Sustained static compression induces apoptotic cell death in the intervertebral disc. (A) Representative sections for terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL), 4′,6-diamidino-2-phenylindole (DAPI), and merged signals and immunohistochemical sections for cleaved caspase-3 and negative control in the loaded disc nucleus pulposus (NP) and annulus fibrosus (AF) at 0, 7, 28, and 56 days of loading (bars, 50 μm). (B) Changes in the percentage of TUNEL-positive cells and cleaved caspase-3-positive cells for in the NP and AF at 0, 7, 28, and 56 days of loading. Data are mean ± standard deviation (n = 6). Two-way analysis of variance with the Tukey-Kramer post hoc test was used. *P < 0.05 when compared between loaded and unloaded conditions. †P < 0.05 when compared between different time points.