Bone is comprised of a mineral and protein scaffold filled with bone cells. This structure is continually broken down and renewed. When the rate of bone loss outpaces the rate of replacement, bones weaken, eventually leading to a condition known as osteoporosis. Many factors can contribute to osteoporosis, including aging, certain medications, and hormonal changes.
Osteoblasts, the cells that build bone, are derived from mesenchymal stem cells in the bone marrow. These skeletal stem cells can also give rise to other types of cells, including fat cells. The bone marrow of older adults has fewer bone-building osteoblasts and more fat cells than that of younger people. The mechanisms responsible for these changes, however, are unknown.