The polarity orientation of cellular microtubules is widely regarded to be important in understanding the control of microtubule assembly and microtubule-based motility in vivo. We have used a modification of the method of Heidemann and McIntosh (Nature (Lond.). 286:517-519) to determine the polarity orientation of axonal microtubules in postganglionic sympathetic fibers of the cat. In fibers from three cats we were able to visualize the polarity of 68% of the axonal microtubules; of these, 96% showed the same polarity orientation. Our interpretation is that the rapidly growing end of all axonal microtubules is distal to the cell body. We support Kirschner's hypothesis on microtubule organizing centers. (J. Cell Biol. 86:330-334), although this interpretation raises questions about the continuity of axonal microtubules. Our results are inconsistent with a number of models for axonal transport based on force production on the surface of microtubules in which the direction of force is determined by the polarity of microtubules.

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