FIGURE 18-5 Microtubules are assembled from microtubule organizing centers (MTOCs). (a–b) The distribution of microtubules in cultured cells, as seen by immunofluorescence microscopy using antibodies to tubulin, in an interphase cell (a) and a cell in mitosis (b). (c–f) Diagrams of the distributions of microtubules in various cells and structures. All of these microtubules are assembled from distinct MTOCs. (c) In an interphase cell, the MTOC is called a centrosome (the nucleus is indicated by a blue oval). (d) In a mitotic cell, the two MTOCs are called spindle poles (the chromosomes are shown in blue). (e) In a neuron, microtubules in both axons and dendrites are assembled from an MTOC in the cell body and then released from it. (f) The microtubules that make up the shaft of a cilium or flagellum are assembled from an MTOC known as a basal body. The polarity of microtubules is indicted by (+) and (−).

[Part (a) courtesy Anthony Bretscher. Part (b) courtesy of Torsten Wittmann.]