Which bone cell is the primary mediator of bone resorption?

• A. Osteocytes
• B. Osteoprogenitor cells
• C. Osteoclasts
• D. Osteoblasts

Answer: (C)

Resorption is carried out by osteoclasts, specialized multinucleated cells derived from the monocyte/macrophage lineage. They attach tightly to the bone surface and create a sealed resorption zone where they pump protons to acidify the microenvironment and release proteolytic enzymes like cathepsin K to digest the organic matrix. This combination of mineral dissolution and matrix breakdown is why osteoclasts are the primary mediators of bone resorption. Other bone cells have different roles in remodeling: osteoblasts form new bone matrix, osteoprogenitor cells are their precursors, and osteocytes act as mechanosensors and regulators within the bone. The activity of osteoclasts is regulated by signals from osteoblast-lineage cells (RANKL stimulates them; osteoprotegerin can inhibit them), but the actual resorptive function is carried out by osteoclasts.