Bone is not static: it is continuously torn down and rebuilt by osteoclasts and osteoblasts working in tightly coupled remodeling units, and the net balance of that cycle determines whether bone mass grows or shrinks.
BMU: resorption -> reversal -> formation
net mass change ~ loading * (formation - resorption)
- Matrix packets — discrete units of bone matrix awaiting resorption or newly deposited.
- Remodeling sites — basic multicellular units (BMUs) where osteoclasts and osteoblasts work in coupled sequence.
- Mechanical loading — mechanical strain (Wolff's law) that upregulates osteoblast formation activity.
- Osteoblast/-clast balance — relative activity of bone-forming vs bone-resorbing cells, tipped by hormones like estrogen and PTH.
This coupled resorption-formation cycle is why astronauts and bedridden patients lose bone density rapidly (loading drops) and why weight-bearing exercise is prescribed for osteoporosis.