Figure 3. Mechanism of osteoclastic bone resorption.

The osteoclast adheres to bone via binding of RGD-containing proteins (green triangle) to the integrin α_vβ₃, initiating signals that lead to insertion into the plasma membrane of lysosomal vesicles that contain cathepsin K (Ctsk). Consequently, the cells generate a ruffled border above the resorption lacuna, into which is secreted hydrochloric acid and acidic proteases such as cathepsin K. The acid is generated by the combined actions of a vacuolar H⁺ ATPase (red arrow), its coupled Cl^– channel (pink box), and a basolateral chloride–bicarbonate exchanger. Carbonic anhydrase converts CO₂ and H₂O into H⁺ and HCO₃^–. Solubilized mineral components are released when the cell migrates; organic degradation products are partially released similarly and partially transcytosed to the basolateral surface for release.