Centrosome and Microtubule-organizing center
The centrosome is located close to the nucleus, and is the starting point of microtubule growth. It consists of a centrosome matrix, enclosing two centrioles, which are situated at a 90° angle towards each other. In motile cells, one of the centrioles serves as the basal body of cilia and flagella during the cell cycle stages except mitosis.
The centrioles together with the pericentriolar matrix around it are called the microtubule-organizing center (MTOC). This matrix is responsible for the minus end formation of the microtubule filaments.
When mitosis begins, after replication of the DNA, the (duplicated) centrosomes move with both their MTOC to opposite sides of the nucleus and form the two poles of the mitotic spindle, separating the chromosomes.
The MTOC always overlaps with the center of the microtubules, making it easy to find. Antibodies that very specifically stain the two centrioles forming the centrosome as well as single spot stainings at the origin of microtubules are usually annotated as centrosome.
Other, larger structures stained at that location are annotated as MTOC. MTOC stainings can be difficult to identify, as it can appear different between cells. It co-localizes with the center of the microtubules but unlike the centrosome it can be both a granular or vesicle like structure.