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Figure 6. Osteoclast progenitor cell proliferation, differentiation, and fusion to bone-resorbing osteoclasts are enhanced during estrogen deficiency because of an increased RANKL/OPG ratio and the increased expression of M-CSF in periosteal osteoblasts and bone marrow stromal cells. Although it has been shown that estrogen can directly regulate M-CSF, it is not clear if the effect on RANKL and OPG expression is caused by a direct effect of estrogen, or if the increase is indirectly due to increased cytokines caused by a lack of estrogen.
