Differentiation of osteoblasts from primary synovial fluid (SF)-derived cells and SF-derived fibroblasts from the fourth passage (P4), from patients with oligoarticular juvenile idiopathic arthritis (oJIA) and polyarticular JIA (pJIA). Osteoblast differentiation was induced by ascorbic acid and β-glycerophosphate, and assessed by alkaline phosphatase (AP) histochemical staining. Total fibroblastic colonies were visualized with methylene blue (MB). (A) Osteoblast and total colonies from representative primary SF-derived cells from children with oJIA and pJIA stained for AP and MB on culture day 21. (B) AP-stained area (median and interquartile range (IQR), lines represent minimum and maximum values) in osteoblast cultures grown from primary SF-derived cells from children with oJIA (n = 18) and pJIA (n = 9); * P < 0.05 vs. oJIA group, Mann-Whitney test. (C) Osteoblast and total colonies from representative SF-derived fibroblast samples from the fourth passage (P4) of SF-derived cells from children with oJIA (n = 7) and pJIA (n = 4) stained for AP and MB on culture day 14. (D) AP activity measured at day 14 in osteoblast cultures derived from P4 SF-derived fibroblast population (median and IQR);* P < 0.05 vs. oJIA, Mann-Whitney test).