This confocal immunofuorescent image shows the organization of the microtubules within an epithelial cell in tissue culture. In this example, the specimen was immunostained with three primary antibodies against tubulin (green), centrin (red), and kinetochores (light blue) and then incubated in a mixture of three different fuorescently tagged secondary antibodies that recognized the primary antibodies. Nuclei were stained (dark blue) with a fuorescent molecule that intercalates into the DNA double helix. Note that the microtubules are focused at the MTOC or centrosome (red) located adjacent to the nucleus. The cell is in the S phase of the cell cycle, as indicated by the presence of both large unduplicated kinetochores and smaller pairs of duplicated kinetochores. 3,000. (Courtesy of Dr. Wilma L. Lingle and Ms. Vivian A. Negron.)
Movement of intracellular organelles is generated by molecular motor proteins associated with microtubules.
In cellular activities that involve movement of organelles and other cytoplasmic structures—such as transport vesicles, mitochondria, and lysosomes—microtubules serve as guides to the appropriate destinations. Molecular motor proteins attach to these organelles or structures and ratchet along the microtubule track (Fig. 2.43). The energy required for the ratcheting movement is derived from ATP hydrolysis. Two families of molecular motor proteins have been identified that allow for unidirectional movement:  Dyneins constitute one family of molecular motors. They move along the microtubules toward the minus end of the tubule. Therefore, cytoplasmic dyneins are capable of transporting organelles from the cell periphery toward the
MTOC. One member of the dynein family, axonemal dynein, is present in cilia and ﬂagella. It is responsible for the sliding of one microtubule against an adjacent micro- tubule of the axoneme that effects their movement.  Kinesins, members of the other family, move along the microtubules toward the plus end; therefore, they are ca
FIGURE 2.43 • The molecular motor proteins associated with microtubules. Microtubules serve as guides for molecular motor proteins. These ATP-driven microtubule-associated motor proteins are attached to moving structures (such as organelles) that ratchet them along a tubular track. Two types of molecular motors have been identifed: dyneins that move along microtubules toward their minus ( ) end (i.e., toward the center of the cell) and kinesins that move toward their plus ( ) end (i.e., toward the cell periphery).
pable of moving organelles from the cell center toward the cell periphery.
Both dyneins and kinesins are involved in mitosis and meiosis. In these activities, dyneins move the chromosomes along the microtubules of the mitotic spindle. Kinesins are simultaneously involved in movement of polar microtubules. These microtubules extend from one spindle pole past the metaphase plate and overlap with microtubules extending from the opposite spindle pole. Kinesins located between these microtubules generate a sliding movement that reduces the overlap, thereby pushing the two spindle poles apart toward each daughter cell (Fig. 2.44).
Actin filaments are present in virtually all cell types.