Staining of mitotic spindle in human cell line HeLa (HPA031240)
Scale bar represents 10µm
Microtubules function as a rail network within the cell used to transport vesicles and other organelles. They start out from microtubule organizing centres (MTOC) and stretch out to the edges of the cell. A characteristic feature of microtubules is that they never reach a steady-state length and thus remain constantly in the process of elongation (polymerization) or shrinkage (depolymerization), a phenomenon known as dynamic instability. This allows the cell to change its shape in response to different environmental conditions.
This process of dynamic instability is much faster at the free ends of the microtubules, 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 disassemble completely and instead form the mitotic spindle which is 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 MTOC. Some proteins are exclusively localised to the growing plus ends of the microtubule, in which case only the tip furthest away from the center is stained. The mitotic spindle can exclusively be detected during cell division as a structure that attaches to the chromosomes and pulls them apart. Size and shape of the mitotic spindle vary and are dependent on the progress of the cell division.