Fig. 2. FNDC5/irisin plays a key role in the effect of physical exercise on IVDD.

a Plasma content of irisin in the exercise group of mice compared to the no-exercise group (N = 6). b Plasma content of irisin in FNDC5^−/− mice compared to FNDC5^WT mice after 4 weeks of physical exercise (N = 6). c Representative X-ray images of the mouse tail intervertebral space at 0 and 4 weeks. d The disc height index of a mouse tail disc at 0 and 4 weeks (N = 6). e The respective histological grades of a mouse tail disc (N = 6). f Representative HE staining and SO staining of discs in FNDC5^−/− and FNDC5^WT mice (bar: 150 μm). g The respective immunohistochemical staining of p16INK4a and cleaved-caspase3 in NP tissues of FNDC5^WT/FNDC5^−/− mice (bar: 50 μm). All data are shown as the mean ± SD. *p < 0.05, **p < 0.01.