Fig. 5. Normal proliferation, but massive apoptosis of Sox5^–/–/Sox6^–/– notochord cells. (A) BrdU labeling of proliferating cells in E11.5 and E13.5 control and Sox5^–/–/Sox6^–/– embryos. Three pairs of littermates were tested per age (1-6). For each embryo, notochord cells were counted in five to nine mid-sagittal sections in the thoracolumbar region, totaling 300-1000 cells. The percentage of BrdU-positive cells is presented as the mean±s.d. for all sections. Representative pictures are shown underneath. BrdU-labeled cells are brown. At both stages, mutant notochord cells proliferate at a rate that is similar to or higher than that of control cells. (B) TUNEL assay in the thoracolumbar region of control and Sox5^–/–/Sox6^–/– embryos. At E11.5, few cells are dying in the control notochord (red, arrowheads), and many more in the mutant notochord. Cells are also dying in intervertebral mesenchyme (IM) in both embryos (arrows). At E13.5, no cells are dying in the control notochord, but massive apoptosis is occurring in the mutant notochord. No cells are dying in control cartilages, but few cells are dying in mutant intervertebral mesenchyme (arrow). VB, vertebral bodies.