The maturation of osteoblasts is promoted by growth factors released from the bone matrix during resorption, as well as by growth factors produced by Protein Tyrosine Kinase inhibitor osteoblast progenitors themselves. Many of the growth factors govern the life-span of osteoblasts and osteoclasts by their effects on apoptosis. Bone loss in sex steroid deficiency or following glucocorticoid excess is caused by alteration of bone cell production and shortening of osteoblast life-span, and by osteoclast life-span alterations. Therapies that prevent or reverse osteoporosis act, at least in part, by preventing osteoblast apoptosis and stimulating osteoclast apoptosis. The following is a partial Inhibitors,research,lifescience,medical list of hormones that regulate apoptosis in bone

cells: Estrogen promotes osteoclast apoptosis, but prevents osteoblast apoptosis.3,4 Glucocorticoid reduces osteoblast number and has a direct anti-apoptotic Inhibitors,research,lifescience,medical effect on the osteoclast.5 Parathyroid hormone (PTH) inhibits osteoblast apoptosis.6 REGULATION OF THE BMU Bone mass maintenance is determined by the net anabolic activity of the BMU,7 when the matrix elaboration of the osteoblasts exceeds the bone resorption by the osteoclasts. The normal function of the BMU causes a continuous remodeling

process of the bone with deposition of bony matrix (osteoid) along the vectors of the generated force Inhibitors,research,lifescience,medical by gravity and attached muscle activity (Wolff’s law)8 and resorption of the bone that is not aligned with these boundaries. A non-physiological propagation of Inhibitors,research,lifescience,medical forces along the bones, such as immobilization of a limb by an external device or low gravity condition on one side, or impaired biochemical control of the BMU, as happens in several pathological conditions, will cause an imbalance in the BMU function with subsequent pathological bone resorption (i.e. osteoporosis) or over-production (i.e. osteopetrosis) or both (e.g. Paget’s disease of the bone).9 All these conditions can lead to significant disability due to excessive bone fragility, with fractures that fail to heal adequately. The genesis of the osteoclast–osteoblast unit from the progenitor stem cells Inhibitors,research,lifescience,medical is regulated by local

and hormonal factors with mutual feedback control (Figure 5). Figure 5 Interactions between BMU components. The macrophage colony-stimulating factor (M-CSF), receptor activator of nuclear factor (NF)-kappaB ligand (RANKL), and osteoprotegerin (OPG) are local factors that are secreted by the mesenchymal progenitors about of the osteoblasts. The former two agents positively regulate the osteoclasts’ transformation from their progenitors. The OPG has a negative feedback action on osteoclast formation by the RANKL inactivation. The osteoclastogenetic local and hormonal agents act in parallel by inducing the osteoclast formation directly and by inactivating the OPG. The programmed degradation of the BMU components, apoptosis, is also controlled by the progenitor cell products.