Fig 8. The spontaneous reversal of the sliding direction of the doublet microtubules of sea urchin sperm axonemes.

(A) Dark-field video micrographs showing the sliding movements of the doublet microtubules. The sea urchin sperm axoneme still beats in the first frame. The microtubule sliding was induced with 0.5 mM MgATP^2- and 20 μg/ml elastase. The numbers indicate the microtubule sliding events that occur on a flagellum. The free-Ca²⁺ solution concentration was adjusted to 10⁻⁹ M. Time interval between successive images is 0.25 s. (B) The profiles of microtubule sliding displacement shown in A. The numbers correspond to those of the microtubule sliding shown in A. The circles represent values of large sliding displacements and squares represent small sliding displacements. The sliding velocity was 31.9 μm/s, as calculated between frames 1 and 5 of the sliding No. 1 and 24.5 μm/s between frames 1 and 3 of the sliding No. 2. Filled circles and squares are values of the sliding displacement obtained from the frames shown in A. Bar = 15 μm.