Decreased mitotic index and apoptosis in Bub1-null embryos. (A) Haematoxylin-eosin-stained sections of brain and liver from embryos harvested 3 days after tamoxifen injection. Enlargements show numerous pyknotic nuclei in tissues from BUB1Δ/Δ embryos. Scale bar, 0.2 mm. (B) Massive apoptosis caused by BUB1 inactivation. BUB1Δ/+ and BUB1Δ/Δ embryos were harvested 3 days after tamoxifen injection, fixed, then sections prepared and stained to detect DNA fragmentation. Scale bar, 0.2 mm. (C) Reduced number of mitotic cells in Bub1-deficient embryos. Embryo sections prepared as in (B) were stained to detect phospho-histone-H3 (green) and DNA (red). Scale bar, 0.5 mm.