What are osteoclasts?

• A. Large cells that enzymatically dissolve bone matrix and are commonly found at sites of active bone remodeling
• B. Cells that lay down bone matrix
• C. Mature bone cells embedded in mineralized matrix
• D. Cells that resorb cartilage

Answer: (A)

Osteoclasts are the bone-resorbing cells. They are large, multinucleated cells derived from the monocyte–macrophage lineage that attach to mineralized bone surfaces and create a sealed, acidic microenvironment with a ruffled border. By pumping protons and releasing proteolytic enzymes like cathepsin K, they enzymatically dissolve the inorganic bone matrix and organic components, forming resorption bays called Howship’s lacunae. This resorption activity is a key part of bone remodeling, occurring at sites where osteoclasts work in concert with osteoblasts to resorb old bone and then replace it with new bone. In contrast, cells that lay down bone matrix are osteoblasts; mature bone cells embedded in mineralized matrix are osteocytes; and cartilage resorption is mediated by chondroclasts, not the typical bone-resorbing osteoclasts.