Centrioles represent the focal point around which the MTOC assembles.
Centrioles, visible in the light microscope, are paired, short, rodlike cytoplasmic cylinders built from nine microtubule triplets. In resting cells, centrioles have an orthogonal
FIGURE 2.50 • Distribution of intermediate flaments in human fetal lung fbroblasts. Distribution of vimentin (red) and actin flaments (green) is shown in cultured fbroblasts from human fetal lung. Vimentin is an intermediate flament protein expressed in all cells of mesenchymal origin. In cultured fbroblasts, vimentin flaments are visible centrally within the cell cytoplasm, whereas the actin flaments are aggregated primary near the cell surface. This immunofuorescent image was obtained using the indirect immunofuorescence techniques in which vimentin flaments were treated with mouse anti-vimentin primary antibodies followed by goat anti-mouse secondary antibodies conjugated with Texas red fuorescent dye. Actin flaments were counter-stained with phalloidin conjugated with a green fuorescent dye. Nuclei were stained blue with Hoechst fuorescent stain. 3,500. (Reprinted with permission from Michael W. Davidson, Florida State University.) orientation: One centriole in the pair is arrayed at a right angle to the other. Centrioles are usually found close to the nucleus, often partially surrounded by the Golgi apparatus, and associated with a zone of amorphous, dense pericentriolar material. The region of the cell containing the centrioles and pericentriolar material is called the microtubule-organizing center or centrosome (Fig. 2.51). The MTOC is the region where most microtubules are formed and from which they are then directed to specific destinations within the cell. Therefore, the MTOC controls the number, polarity, direction, orientation, and organization of microtubules formed during the interphase of the cell cycle. During mitosis, duplicated MTOCs serve as mitotic spindle poles. Development of the MTOC itself depends solely on the presence of centrioles. When centrioles are missing, the MTOCs disappear, and formation of microtubules is severely impaired.
The pericentriolar matrix of MTOC contains nume rous ring-shaped structures that initiate microtubule formation.
The MTOC contains centrioles and an amorphous pericentriolar matrix of more than 200 proteins, including tubulin that is organized in ring-shaped structures. Each -tubulin ring serves as the starting point (nucleation site) for the growth of one microtubule that is assembled from tubulin dimers; and -tubulin dimers are added with specific orientation to the -tubulin ring. The minus end of the micro-tubule remains attached to the MTOC, and the plus end