Cellular and molecular mechanisms of osteoporosis.

Osteoblasts and osteoclasts are derived from progenitors originating in the bone marrow, and the process of bone remodeling is controlled by growth factors and cytokines which regulate the birth and death of these cells. An overproduction of osteoclasts relative to the need for remodeling, and an undersupply of osteoblasts relative to the need for cavity repair, represent the fundamental pathophysiologic changes in postmenopausal and age-related osteopenia, respectively. As in these two forms of the disease, the osteoporosis induced by glucocorticoid excess is also caused by changes in the birth and death of bone cells, and in particular a decrease in osteoblastogenesis in the bone marrow, and an increased rate of osteoblast and osteocyte apoptosis.