The Centrosome and Cilium

The centrosome, containing a pair of centrioles, is conspicuous in preneuronal proliferative cells during early neural development. It is seldom noted in light micrographs of adult neurons but sometimes seen in electron micrographs (Fig. 28, right), usually associated with a cilium. Because adult neurons, with a few

Figure 28 Cilium and centrosome: left EM shows a longitudinal section of a cilium arising from a neuron. At its base is a basal body (bb) with attached parabasal body. The microtubules in the ciliary shaft have a 9 + O configuration and continue into the basal body. Right EM shows part of the perikaryon of a cerebellar granule cell containing two longitudinally sectioned centrioles (arrows). The wall of each is formed by nine triplets of short tubules oriented parallel to its long axis. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford Unviersity Press, Inc. (EMs from collections of authors).

Figure 28 Cilium and centrosome: left EM shows a longitudinal section of a cilium arising from a neuron. At its base is a basal body (bb) with attached parabasal body. The microtubules in the ciliary shaft have a 9 + O configuration and continue into the basal body. Right EM shows part of the perikaryon of a cerebellar granule cell containing two longitudinally sectioned centrioles (arrows). The wall of each is formed by nine triplets of short tubules oriented parallel to its long axis. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford Unviersity Press, Inc. (EMs from collections of authors).

exceptions that may reflect the residual presence of antecedent cell types, do not divide, the role of the centrosome is puzzling: possibly it is a nondisposable relic of the mitotic apparatus of parental cells. But the recognition of the centrosome in neurons and in other cells as the microtubule-organizing center gives a new perspective. A cloud of proteins surrounds the centrioles. Microtubules attach to it at their minus ends and grow from it by addition to their plus ends.

The cilium (Fig. 28, left) has nine radial pairs of microtubules, distally dwindling to eight, with short connections but no arms or spokes between them as in other cilia. Distally a central pair appears. In kinocilia it is thought to switch the other pairs from power stroke to recovery stroke. Here it seems nonmotile.

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