Microtubule organizing center
The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: The organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spindle apparatus separating the chromosomes during cell division. The MTOC is the site of microtubule nucleation and can be visualized in cells by immunohistochemical detection of γ-tubulin. In animals, the two most important types of MTOCs are the basal bodies associated with cilia and certain intercellular junctions in epithelial cells, and the centrosome associated with spindle formation.
Most animal cells during interphase have one centrosome, usually located near the nucleus, and generally associated closely with the Golgi apparatus. The centrosome usually has a pair of centrioles. Microtubules are anchored with their "minus" ends in the centrosome, and because microtubules dissociate preferentially at this end, this anchoring has a stabilizing effect, and MTOC-associated microtubules can grow very quickly. The polarity of the microtubules is important for membrane bound transport, as the motor proteins kinesin and dynein typically move preferentially in either the "plus" or "minus" direction along a microtubule, allowing vesicles to be directed to or from the endoplasmic reticulum and Golgi apparatus.
In epithelial cells, MTOCs also anchor and organize the microtubules that make up cilia. As with the centrosome, these MTOCs stabilize and give direction to the microtubules, in this case to allow unidirectional movement of the cilium itself, rather than vesicles moving along it.
Spindle pole body
In yeasts and some algae, the MTOC is embedded into the nuclear envelope as a spindle pole body. In these organisms, the nuclear envelope does not break down during mitosis and the spindle pole body serves to connect cytoplasmic with nuclear microtubules.
Plant cells lack centrosomes or spindle pole bodies. Instead, the nuclear envelope itself appears to function as the main MTOC for microtubule nucleation and spindle organization during plant cell mitosis.