Diagram illustrates the mechanism by which the core of actin filaments is remodeled. Actin polymerization and espin cross-linking into the barbed (plus) end of actin filaments occurs at the tip of the stereocilia. Disassembly and actin filaments depolymerization occurs at the pointed (minus) end of actin filament near the base of the stereocilium. When the rate of assembly at the tip is equivalent to the rate of disassembly at the base, the actin molecules undergo an internal rearward flow or treadmilling, thus maintaining the constant length of the stereocilium. (Reprinted with permission from Rzadzinska AK, Schneider ME, Davies C, Riordan GP, Kachar B. An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal. J Cell Biol 2004;164:887–897.) of microtubules in the axoneme and lack of microtubule-concentrated in the area that surrounds the primitive associated motor proteins. They function as chemosen-node, hence their name nodal cilia. They have a similar axsors, osmosensors, and mechanosensors, and they onemal internal architecture as primary cilia; however, mediate light sensation, odorant, and sound perception in they are distinct in their ability to perform rotational multiple organs in the body. It is now widely accepted movement. They play an important role in early embrythat primary cilia of cells in developing tissues are essen-onic development. tial for normal tissue morphogenesis.
 Nodal cilia are found in the embryo on the bilaminar The functional and structural features of all three types of embryonic disc at the time of gastrulation. They are cilia are summarized in Table 5.2.
Motile cilia are capable of moving fluid and particles along epithelial surfaces.
Motile cilia possess an internal structure that allows them to move. In most ciliated epithelia, such as the trachea, bronchi, or oviducts, cells may have as many as several hundred cilia arranged in orderly rows. In the tracheobronchial tree, the cilia sweep mucus and trapped particulate material toward the oropharynx where it is swallowed with saliva and eliminated from the body. In the oviducts, cilia help transport ova and fluid toward the uterus.
Cilia give a “crew-cut” appearance to the epithelial surface.
In the light microscope, motile cilia appear as short, fine, hairlike structures, approximately 0.25 m in diameter and 5 to10 m in length, that emanate from the free surface of the cell (Fig. 5.6). 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.
Motile cilia contain an axoneme, which represents 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, called axoneme (Fig. 5.7a). A cross-sectional view reveals a characteristic configuration of nine pairs or doublets of circularly arranged microtubules surrounding two central microtubules (Fig. 5.7b).
FIGURE 5.6 • Ciliated epithelium. Photomicrograph of an H&E–stained specimen of tracheal pseudostratified ciliated epithelium. The cilia (C) appear as hairlike processes extending from the apical surface of the cells. The dark line immediately below the ciliary processes is produced by the basal bodies (BB) associated with the cilia. 750.