Osteoclast
A large cell responsible for breaking down and resorbing bone tissue. Calcitonin-salmon inhibits osteoclast activity, reducing bone resorption. The balance between osteoblast (bone-building) and osteoclast (bone-resorbing) activity determines net bone density.
Technical Context
Osteoclasts are large, multinucleated cells derived from monocyte/macrophage precursors through RANKL-RANK-OPG signalling: osteoblasts express RANKL (receptor activator of NF-κB ligand) which binds RANK on osteoclast precursors → NF-κB and MAPK activation → differentiation and fusion into mature osteoclasts. OPG (osteoprotegerin), a decoy receptor for RANKL produced by osteoblasts, inhibits osteoclastogenesis. Mature osteoclasts resorb bone by: creating a sealed resorption lacuna (ruffled border), secreting HCl (dissolving mineral) and cathepsin K (degrading organic matrix). Calcitonin-salmon directly inhibits osteoclast activity by binding calcitonin receptors on osteoclasts → cAMP elevation → disruption of ruffled border → cessation of bone resorption. This acute anti-resorptive effect explains calcitonin's use in hypercalcaemia emergencies. PTH paradoxically activates osteoclasts (via osteoblast RANKL production) with continuous exposure but predominantly activates osteoblasts with intermittent exposure.