The terminal web is composed of actin flaments stabilized by spectrin (468 kilodaltons), which also anchors the terminal web to the apical cell membrane (Fig. 5.3b). The presence of myosin II and tropomyosin in the terminal web explains its contractile ability; these proteins decrease the diameter of the apex of the cell, causing the microvilli, whose stiff actin cores are anchored into the terminal web, to spread apart and increase the intermicrovillous space.
The functional and structural features of microvilli are summarized in Table 5.2.
Stereocilia are unusually long, immotile microvilli.
Stereocilia are not widely distributed among epithelia. They are, in fact, limited to the epididymis, the proximal part of the ductus deferens of the male reproductive system, and the sensory (hair) cells of the inner ear. They are included in this section because this unusual surface modification is traditionally treated as a separate structural entity.
FIGURE 5.3 • Molecular structure of microvilli. a. High magnification of microvilli from Figure 5.2c. Note the presence of the actin filaments in the microvilli (arrows), which extend into terminal web in the apical cytoplasm. 80,000. b. Schematic diagram showing molecular structure of microvilli and the location of specific actin filament–bundling proteins (fimbrin, espin, and fascin). Note the distribution of myosin I 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.
Stereocilia of the genital ducts are extremely long processes that extend from the apical surface of the cell and facilitate absorption. Unique features include an apical cell protrusion from which they arise and thick stem portions that are interconnected by cytoplasmic bridges. Because electron microscopy reveals their internal structure to be that of unusually long microvilli, some histologists now use the term stereovilli (Fig. 5.4a). Seen in the light microscope, these processes frequently resemble the hairs of a paint brush because of the way they aggregate into pointed bundles.
Like microvilli, stereocilia are supported by internal bundles of actin flaments that are cross-linked by fmbrin. The actin filaments’ barbed (plus) ends are oriented toward the tips of the stereocilia and the pointed (minus) ends at the base. This organization of actin core shares many construction principles with the microvilli, yet it can be as long as 120 m.
Stereocilia develop from microvilli by the lateral addition of actin filaments to the actin bundle as well as elongation of the actin filaments. Unlike microvilli, an 80kilodalton actin-binding protein, ezrin, closely associated with the plasma membrane of stereocilia, anchors the actin filaments to the plasma membrane. The stem portion of the stereocilium and the apical cell protrusion contain the crossbridge–forming molecule -actinin (Fig. 5.4b). A striking difference between microvilli and stereocilia, other than size and the presence of ezrin, is the absence of villin from the tip of the stereocilium.
Stereocilia of the sensory epithelium of the ear have some unique characteristics.
Stereocilia of the sensory epithelium of the ear also derive from microvilli. They are exquisitely sensitive to mechanical vibration and serve as sensory mechanoreceptors rather than absorptive structures. They are uniform in diameter and organized into ridged bundles of increasing heights, forming characteristic staircase patterns (Fig. 5.5a). Their internal structure is characterized by the high density of actin flaments extensively cross-linked by espin, which is critical to normal structure and function of stereocilia. Stereocilia of sensory epithelia lack both ezrin and -actinin.