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Fig. 2 Cell cycle status of zebrafish nz171 mutant embryos compared with wild-type siblings. (A,B) BrdU-labeling to detect cells in S-phase in forebrain and eyes of flat-mounted 18-somite embryos, anterior to top. (C-F) Dorsal (C,D) and lateral (E,F) views of anti-phosphohistone H3 (pH3)-labeling to detect cells in M phase at the stages indicated (anterior to the left). (G-J) Disordered mitotic chromosomes in nz171 mutants. (G,H) Transverse sections bisecting the yolk extension of wild-type (G) and nz171 mutant (H) embryos stained with Giemsa (dorsal to top). Whereas recognizable structures (s, somite; da, dorsal aorta) and normal nuclei (example marked by asterisk) are visible in wild-type embryos, no such structures are visible in nz171 mutants, which have abnormal nuclei with condensed, disorganized chromosomes. pH3-labeled chromosomes of a similar configuration to H (blue arrowheads) appear in nz171 (J) but not in wild-type (I); lateral flat-mounted embryos, mid-trunk (high magnification inset). Scale bar: 20 µm. (K,L) TUNEL labeling indicates prevalent cell death in the ICM and neuronal regions of nz171 embryos; lateral views of tail regions, anterior to the left, showing the ICM region just above the yolk extension (ye).