Osteoblasts and osteoclasts
The two opposing cell types whose balance determines whether bone is built or lost.
Osteoblasts build bone. They arise from mesenchymal stem cells and secrete the collagen-rich matrix that subsequently mineralises. Some become entombed in their own matrix and mature into osteocytes — the mechanosensing network that tells the skeleton where load is being carried. Osteoclasts, by contrast, descend from the blood cell lineage and dissolve mineral and matrix.
The two are linked by signalling rather than merely coexisting. Osteoblast-lineage cells produce RANKL, which drives osteoclast formation, and osteoprotegerin, which blocks it. The ratio between them is a principal control point of remodeling — and the target of several established osteoporosis drugs.
Directing stem cells toward the osteoblast lineage is the goal of most bone tissue engineering, which is why osteoinductive signals such as BMP-2 and BMP-7 matter so much. See Stem cell differentiation for how that commitment is controlled.
Related articles
- BoneBone remodelingThe continuous cycle of resorption and formation that rebuilds the skeleton throughout life.
- CellsMesenchymal stem cellsThe most widely studied cells in regenerative medicine — and a reassessment of what they actually do.
- BoneBMP-2: bone morphogenetic protein 2The osteoinductive growth factor that moved from a laboratory curiosity to approved clinical use — and the lessons of its dosing.
- CellsStem cell differentiationHow an uncommitted cell chooses a fate — under chemical, mechanical, and spatial instruction.
- BoneOsteoporosis and bone densityWhat happens when the remodeling cycle tips toward resorption, and why density is only part of the story.
- BoneBMP-7: osteogenic protein-1The other clinically applied bone morphogenetic protein, and what comparing it with BMP-2 reveals.