Since stereocilia can be easily damaged by overstimulation, they have a molecular mechanism to continuously renew their structure, which needs to be maintained in proper working condition for a lifetime. Using fluorescent-labeled actin molecules, researchers found that actin
TABLE Summary of Apical Domain Modifications in the Epithelial Cells 5.2
FIGURE 5.4 • Molecular structure of stereocilia. a. Electron micrograph of stereocilia from the epididymis. The cytoplasmic projections are similar to microvilli, but they are extremely long. 20,000. b. Schematic diagram showing the molecular structure of stereocilia. They arise from the apical cell protrusions, having thick stem portions that are interconnected by cytoplasmic bridges. Note the distribution of actin filaments within the core of the stereocilium and the actin-associated proteins, fimbrin, and espin, in the elongated portion (enlarged box); and -actinin in the terminal web, apical cell protrusion, and occasional cytoplasmic bridges between neighboring stereocilia.
monomers are being constantly added at the tips and removed at the base of the stereocilia while the entire bundle of actin filaments moves toward the base of the stereocilium (Fig. 5.5b,c). This treadmilling effect of the actin core structure is highly regulated and depends on the length of the stereocilium.
The functional and structural features of stereocilia in comparison to microvilli and cilia are summarized in Table 5.2.
Cilia are common surface modifications present on nearly every cell in the body. They are hairlike extensions of the apical plasma membrane containing an axoneme, the microtubulebased internal structure. The axoneme extends from the basal body, a centriole-derived, microtubuleorganizing center (MTOC) located in the apical region of a ciliated cell. The basal bodies are associated with several accessory structures that assist them with anchoring into cell cytoplasm. Cilia, including basal bodies and basal body–associated structures, form the ciliary apparatus of the cell.
In general, cilia are classified as motile, primary, or nodal.
Based on their functional characteristics cilia are classified into three basic categories:  Motile cilia have been historically the most studied of all cilia. They are found in large numbers on the apical domain of many epithelial cells. Motile cilia and their counterparts, fagella, possess a typical 9  2 axonemal organization with microtubule-associated motor proteins that are necessary for the generation of forces needed to induce motility.
 Primary cilia (monocilia) are solitary projections found on almost all eukaryotic cells. The term monocilia imp lies that only a single cilium per cell is usually present. Primary cilia are immotile because of different arrangements
FIGURE 5.5 • Dynamic turnover of an internal architecture of sterocilia. a. This scanning electron micrograph shows stereocilia of sensory epithelium of the inner ear. They are uniform in diameter and organized into ridged bundles of increasing heights. 47,000.
b.
Confocal microscopy image shows incorporation of the -actin green fluorescent protein (GFP) and espin-GFP to the tip of the stereocilia (green). Actin filaments in the core of the stereocilia are counterstained with rhodamine/phalloidin (red). 35,000 c.