Which cell type resorbs bone by creating an acidic microenvironment through proton pumps and carbonic anhydrase activity?

• A. Osteoblasts resorb bone by alkaline secretions.
• B. Osteocytes resorb bone by phagocytosis.
• C. Chondrocytes resorb bone by enzymatic digestion.
• D. Osteoclasts resorb bone by pumping protons and carbonic anhydrase–driven acid release.

Answer: (D)

Bone resorption hinges on how osteoclasts create an acidic microenvironment at the bone surface. These cells pump hydrogen ions into the space between the cell and the bone using proton pumps (H+-ATPases) and generate protons via carbonic anhydrase activity, creating a highly acidic resorption lacuna. That low pH dissolves the mineral component of bone and helps activate enzymes that break down the organic matrix, allowing bone to be resorbed and recycled.

Osteoclasts are the cells specialized for this process. In contrast, osteoblasts build bone and secrete substances that promote mineralization, not acid-mediated resorption. Osteocytes are mature bone cells embedded in the matrix that regulate remodeling signals but do not resorb bone themselves. Chondrocytes are cartilage cells and are not involved in bone resorption.