Two kinds of bone cells and their roles.

Two kinds of bone cells and their roles.

• A. Osteoblasts—build up bones; Osteoclasts—break down bones
• B. Osteocytes—store calcium; Osteoblasts—break down bones
• C. Osteoclasts—build bones; Osteoblasts—resorb bone
• D. Chondrocytes—convert cartilage to bone; Osteoblasts—deposit minerals

Answer: (A)

Bone remodeling relies on two main players with opposite actions: osteoblasts and osteoclasts. Osteoblasts are the builders—they synthesize the bone matrix (osteoid) and promote mineralization to form new bone. They originate from mesenchymal stem cells and, as they work, some become embedded in the matrix as osteocytes or become lining cells on bone surfaces. Osteoclasts are the breakers—they resorb bone by attaching to the bone surface, creating an acidic environment and releasing enzymes that dissolve mineral and organic matrix. This resorption clears old or damaged bone so osteoblasts can replace it with fresh bone, helping regulate calcium levels in the blood. The other options mix up these roles. Osteocytes are mature osteoblasts embedded in bone and serve more in maintenance and signaling rather than storing calcium directly. The idea that osteoclasts build and osteoblasts resorb is opposite of their true functions. Chondrocytes form cartilage, not bone, and while cartilage can be a template in development, the ongoing resident bone cells responsible for remodeling are osteoblasts and osteoclasts. The statement that osteoblasts build up bones and osteoclasts break down bones best captures the essential two-sided process of bone turnover.

Bone remodeling relies on two main players with opposite actions: osteoblasts and osteoclasts. Osteoblasts are the builders—they synthesize the bone matrix (osteoid) and promote mineralization to form new bone. They originate from mesenchymal stem cells and, as they work, some become embedded in the matrix as osteocytes or become lining cells on bone surfaces. Osteoclasts are the breakers—they resorb bone by attaching to the bone surface, creating an acidic environment and releasing enzymes that dissolve mineral and organic matrix. This resorption clears old or damaged bone so osteoblasts can replace it with fresh bone, helping regulate calcium levels in the blood.

The other options mix up these roles. Osteocytes are mature osteoblasts embedded in bone and serve more in maintenance and signaling rather than storing calcium directly. The idea that osteoclasts build and osteoblasts resorb is opposite of their true functions. Chondrocytes form cartilage, not bone, and while cartilage can be a template in development, the ongoing resident bone cells responsible for remodeling are osteoblasts and osteoclasts. The statement that osteoblasts build up bones and osteoclasts break down bones best captures the essential two-sided process of bone turnover.