20.4: Bone Remodeling
Even in the adult skeleton, bones are active tissues, continuously undergoing changes in architecture. This process, called bone remodeling, consists of equal phases of bone resorption—its removal—and depositing new tissue.
When mature bone cells—osteocytes—sense mechanical stress, they signal cells to the bone site, for instance, the ends of the femur, which are replaced every six months.
During resorption, one cell type, osteoclasts, clings tightly to the surface. By secreting lysosomal enzymes and hydrogen protons, they can degrade the organic components—creating erosion cavities as they digest the old or damaged matrix. Calcium is also released into the blood and plays a role in hormonal feedback loops.
Once the material is dissolved, the osteoclasts self-destruct via apoptosis, which prevents further destruction of the bone. To initiate reversal, mononuclear cells appear on the surface in preparation for the next step.
In the formation phase, osteoblasts move into the cavity and deposit new bone through the ossification of organic matrix known as osteoid. This part includes collagen fibers that contribute to the structure and flexibility. After matrix synthesis, osteoblasts can differentiate into flattened bone lining cells or become buried in the bone as osteocytes.