FIGURE 5.7 • Molecular structure of cilia. This figure shows a three-dimensional arrangement of microtubules within the cilium and the basal body. Cross section of the cilium (right) illustrates the pair of central microtubules and the nine surrounding microtubule doublets (9  2 configuration). The molecular structure of the microtubule doublet is shown below the cross section. Note that the A microtubule of the doublet is composed of 13 tubulin dimers arranged in a side-by-side configuration (lower right), whereas the B microtubule is composed of 10 tubulin dimers and shares the remaining dimers with those of the A microtubule. The dynein arms extend from the A microtubule and make temporary cross-bridges with the B microtubule of the adjacent doublet. The basal body is anchored by the striated rootlet within the cell cytoplasm. Note the presence of the basal foot in the midsection of the basal body. The cross section of the basal body (lower left) shows the arrangement of nine microtubule triplets. These structures form a ring connected by nexin molecules. Each microtubule doublet of the cilium is an extension of two inner A and B microtubules of the corresponding triplet. The C microtubule is shorter and extends only to the transitional zone. Inset a. Electron micrograph of longitudinally sectioned cilia from the oviduct. The internal structures within the cilia are microtubules. The basal bodies appear empty because of the absence of the central pair of microtubules in this portion of the cilium. 20,000. Inset b. Electron micrograph of cross section of the cilium showing corresponding structures with drawing below. 180,000.
FIGURE 5.8 • Ciliated surface of the respiratory mucosa. Electron micrograph shows a longitudinally sectioned cilia from a respiratory epithelium of the nasal cavity. At this magnification, most of the basal bodies (BB) appear empty because of the absence of the central pair of microtubules in this portion of the cilium. Structural details of the basal body and basal body–associated structures are well visible on this section as well as on the higher magnification insert. Note that almost all basal bodies on this section possess striated rootlets (SR). They anchor the basal bodies deep within the apical cell cytoplasm. Each basal body has a single asymmetric basal foot (BF) projecting laterally; several are well visible on this section. The transitional zone (TZ) extends from the upper end of the basal body into the axomene (Ax) which is formed by a 9  2 microtubular arrangement. A central pair of microtubules is present on most of these sections. In addition, an alar sheath (arrowheads) provides a winglike extension between the transitional zone and plasma membrane. The first and second basal bodies from the right have well preserved alar sheaths. 15,000. Inset 25,000. (Courtesy of Dr. Jeffrey L. Salisbury.)  The basal foot is an accessory structure that is usually Localization of myosin molecules associated with basal feet found in the midregion of the basal body (see Fig 5.8). supports this hypothesis. Since in the typical epithelial ciliated cells all basal feet are  The striated rootlet is composed of longitudinally aligned oriented in the same direction (Fig. 5.9), it has been hy-protofilaments, containing rootletin (a 220-kilodalton pothesized that they function in coordinating ciliary protein). Striated rootlet projects deep into cytoplasm and movement. They are most likely involved in adjusting firmly anchors the basal body within the apical cell cytobasal bodies by rotating them to the desired position. plasm (see Fig 5.8).