Microtubules Radiate from Centrosomes and Organize Certain Subcellular Structures

Like microfilaments and intermediate filaments, microtubules are not randomly distributed in cells. Rather, micro-tubules radiate from the centrosome, which is the primary microtubule-organizing center (MTOC) in animal cells (Figure 5-33). As detailed in Chapter 20, the two ends of a mi-crotubule differ in their dynamic properties and are commonly designated as the (+) and (—) ends. For this reason, microtubles can have two distinct orientations relative to one another and to other cell structures. In many nondi-viding animal cells, the MTOC is located at the center of the cell near the nucleus, and the radiating microtubules are all oriented with their (+) ends directed toward the cell periphery. Although most interphase animal cells contain a single perinuclear MTOC, epithelial cells and plant cells contain hundreds of MTOCs. Both of these cell types exhibit distinct

▲ FIGURE 5-33 Fluorescence micrograph of a Chinese hamster ovary cell stained to reveal microtubles and the

MTOC. The microtubules (green), detected with an antibody to tubulin, are seen to radiate from a central point, the microtubule-organizing center (MTOC), near the nucleus. The MTOC (yellow) is detected with an antibody to a protein localized to the centrosome. [Courtesy of R. Kuriyame.]

functional or structural properties or both in different regions of the cell. The functional and structural polarity of these cells is linked to the orientation of microtubules within them.

Findings from studies discussed in Chapter 20 show that the association of microtubules with the endoplasmic retic-ulum and other membrane-bounded organelles may be critical to the location and organization of these organelles within the cell. For instance, if microtubules are destroyed by drugs such as nocodazole or colcemid, the ER loses its networklike organization. Microtubules are also critical to the formation of the mitotic apparatus—the elaborate, transient structure that captures and subsequently separates replicated chromosomes in cell division.

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