TNFSF11 and rheumatoid arthritis: Indeed, many studies revealed a higher expression of RANKL in synovial tissues and synovial fibroblasts from RA compared to OA patients and exclusively co-cultures with synovial fibroblasts from patients with RA promote osteoclast development in vitro, indicating that upregulation of RANKL in activated synovial fibroblasts may facilitate the differentiation of osteoclasts [109, 110].