Polarity of some motility-related microtubules.

We have investigated the structural polarity of microtubules from several systems in which these fibers are thought to contribute to cell motility. By using a method for displaying microtubule polarity in the electron microscope, we find that both the A and B subfibers of Tetrahymena ciliary outer doublets and the inner pair of single microtubules are all oriented with their plus ends (i.e., their fast-growing ends) distal to the basal body. All of the microtubules in the axopodia of the heliozoan Actinosphaerium and all of the microtubules in the processes of melanophores from the angelfish Pterophyllum are likewise oriented with their plus ends distal to the cell centers. These results suggest that cellular systems for motility, and even those capable of bidirectional motility, can be constructed from microtubules of a single polarity.