Fig. 5. Rappaport furrows in C. elegans. (A) A sequence of DIC images in which the spindle-side furrow failed to abscise (frame 3), yielding a toroidal binucleate cell. Since no cleavage scar exists to orient the posterior spindle, at second cleavage both spindles develop in parallel. Furrows form both across each spindle (arrowheads) and between the two pairs of spindle poles (arrowheads with `r' for Rappaport furrow). Both the spindle-crossing and Rappaport furrows may ingress completely (frame 5) and remain stable, but Rappaport furrows are less likely to do so (one has regressed in frame 6; the other is stable). (B) A wild-type, perforated GFP-myosin-expressing zygote that exhibits furrowing between two unconnected spindle poles. Frames 1-3 show that this cell exhibited myosin recruitment to both the spindle-side and far-side cortex, and that as usual only the spindle-side furrow completed, yielding a U-shaped binucleate cell (frame 4). In this case the posterior spindle failed to align with the AP axis; asterisks in frame 5 indicate spindle poles. During cytokinesis three furrows initiate: around the anterior arm of the U, around the posterior arm, and between the two spindles (arrowheads in frame 7). The middle furrow represents an independent induction since the posterior arm of the U exhibits myosin recruitment to the probe cortex (arrowhead in frame 8). (C) A perforated, PAR-2-depleted zygote. Frame 1 shows that a furrow closed on the spindle side, yielding a binucleate U-shaped cell. Asterisks in frame 3 indicate the spindle poles. Furrowing begins across each spindle (frame 4), but myosin is also recruited between them, eventually resulting in a shallow but persistent furrow (frames 5 and 6). Bars, 10 µm; ss, spindle side; fs, far side from the perforation; r, Rappaport furrow.