Consistent with our findings, Tomaszewski et al. observed that OPN expression was down-regulated in degenerated and calcified endplates,27 while Xiao et al. found that OPN expression was up-regulated in ossified endplates of IDD.28 It is possible that OPN from different cell populations may have various biological effects during the process of different stages of degeneration. This evidence concerns the gene SPP1 and intervertebral disk degenerative disorder.