimitotic alkaloids are useful as cancer chemotherapy agents .
Taxol increases the formation of microtubules and stabilizes them so that there is no free tubulin for the formation of mitotic spindles. Vinblastine causes the disassembly of formed microtubules and causes the aggregation of crystaline tubulin. Because tumor cells multiply faster than normal cells they are more susceptible to antimitotic drugs. Some body systems are more affected than others based on their normal turnover rate. For instance, the cells of the gastrointestinal lining and blood forming system have rapid turnover rates and are therefore susceptible to the inhibition of mitosis caused by the chemotherapy agents.