Application of the 8 hours/16 hours/8 hours stretch protocol to human intervertebral disc cells. (A) Representative morphological images of static and high mechanical strain (HMS) cultured human annulus fibrosus (AF) and nucleus pulposus (NP) cells. Scale bar: 200 μm. (B) Gene expression analysis of both AF and NP cells immediately after the stretch protocol ended. IVD, Intervertebral disc; NGF, Neuronal growth factor; TLR, Toll-like receptor; TNF, Tumour necrosis factor. Error bars represent SEM. Three independent experiments were performed. *P < 0.05 and **P < 0.01 by Student’s t-test.