Detection of SIRT1 expression and cellular senescence in human degenerative NP tissues. (A) Preoperative lumbar MRI images of patients’ IVD tissues were classified via Pfirrmann’s grading system. Degenerative IVDs scored III-IV were defined as early stage of IVDD, and the discs scored V were defined as advanced stage of IVDD. Red arrows refer to the position of the acquired NP tissue. (B) The degree of senescence of NP cells isolated from IVDs with different grades of degeneration was detected and analyzed using SA-β-Gal staining (200×). (C) The expression level of SIRT1 in NP tissues obtained from patients with different Pfirrmann grades was detected by immunohistochemistry (200×). *P<0.05 versus the Pfirrmann III group. #P<0.05 versus the Pfirrmann IV group. Black bars=100 μm.