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MTOC or microtubule organizing center is a structure found in all plant and animal cells from which microtubules radiate. The two most important types of MTOCs are the basal bodies associated with cilia and certain intercellular junctions in epithelial cells, and the centrosome.
Most animal cells during interphase have one MTOC, usually located near the nucleus, and generally associated closely with the Golgi apparatus during interphase. This MTOC is the centrosome which usually has a pair of centrioles. Microtubules are anchored with their "minus" ends in the centrosome, and becuase microtubules dissociate preferentially at this end, this anchoring has a stabalizing effect, and MTOC aqssociated microtubules can grow very quickly. This directionality is also 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 stabalize and give direction to the microtubules, in this case to allow unidirectional movement of the cilium itself, rather than vesicles moving along it.
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