FIGURE 11.18 • Photomicrograph of smooth muscle from a human colon. The smooth muscle shown in this micrograph is arranged in two layers. On the left, the muscle cells are cut in longitudinal section; on the right, they are cut in cross section. Smooth muscle cells are elongate and have tapering ends. Note that the nuclei in the longitudinally sectioned muscle cells appear elongate and also exhibit tapering ends, thus matching the shape of the cell. In contrast, the nuclei in the cross-sectioned muscle cells are circular in profile. Also, some of the cross-sectioned cells appear to lack a nucleus, a reflection that the section passed through one of the ends of the cell. Also note that the longitudinally sectioned muscle cells are not easily delineated from one another, which is on account of the way they lie over one another within the thickness of the section. 400.
the cytoplasmic organelles are concentrated at each end of the nucleus. These include numerous mitochondria, some cisternae of the rER, free ribosomes, glycogen granules, and a small Golgi apparatus.
Structure of Smooth Muscle
Smooth muscle cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments.
The remaining sarcoplasm is filled with thin filaments that form a part of the contractile apparatus. Thick myosin filaments are scattered throughout the sarcoplasm of a smooth muscle cell. They are extremely labile and tend to be lost during tissue preparation. Special techniques can be used, however, to retain the structural integrity of the thick filaments and thus demonstrate them with the TEM. The thin
FIGURE 11.19 • Electron micrograph of smooth muscle cells.
This electron micrograph shows parts of three smooth muscle cells. The nucleus of one cell is in the lower part of the micrograph. The bulk of the cytoplasm is occupied by thin (actin) filaments, which are just recognizable at this magnification. The -actinin–containing cytoplasmic densities, or dense bodies, are visible among the myofilaments (arrows). Elements of the sarcoplasmic reticulum (SR) and the pinocytotic vesicles (PV ) are also indicated. The other two cells in the middle and upper part of the micrograph possess visible gap junctions (GJ) that allow communication between adjacent cells. The small dark particles are glycogen. 25,000. Inset. Enlargement of the gap junction. Note the presence of pinocytotic vesicles. 35,000.
filaments in a smooth muscle cell are attached to cytoplasmic densities or dense bodies that are visible among the filaments (Fig. 11.20). These structures are distributed throughout the sarcoplasm in a network of intermediate filaments containing the protein desmin. Intermediate filaments are part of the cytoskeleton of the cell. Note that vascular smooth muscle contains vimentin filaments in addition to desmin filaments.
The components of the contractile apparatus in smooth muscle cells are the following.  Thin filaments contain actin, the smooth muscle isoform of tropomyosin, and two smooth muscle–specific proteins, caldesmon and calponin. No troponin is associ ated with smooth muscle tropomyosin. Actin is involved in the force-generating interaction with myosin II molecules. Research suggests that the tropomyosin position on the actin filament is regulated by phosphorylation of myosin heads. Caldesmon (120 to 150 kilodaltons) and calponin (34 kilodaltons) are actin-binding proteins that block the myosin-binding site. The action of these proteins is Ca2-dependent and is also controlled by the phosphorylation of myosin heads.