Figure 2: Cytokinetic rings of mammalian cells exhibit regular patterns.

(a) Myosin is distributed in regular clusters around the perimeter of the cytokinetic ring. Right: overlay of 25 frames, starting from the frame shown on the left. The clusters move radially, though little and collective rotations are visible for larger diameter. Time between images 10 s. (b) Overlay of 26 subsequent frames of a closing ring visualized by the fluorescence labelling of myosin and actin. Time between images 45 s, superimpositions of five z-planes, plane distance 1.3 μm. (c) The cytokinetic ring visualized by the labelling of myosin and mDia2-formin. Formin colocalizes with myosin (arrows). Overlay of 17 frames, time between frames 10 s. (d) Characterization of the myosin pattern in the control (blue) and after incubation with the cytoskeleton drugs blebbistatin (orange) and latrunculin A (purple). The cluster contrast stays about constant. The cluster density is increasing while the ring is constricting, but not as much as expected for a constant number of clusters (dashed line). Cluster density of cells treated with the two drugs is reduced. The cluster size decreases as the ring closes. Blue points represent averages, crosses correspond to single data points, error bars indicate the s.d., time-lapses of 11 closing rings were analysed. Each orange (blebbistatin) or purple (latrunculin A) point is obtained from averaging the cluster parameters from a fixed ring (blebbistatin: N=25, latrunculin A: N=28). (e) Time-lapse of a cell before, during and after formation of the cytokinetic ring visualized for myosin, below a zoomed region. Scale bar, 5 μm. (f) Fourier spectrum of the intensity profiles from the ring shown in e. At t=0 s, higher frequency structures appear. Scale bar, 5 μm in a–c,e; t=0 s is the onset of constriction.