FIGURE 5.9 • Basal bodies and cilia. This diagnostic electron micrograph obtained during biopsy of the nasal mucosa from a child undergoing evaluation for primary ciliary dyskinesia shows a normal appearance of basal bodies (BB) and cilia (C). It is an oblique section through the apical part of ciliated cells. Basal bodies seen in cross section appear as more dense structures than sectioned oblique and longitudinal profiles of the cilia above. Several profiles of microvilli (Mv) are visible at the apical cell surface. 11,000. Inset. Three basal bodies sectioned at the level of basal feet (BF). Note that all basal feet are oriented in the same direction. They most likely rotate the basal body to a desired angle in an effort to coordinate ciliary movement. 24,000. (Courtesy of Patrice C. Abell Aleff.)
Cilia movement originates from the sliding of microtubule doublets, which is generated by the ATPase activity of the dynein arms.
Ciliary activity is based on the movement of the doublet microtubules in relation to one another. Ciliary movement is initiated by the dynein arms (see Fig. 5.7b). The ciliary dynein located in the arms of the A microtubule forms temporary cross-bridges with the B microtubule of the adjacent doublet. Hydrolysis of ATP produces a sliding movement of the bridge along the B microtubule. The dynein molecules produce a continuous shear force during this sliding directed toward the ciliary tip. Because of this ATP-dependent phase, a cilium that remains rigid exhibits a rapid forward movement called the effective stroke. At the same time, the passive elastic connections provided by the protein nexin and the radial spokes accumulate the energy necessary to bring the cilium back to the straight position. Cilia then become flexible and bend toward the lateral side on the slower return movement, the recovery stroke.
However, if all dynein arms along the length of the A microtubules in all nine doublets attempted to form temporary cross-bridges simultaneously, no effective stroke of the cilium would result. Thus, regulation of the active shear force is required. Current evidence suggests that the central pair of microtubules in 9  2 cilia undergo rotation with respect to the nine outer doublets. This rotation may be driven by another motor protein, kinesin, which is associated with the central pair of microtubules. The central microtubule pair can act as a “distributor” that progressively regulates the sequence of interactions of the dynein arms to produce the effective stroke.
Cilia beat in a synchronous pattern.
Motile cilia with a 9  2 pattern display a precise and synchronous undulating movement. Cilia in successive rows start their beat so that each row is slightly more advanced in its cycle than the following row, thus creating a wave that sweeps across the epithelium. As previously discussed, basal feet of basal bodies are most likely responsible for synchronization of ciliary movement. During the process of cilia formation, all basal feet became oriented in the same direction of effective stroke by rotating basal bodies. This orientation allows cilia to achieve a metachronal rhythm that is responsible for moving mucus over epithelial surfaces or facilitating the flow of fluid and other substances through tubular organs and ducts.
Primary cilia are nonmotile and contain a 9  0 pattern of microtubules.
Differing from motile cilia with the 9  2 pattern of microtubules is another type of cilia that display a 9  0 microtubule arrangement. Cilia with this pattern have the following characteristics:  They are nonmotile and passively bend by a flow of the fluid.
to generate motile force.  The central pair of microtubules is missing.  The axoneme originates from a basal body that resembles a mature centriole positioned orthogonally in relation to its immature counterpart.  Primary cilium formation is synchronized with cell cycle progression and centrosome duplication events.