by stimulating osteoclast activity and differentiation from progenitor cells.  Vitamin D ,  parathyroid hormone  and stimulation from osteocytes induce osteoblasts to increase secretion of RANK- ligand  and  interleukin 6 , which cytokines then stimulate increased reabsorbtion of bone by osteoclasts. These same compounds also increase secretion of  macrophage colony-stimulating factor  by