The Axonemes of Cilia and Flagella
An axoneme is the specialised bundle of microtubules found within the membrane of a eukaryotic flagellum or cilium, organelles which are distinguished from one another really only on the basis of details of their modes of beating. Axonemes and basal bodies which are closely related in structure, are found throughout the animal kingdom, in algae, protists, fungi and primitive plants such as ferns. The bundles are highly ordered and very stable in comparison with microtubules in general. There are many other motile organdies consisting of bundles of stable microtubules in different arrangements, found in numerous invertebrate species. There are also variations on the usual ‘9 + 2’ arrangement (shown for a sperm tail in Figure 1.13) throughout the animal kingdom. However, the ‘9 + 2’ axoneme seems to have been selected during evolution as the optimum, geared for maximum efficiency. It is to microtubules what a striated muscle is to actin: a powerful machine in which efficiency is maximised, at the expense of flexibility in the redeployment of the components.
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