Microtubules function as a rail network, transporting vesicles to different locations within the cell. They start out from the microtubule organizing centre (MTOC) at the centrosome and stretch out to the edges of the cell. At high enough concentrations of the tubuli subunits, the microtubules will spontaneously self assemble, and when the concentration dips they will disassemble. This process of dynamic instability is much faster at the free ends of the tubules, thus referred to as the plus ends, whereas the slowly changing minus ends are joined together at the MTOC.
During mitosis the microtubule network will completely disassemble and instead form what will become the mitotic spindle, responsible for separating the duplicated chromosomes into the two 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, even though it need not be stained.
Sometimes it is also possible to stain the growing plus ends of the microtubule, in which case only the tip furthest away from the center is stained.