Microtubules are one of three principal components of the cytoskeleton. Microtubules are assembled from heterodimers of two globular subunits, called alfa- and beta tubulin, which polymerize and associate laterally to form a hollow tube with a diameter of about 25 nm. Due to the end-to-end linear arrangement of the alpha- and beta-tubulin dimers, the filaments have a polarity, in which beta subunits are exposed at the plus ends and alpha subunits are exposed at the minus ends. Microtubules are highly dynamic, and are constantly undergoing polymerization and depolymerization from their respective ends. By changes in the relative rates of polymerization and depolymerization, individual microtubules can rapidly alter between phases of growth and shrinkage, in a phenomenon known as dynamic instability. The dynamic properties differ between the ends, and are further regulated by interactions with other proteins. Microtubules are generally nucleated from gamma tubulin at the centrosome, which in mesenchymal cells continues to anchor the minus ends of microtubules, while the plus ends radiate towards the periphery. In epithelial cells, microtubules are instead organized along the apical-basal axis. Microtubules provide mechanical support to cells, but also play a prominent role in intracellular polarization, organization and transport. They provide directional routes for transport of organelles, vesicles and macromolecular complexes with the help of motor proteins. During mitosis, microtubules are organized into the mitotic spindle and plays a vital role in the separation of chromosomes into the daughter cells.
Immunofluorescent staining of microtubules shows thin strands that stretch throughout the whole cell. It is almost always possible to detect the center from which they all originate, the centrosome.
Read more about the proteome of microtubules.