Which molecule is expressed by osteoblasts to activate osteoclasts, facilitating bone resorption?

• A. Osteoprotegerin
• B. RANKL
• C. M-CSF
• D. Osteocalcin

Answer: (B)

Bone remodeling hinges on signals from osteoblasts that control osteoclasts. The key molecule osteoblasts express to activate osteoclasts is RANKL. When RANKL sits on the surface of osteoblasts and binds to its receptor RANK on osteoclast precursors (and on active osteoclasts), it promotes their differentiation, activation, and survival, driving bone resorption. This direct activation is the main way osteoblasts regulate osteoclasts during remodeling.

There’s also a balancing factor: osteoprotegerin (OPG) acts as a decoy receptor that binds RANKL, preventing it from activating RANK. This dampens osteoclast formation and activity, tipping the scale toward bone formation when OPG is higher. Another molecule, M-CSF, is important for the early development and survival of osteoclast precursors, supporting their formation, but the primary signal from osteoblasts that triggers osteoclast activation is RANKL. Osteocalcin, on the other hand, is a marker produced by osteoblasts related to bone formation, not a driver of osteoclast activation.