Fig. 4. DNA damage triggers a Chk2-dependent cellularization block. (A-H) Control (w¹¹¹⁸) and mnk mutant embryos were injected with bleomycin, and cellularization was monitored by time-lapse transmitted light and laser scanning confocal microscopy. Rhodamine-conjugated tubulin was co-injected as a marker for cell cycle stage. Nuclei exclude fluorescent rhodamine-conjugated tubulin and thus appear as dark circles on confocal imaging (Sibon et al., 1997). Recordings were started at the beginning of interphase 14 (0 seconds, 0s). Immediately after injection, nuclei were in a uniform monolayer in both wild-type (w¹¹¹⁸) and mnk mutant embryos (A,E). By 1200 seconds (1200s), the nuclear monolayer in w¹¹¹⁸ controls was disorganized and some nuclei had dropped into the interior of the embryo (B), and these embryos did not cellularize (C,D). By contrast, all mnk mutant embryos injected with bleomycin consistently maintained a uniform nuclear monolayer and cellularized (E-H). (F,G) Plasma membrane invagination at the cellularization front is indicated (arrows). (D,H) Rhodamine-conjugated tubulin is excluded from interphase nuclei and was used as a cell cycle marker. (Also see Movies 1-5 in the supplementary material.) Scale bar: 50 µm.