Osteoblast

Osteoblasts are bone-forming cells derived from mesenchymal progenitors under the transcription factors RUNX2 and osterix. They secrete osteoid, an unmineralised matrix of type I collagen with osteocalcin and other non-collagenous proteins, then mineralise it with hydroxyapatite, using alkaline phosphatase to remove the pyrophosphate that would otherwise block crystal growth. Most osteoblasts finish by dying or flattening into quiescent lining cells; a minority become entrapped as osteocytes, the long-lived mechanosensors that sit in lacunae and secrete sclerostin, a brake on further formation.

This biology has yielded real drugs. Teriparatide, the 1-34 fragment of parathyroid hormone approved in 2002, is anabolic when given intermittently even though continuous exposure is catabolic, a difference in signalling pattern rather than in molecule. Romosozumab, an antibody against sclerostin approved in 2019, raises formation and lowers resorption at once, and carries a boxed warning for cardiovascular events. Formation is tracked in blood by P1NP, the propeptide clipped from type I procollagen as it is deposited, and by bone-specific alkaline phosphatase.

The practical distinction is between a marker and an outcome. A rise in P1NP shows matrix is being made; whether it arrives where it is mechanically needed, mineralises properly and reduces fractures is a separate question that only a fracture-endpoint trial answers.

The error follows from that. Compounds are promoted for bone healing on the strength of osteoblast proliferation or alkaline phosphatase activity in cell culture, which is an easy assay to move and a poor predictor. Growth hormone secretagogues in particular are called bone-building on the basis of turnover markers, with no fracture or union endpoint behind the claim.