osteoblast

Osteoblasts are the construction crew of bone. Their job is to build: they secrete the soft protein scaffold of new bone, mostly collagen, and then seed it with mineral so it hardens into the rigid material that supports your body.

These cells arise from stem cells in the marrow and respond to a host of hormones, including PTH, calcitriol, growth hormone, and sex steroids, as well as to mechanical loading from exercise. Crucially, osteoblasts also direct their counterparts: by producing RANKL they switch on osteoclasts, and by producing the decoy osteoprotegerin they switch them off. This lets osteoblasts orchestrate the whole remodeling cycle, not just the building half.

Once an osteoblast finishes laying down bone, it can become trapped in the matrix it made, maturing into an osteocyte, the long-lived sensor cell that detects mechanical strain and signals when repair is needed. Drugs that stimulate osteoblast activity, such as teriparatide and romosozumab, are valued precisely because most osteoporosis treatments only slow bone loss rather than building new bone.