Figure 251

Electron micrograph showing parent and daughter centrioles in a fibroblast. Note that the transverse-sectioned centriole in each of the pairs reveals the triplet configuration of microtubules. The lower right centriole represents a midlongitudinal section, whereas the upper left centriole has also been longitudinally sectioned, but along the plane of its wall. x90,000. (Courtesy of Drs. Manley McGill, D. P. Highfield, T. M. Monahan, and Bill R. Brlnkley.)

misoriented spindle (anastral bipolar spindle)

The microtubule triplets of the centriole surround an internal lumen. The distal part of the lumen (away from the nucleus) contains a 20-kDa Ca2+-binding protein, centrin (Fig. 2.52). The proximal part of the lumen (close to the nucleus) is lined by y-tubulin, which provides the template for the arrangement of the triplet microtubules. A group of proteins generally known as pericentrins has also been associated with the centrioles. Newly identified protein p210 forms a ring of molecules that appears to link the distal end of the centriole to the plasma membrane. Filamentous proximal and distal proximal and distal

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