PTHLH and rheumatoid arthritis: The findings that the expression level of BGLAP/osteocalcin gene is higher and the expression levels of BMP6, TGF-β1, PTHLH, and PTN genes are lower in the hTERT-OA 13A FLSs compared to hTERT-RA 516 FLSs are consistent with the findings that the protein level of BGLAP/osteocalcin is significantly higher [46] and the protein levels of BMP6, PTHLH, PTN, and TGF-β1 are significantly lower in the synovial fluid of OA patients compared to RA patients [43, 45, 47–49].