Optimization of platelet concentration for CPC chondrogenesis. (A) Immunofluorescence analysis of the secretion of Col-I, Col-II, Col-X, and Sox-9 in CPCs exposed to different concentrations of PRP. Scale bars represent 10 μm. (B) Col-I, Col-II, Col-X, and Sox-9 positive cell ratios after the PRP stimulation. (C) Col-I, Col-II, Col-X and Sox-9 positive cell numbers after the PRP treatment. Data are presented as mean ± SEM, n = 3, *P < 0.05, **P < 0.01, ***P < 0.001 and #P > 0.05.