Bone Remodeling and Repair

Bone remodeling occurs for various purposes, including repairing minor damage and regulating blood calcium levels.

During bone remodeling, osteoclasts are connective tissue cells that destroy the old matrix and osteoblasts are the cells that deposit a new one.

Bone remodeling occurs in three distinct steps--bone resorption, reversal, and formation.

When a bone first forms, osteoblasts are buried in the newly formed matrix and transform into osteocytes. Osteocytes sense mechanical stress and signal osteoclasts to the bone site.

Osteoclasts are multinucleated cells that initiate the bone resorption phase. They have finger-like projections containing vesicles that secrete enzymes for the degradation of organic bone components.

The degradation creates shallow depressions called resorption bays and releases calcium into the bloodstream.

In the reversal phase, the osteoclasts self-destruct via apoptosis, and mononuclear phagocytes appear at the degradation site. These cells prepare the degradation site for bone formation.

In the formation phase, the osteoblasts move into the resorption bays and deposit new bone through the ossification of the organic matrix, leading to bone growth.