4-aminobiphenyl (4-ABP) induces chondrogenic differentiation of umbilical cord mesenchymal stem cells (UC-MSC). (A) HPLC/MS analysis of kartogenin (KGN), 4-ABP and phthalic acid (PA) in UC-MSC culture medium and UC-MSC after KGN (10 μM) treatment for 24 h. (B-F) Chondrocyte-specific gene and protein expression after KGN, 4-ABP or PA treatment for 4 days (B) and 20 days (C-F) in UC-MSC under monolayer culture. UC-MSC pellets diameters (G) and type II collagen expression and analysis (H,I) after chemical treatment for 20 days. Scale bars = 50 μm. *p < 0.05, **p < 0.01, ***p < 0.001 versus vehicle; #p < 0.05, ##p < 0.01, ###p < 0.001 versus KGN. n = 6 per group.