Cilia are long, highly motile apical structures, larger than microvilli, and containing internal arrays of microtubules not microfilaments (Figure 1). In addition to cilia on epithelial cells, most (if not all) other cell types have at least one short projection called a primary cilium, which is not motile but is enriched with receptors and signal transduction complexes for detection of light, odors, motion, and flow of liquid past the cells.
Fig1. Cilia
Motile cilia are abundant on cuboidal or columnar cells of many epithelia. Typical cilia are 5-10 μm long and 0.2 μm in diameter, which is much longer and two times wider than a typical microvillus. As shown in Figure 2, each cilium has a core structure consisting of nine peripheral micro tubule doublets (in which a few tubulin protofilaments are shared) arrayed around two central microtubules. This 9 + 2 assembly of microtubules is called an axoneme (Gr. axon, axis + nema, thread). As with other microtubules, kinesin and cytoplasmic dynein motors move along the peripheral micro tubules for the transport of molecular components into and out of these structures.
Fig2. Ciliary axoneme.
Microtubules of axonemes are continuous with those in basal bodies, which are apical cytoplasmic structures just below the cell membrane (Figures 1 and 2). Basal bodies have a structure similar to that of centrioles, with triplets of microtubules and dynamic tubulin protofilaments forming rootlets anchoring the entire structure to the cytoskeleton.
Cilia exhibit rapid beating patterns that move a current of fluid and suspended matter in one direction along the epithelium. Ciliary motion occurs through successive changes in the conformation of the axoneme, in which various accessory proteins make each cilium relatively stiff, but elastic. Complexes with axonemal dynein bound to one microtubule in each doublet extend as “arms” toward a microtubule of the next doublet. With energy from ATP dynein-powered sliding of adjacent doublets relative to each other bends the axoneme and a rapid series of these sliding movements pro duces the beating motion of the cilium. The long flagellum that extends from each fully differentiated sperm cell has an axonemal structure like that of a cilium and moves with a similar mechanism.
