TNFSF11 and osteoporosis: An emerging phenomenon during osteoporosis is spontaneous osteoclastogenesis (SO), defined as the differentiation of peripheral blood mononuclear cells (PBMCs) into mature, multinucleated, bone-resorbing cells in the absence of exogenous factors, e.g., receptor activator of nuclear factor kappa-B ligand (RANKL) or macrophage colony-stimulating factor (M-CSF) (8–10).