Dose-response effect of growth factors on proliferation of human NP cells (10%FBS/F12) (n = 20). Cells were incubated with increasing concentrations (0.1–500 μg/L) of TGF-β1 and IGF-I in 10%FBS/F12 medium for 72 hours. The proliferative response was evaluated by MTT assay. Results were expressed as the mean optical density of MTT absorbance in growth factors and control wells. * P < 0.05, ** P < 0.01, *** P < 0.001 versus control.