[Molecular basis of sperm movement (author's transl)]

Sperm flagella like somatic cilia consist of an axoneme of 9 + 2 microtubules. Several projections protrude form the A subfiber of each microtubule doublets: the 2 rows of Dynein arms, the Radial Spokes and the Nexin Links. All the axonemal components constitute the flagellar motor. The energy necessary to generate microtubule sliding, responsible of the flagellar beat, is provided by Mg2+-ATP hydrolysis by Dynein ATPase. The Dynein ATPase is a group microtubule A called Dynein arms. Mg2+-ATP binding and hydrolysis generate transient cross-bridges that cause the microtubule doublets walk one upon another. The transformation of sliding to bending is produced by the Radial Spokes which eventually go through a cycle of transient attachment to the central sheath. The flagellar movement is controlled by intra-flagellar factors mostly Ca2+, responsible for the symmetry of the beat and the arrest of flagellar motility. Further, cAMP increase is associated in certain conditions with the initiation or increase of sperm motility. Extrinsic factors seem to play a major role in the control of sperm motility, such as proteins that coat the sperm membrane or factors associated with the egg, like the peptide "speract", extracted from sea urchin egg, which stimulates sperm respiration rate and increases cAMP and cGMP concentrations.