Molecular structure of microvilli, a. High magnification of microvilli from Figure 4.2c. Note the presence of the actin filaments in the microvilli (arrows), which extend into the apical cytoplasm, x 80,000. b. Schematic diagram showing molecular structure of microvilli and the villin villin

location of specific actin filament-bundling proteins. Note the distribution of myosin 1 within the microvilli and myosin II within the terminal web. The spectrin molecules stabilize the actin filaments within the terminal web and anchor them into the apical plasma membrane.

Cilia give a "crew-cut" appearance to the epithelial surface

In the light microscope, cilia appear as short, fine, hairlike structures emanating from the free surface of the cell (Fig. 4.5). A thin, dark-staining band is usually seen extending across the cell at the base of the cilia. This dark-staining band represents structures known as basal bodies. These structures take up stain and appear as a continuous band when viewed in the light microscope. When viewed with the EM, however, the basal body of each cilium appears as a distinct individual structure.

Cilia contain an organized core of microtubules arranged in a 9 + 2 pattern

Electron microscopy of a cilium in longitudinal profile reveals an internal core of microtubules (Fig. 4.6a). A

cross-sectional view reveals a characteristic configuration of nine pairs or doublets of circularly arranged microtubules surrounding two central microtubules (Fig. 4.6b).

The microtubules composing each doublet are constructed so that the wall of one microtubule, designated the B microtubule, is actually incomplete; it shares a portion of the wall of the other microtubule of the doublet, the A microtubule. The A microtubule is composed of 13 tubulin dimers, arranged in side-by-side configuration, whereas the B microtubule is composed of 10 tubulin dimers. When seen in cross section at high resolution, each doublet exhibits a pair of "arms" that contain ciliary dynein, a microtubule-associated motor protein. This motor protein uses the energy of adenosine triphosphate (ATP) hydrolysis to move along the surface of the adjacent microtubule (see Fig. 4.6b). The dynein arms occur at 24-nm intervals along the length of the A microtubule and extend out to form temporary

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