Modulatory mechanisms of sliding of nine outer doublet microtubules for generating planar and half-helical flagellar waves.

It has been shown that sperm flagellar motility is generated and modulated by metachronal sliding and two types of synchronous sliding of the outer doublet microtubules. Metachronal sliding propagates around the axoneme circumferentially from one doublet to another along the flagellum, whereas the two types of synchronous sliding occur synchronously throughout an extended region along the doublet microtubules. Oscillatory synchronous sliding occurs between most pairs of the nine doublet microtubules, whereas non-oscillatory synchronous sliding occurs between a specific pair of the nine doublet microtubules. These types of sliding coexist in the flagellum and create beat cycles of flagellar movement. The circumferential propagation of active sliding around the nine doublet microtubules in the metachronal sliding suggests that it is easier for a flagellum to produce helical waves than planar waves. Most sperm flagellar movements are planar to a certain extent. Therefore, mechanisms that modulate the helical waves into planar waves may be present. Structures such as the central pair microtubules in 9 + 2 sperm flagella and the fusion of fibrous-sheath and 3-,8-doublet microtubules in mammalian sperm flagella partition the nine outer doublet microtubules into two groups. Accordingly, the sliding between these two groups generates planar flagellar waves. A similar effect is caused by the sliding between a specific pair of the nine doublet microtubules of the non-oscillatory synchronous sliding, occurring in a Ca2+ concentration-dependent manner. These hard- and soft-wired systems produce the nearly planar flagellar waves required for the efficient propulsion of spermatozoa.