Relationship Between Flagellar Movement and Head Trajectory at Higher Frame Rates: Is This Still a Valid Approach for CASA?

Abstract

Metachronal sliding and two types of synchronous sliding of the outer doublet microtubules have been found to generate and modulate sperm flagellar motility. The metachronal sliding propagates around the axoneme circumferentially, from one doublet to another along the flagellum, whereas the two types of synchronous sliding take place synchronously throughout an extended region along the doublet microtubules: oscillatory synchronous sliding occurs between most pairs of the doublet microtubules, and non-oscillatory synchronous sliding occurs between a specific pair of the doublet microtubules. Moreover, sperm hyperactivation is the conversion mode of synchronous sliding from the non-oscillatory synchronous sliding to the oscillatory synchronous sliding under a constant rate of microtubule sliding. Therefore, the most important feature of the hyperactivated sperm motility is a low beat frequency caused by large microtubule displacement of the oscillatory synchronous sliding. The circumferential propagation of active sliding around the nine doublet microtubules of the metachronal sliding suggests that it is easier for the flagellum to generate helical waves than planer waves. In fact, most spermatozoa beat with three-dimensional flagellar waves to a greater or lesser extent. The spermatozoa with a large three-dimensional component rotate around their long axes during swimming. Accordingly, increasing the image sampling frequency is necessary to improve the accuracy of motility analysis of the spermatozoa rotating around their long axes and develop a new analysis method.