Which cell type resorbs bone?

• A. Osteoclasts
• B. Osteoblasts
• C. Osteocytes
• D. Osteoprogenitors

Answer: (A)

Bone resorption is carried out by osteoclasts, specialized multinucleated cells derived from the monocyte–macrophage lineage. These cells attach to the bone surface and create a sealed resorption compartment. They secrete acid and proteolytic enzymes, such as acid phosphatase and cathepsin K, into this space, dissolving the mineral component and digesting the organic matrix. This resorption clears old or damaged bone so osteoblasts can subsequently lay down new bone. In contrast, osteoblasts build bone by secreting osteoid that becomes mineralized, osteocytes are mature bone cells embedded within the matrix that regulate remodeling, and osteoprogenitors are stem cells that differentiate into osteoblasts.