FIGURE 2.52 • Basal bodies and cilia. This electron micro-graph shows the basal bodies and cilia in cross-sectional profle as seen in an oblique section through the apical part of a ciliated cell in the respiratory tract. Note the 9 2 microtubule arrangement of the cilia in which nine microtubules at the periphery of the cilia surround two central microtubules. The basal bodies lack the central tubule pair. On several cross sections, the basal foot is visible as it projects laterally from the basal body (asterisks).
28,000. (Courtesy of Patrice C. Abell-Aleff.) formed by MTOC-independent mechanisms and consists of microtubules that originate from the chromosomes. Recent experimental data indicate that in the absence of centrioles, astral microtubules fail to develop, causing errors in mitotic spindle orientation (Fig. 2.54). Thus, the primary role of centrioles in mitosis is to position the mitotic spindle properly by recruiting the MTOC from which astral microtubules can grow and establish the axis for the developing spindle.
The dominant feature of centrioles is the cylindrical array of triplet microtubules with associated proteins.
The TEM reveals that each rod-shaped centriole is about 0.2 m long and consists of nine triplets of microtubules that are oriented parallel to the long axis of the organelle and run in slightly twisted bundles (Fig. 2.55). The three micro-
FIGURE 2.53 • Two pathways of basal body formation. In the centriolar pathway, a pair of existing centriole serves as an organizing center for the duplication of new centrioles. Utilizing this pathway, ciliated cells have the ability to assemble large number of centrioles in the vicinity of an old mature centriole. In the acentriolar pathway, which plays a major role in the formation of basal bodies in ciliated cells, new centrioles are formed de novo from fbrous granules located in close proximity of nonmicrotubular structures called deuteresomes. Both pathways give rise to procentrioles, which mature as they migrate to the appropriate site near the apical cell membrane, where they become basal bodies. Fibrous granules contribute to the formation of the striated rootlet. (Based on Hagiwara H, Ohwada N, Takata K. Cell biology of normal and abnormal ciliogenesis in the ciliated epithelium. Int Rev Cytol 2004;234:101–139.) tubules of the triplet are fused, with adjacent microtubules sharing a common wall. The innermost or A microtubule is a complete ring of 13 protofilaments containing and tubulin dimers; the middle and outer B and C micro-tubules, respectively, appear C-shaped because they share tubulin dimers with each other and with the A microtubule. The microtubules of the triplets are not equal in length. The C microtubule of the triplet is usually shorter than A and B.