FIGURE 2.40 • Polymerization of microtubules. On the left, the diagram depicts the process of polymerization and depolymerization of tubulin dimers during the process of microtubule assembly. Each tubulin dimer consists of an -tubulin and a -tubulin subunit. On the right is a diagram showing that each microtubule contains 13 tubulin dimers within its cross section. The minus ( ) end of the microtubule contains a ring of -tubulin, which is necessary for microtubule nucleation. This end is usually embedded within the MTOC and possesses numerous capping proteins. The plus ( ) end of the microtubule is the growing end to which tubulin dimers bound to guanosine triphosphate (GTP) molecules are incorporated. Incorporated tubulin dimers hydrolyze GTP, which releases the phosphate groups to form polymers with guanosine diphosphate (GDP)–tubulin molecules.
crotubules linking peripheral structures and organelles with the MTOC. As mentioned earlier, the association of a micro-tubule with MAPs, such as occurs within the axoneme of a cilium or ﬂagellum, effectively blocks this dynamic instability and stabilizes the microtubules.
FIGURE 2.41 • Three-dimensional reconstruction of an intact microtubule. This image was obtained using cryo-electron microscopy. Tomographic (sectional) images of a frozen hydrated microtubule were collected and digitally reconstructed at a resolution of 8 angstroms (Å). The helical structure of the -tubulin molecules is recognizable at this magnifcation. 3,250,000. (Courtesy of Dr. Kenneth Downing.)
The structure and function of microtubules in mitosis and in cilia and ﬂagella are discussed later in this chapter and in Chapter 5.
Microtubules can be visualized in the light microscope and are involved in intracellular transport and cell motility.
Microtubules may be seen in the light microscope by using special stains, polarization, or phase-contrast optics. Because of the limited resolution of the light microscope, in the past microtubules were erroneously called fibers, such as the “fibers” of the mitotic spindle. Microtubules may now be distinguished from filamentous and fibrillar cytoplasmic components even at the light microscopic level by using antibodies to tubulin, the primary protein component of microtubules, conjugated with ﬂuorescent dyes (Fig. 2.42).
In general, microtubules are found in the cytoplasm, where they originate from the MTOC; in cilia and ﬂagella, where they form the axoneme and its anchoring basal body; in centrioles and the mitotic spindle; and in elongating processes of the cell, such as those in growing axons.
Microtubules are involved in numerous essential cellular functions:  Intracellular vesicular transport (e.g., movement of secre tory vesicles, endosomes, and lysosomes),  Movement of cilia and ﬂagella,  Attachment of chromosomes to the mitotic spindle and their movement during mitosis and meiosis,  Cell elongation and movement (migration), and  Maintenance of cell shape, particularly its asymmetry.
FIGURE 2.42 • Staining of microtubules with fuorescent dye.