Osteoblast

Osteoblasts are the bone-building cells. They secrete the collagen matrix that mineral crystallises onto, and some become embedded in the bone they make, maturing into osteocytes that sense mechanical strain and signal for further remodelling.

It matters because osteoblast activity is what mechanical loading recruits. Strain on bone — from heavy lifting, jumping, running — is detected by osteocytes, which signal osteoblasts to reinforce the loaded region. That is the cellular version of 'bone adapts to stress'.

In practice bone remodels far more slowly than muscle, on a timescale of many months, so density changes are measured over years rather than training blocks. The consolation is that the adaptation, once made, is durable.