Sustained static compression induces transient activation of apoptosis through the death receptor pathway and persistent activation of apoptosis through the p53-mediated mitochondrial pathway in intervertebral disc cells. (A) Representative immunohistochemical sections for cleaved caspase-8, cleaved caspase-9, p53-regulated apoptosis-inducing protein 1 (p53AIP1), 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 cleaved caspase-8-positive cells, cleaved caspase-9-positive cells, and p53AIP1-positive cells 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.