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, which separate the chromosomes during cell division. The MTOC is a major site of microtubule nucleation and can be visualized in cells by immunohistochemical detection of γ-tubulin. The morphological characteristics of MTOCs vary between the different eukaryote groups. In animal cells, the two most important types of MTOCs are the basal bodies associated with cilia and flagella, and the centrosome associated with spindle formation.
Organization
thumb|Microtubule arrangement in a [[9+2 axoneme of bronchiolar cilia]]
Microtubule-organizing centers function as the site where microtubule formation begins, as well as a location where free-ends of microtubules attract to. Within the cells, microtubule-organizing centers can take on many different forms. An array of microtubules can arrange themselves in a pinwheel structure to form the basal bodies, which can lead to the formation of microtubule arrays in the cytoplasm or the 9+2 axoneme. Other arrangements range from fungi spindle pole bodies to the eukaryotic chromosomal kinetochores (flat, laminated plaques). MTOCs can be freely dispersed throughout the cytoplasm or centrally localized as foci. The most notable MTOCs are the centrosome at interphase and the mitotic spindle poles.
Centrioles can act as markers for MTOCs in the cell.
Centrosomes
The centrosome is the most well understood MTOC.
Movements of the microtubules are based on the actions of the centrosome. Instead, the nuclear envelope itself appears to function as the main MTOC for microtubule nucleation and spindle organization during plant cell mitosis.
Signal transduction
The MTOC reorients itself during signal transduction, primarily during wound repair or immune responses. The MTOC is relocalized to a position between the edge of the cell and the nucleus in cells like macrophages, fibroblasts, and endothelial cells. Organelles like the Golgi apparatus aid in the reorientation of the MTOC which can occur rapidly. Transduction signals cause microtubules to grow or contract, as well as cause the centrosome to become motile. The MTOC is located in a perinuclear position and contains the negative ends of microtubules while the positive ends grow rapidly towards the edge of the cell. The Golgi apparatus reorients along with the MTOC, and together cause the cell to seemingly send a polarized signal.