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The motile apparatus of the sperm. (A) Cross section of the flagellum of a mammalian spermatozoon, showing the central axoneme and the external fibers. (B) Interpretive diagram of the axoneme, showing the “9 + 2” arrangement of the microtubules and other flagellar components. The schematic diagram shows the association of tubulin protofilaments into a microtubule doublet. The first (“A”) portion of the doublet is a normal microtubule comprising 13 protofilaments. The second (“B”) portion of the doublet contains only 11 (occasionally 10) protofilaments. The dynein arms contain the ATPases that provide the energy for flagellar movement. (C) A three-dimensional model of an “A” microtubule. The α- and β-tubulin subunits are similar but not identical, and the microtubule can change size by polymerizing or depolymerizing tubulin subunits at either end. (A, photograph courtesy of D. M. Phillips; B after De Robertis et al. 1975 and Tilney et al. 1973.)