Osteoclast

Osteoclasts are multinucleated cells formed by fusion of monocyte-macrophage lineage precursors under two signals: macrophage colony-stimulating factor and RANK ligand. Osteoblast-lineage cells supply both, and they also secrete osteoprotegerin, a decoy receptor that soaks up RANK ligand, so the RANKL to osteoprotegerin ratio is the switch that sets resorption. An activated osteoclast clamps to bone through a sealing zone, folds its membrane into a ruffled border, and pumps protons into the enclosed space to dissolve hydroxyapatite; cathepsin K then digests the exposed collagen, leaving a resorption pit.

The pathway is heavily drugged. Bisphosphonates bind mineral, are taken up during resorption and disable the osteoclast from within. Denosumab, a monoclonal antibody against RANK ligand, was approved in 2010 and works entirely outside the bone matrix, which is why its effect reverses on stopping: discontinuation without follow-on therapy produces a rebound in turnover and a documented risk of multiple vertebral fractures. Odanacatib, a cathepsin K inhibitor, reduced fractures in a large phase 3 programme and was still abandoned in 2016 over an excess of stroke.

What these examples teach is coupling. Resorption and formation are yoked, so suppressing osteoclasts also suppresses osteoblasts over time, and bone that is never resorbed is never replaced. That is the mechanism behind the rare complications of long-term suppression, atypical femoral fracture and osteonecrosis of the jaw, and behind the practice of drug holidays.

The misreading is to treat a fall in a resorption marker such as CTX, or a rise in bone mineral density, as equivalent to a stronger skeleton. Density is a surrogate; the drugs above are trusted because they reduced fractures in randomised trials, not because they moved a scan.