Regulation of osteoclast formation. A range of hormones and cytokines, targeting to stromal cells, enhance expression of M-CSF and RANK-L, the pivotal osteoclastogenic molecules, and the proinflammatory cytokines IL-1, IL-6, and TNF. These proteins stimulate both formation and activity of mature polykaryons. HSC, hematopoietic stem cell; OC, osteoclast.