Cilia are present in almost all organs and play a significant role in the human body. There is increasing evidence that cilial dysfunction is involved in many human disorders. Sev-eral hereditary disorders grouped under the general name of primary ciliary dyskinesia (PCD), also known as immotile cilia syndrome, affect the function of cilia. PCD represents a group of autosomal recessive hereditary disorders affecting 1 in 20,000 individuals at birth. The clinical features of PCD reflect the distribution of motile cilia. For instance, the mucociliary transport that occurs in the respiratory epithelium is one of the important mechanisms protecting the body against invading bacteria and other pathogens. Motile cilia covering the epithelium of the respiratory tract are responsible for the clearance of the airway. Failure of the mucociliary transport system occurs in Kartagener’s syndrome, which is caused by a structural abnormality that results in absence of dynein arms (Fig. F5.2.1). In addition, EM examination of basal bodies from individuals with the Kartagener’s syndrome often reveals misoriented basal feet pointing in different di-rections. Young’s syndrome, which is characterized by malformation of the radial spokes and dynein arms, also af-fects ciliary function in the respiratory tract. The most prominent symptoms of PCD are chronic respiratory dis -tress (including bronchitis and sinusitis), otitis media (in-flammation of the middle ear cavity), persistent cough, and asthma. Respiratory problems are caused by severely impaired or absent ciliary motility that results in reduced or absent mucociliary transport in the tracheobronchial tree. Flagellum of the sperm, cilia of the efferent ductules in the testis, and cilia of the female reproductive system share the same organization (9 2) pattern with the cilia of the respiratory tract. Therefore, males with PCD are sterile be-cause of immotile flagella. In contrast, some females with the syndrome may be fertile; however, there is an increased incidence of ectopic pregnancy. In such individuals, the FIGURE F5.2.1 • Electron micrograph of the cilium from an individual with primary cilary dyskinesia (PCD). Note the absence of dynein arms on microtubule doublets. 180,000. (Courtesy of Patrice Abell-Aleff.) ciliary movement may be sufficient, though impaired, to permit transport of the ovum through the oviduct to the uterus. Some individuals with PCD may also develop symp-toms of hydrocephalus internus (accumulation of fluid in the brain) or transient dilatation of inner brain ventricles. The ependymal cells lining the cerebrospinal fluid–filled spaced in the brain possess motile cilia with a 9 2 pat-tern. These cilia may be important for the circulation of cerebrospinal fluid through the narrow spaces between the brain ventricles. About 50% of patients with diagnosed PCD have situs inversus (a condition in which the organs of the viscera are transposed through the sagittal plane), providing a link be-tween left–right asymmetry and nodal cilia. Diagnosis of PCD in individuals with clinical syn-dromes compatible with PCD can be established by EM (see Fig. F5.2.1).
merization of and -tubulin molecules. A growing projection of the apical cell membrane become visible and contains the nine doublets found in the mature cilium. During the elongation stage of motile cilia, the assembly of two single, central microtubules starts in the transitional zone from  -tubulin rings. The subsequent polymerization of tubulin molecules occurs within the ring of doublet microtubules, thus yielding the characteristic axonemal 9  2 arrangement. Subsequently, the axoneme grows upward from the basal body, pushing the cell membrane outward to form the mature cilium.