Osteoblast

Osteoblasts are the cells that build bone. They secrete the collagen rich matrix that gives bone its framework and then mineralise it with calcium phosphate, which is what makes bone both stiff and resistant to shattering.

They work in partnership with osteoclasts, which remove old bone, in a continuous remodelling cycle. Resorption comes first, formation follows, and the balance between the two over years is what your bone density scan is measuring.

Some osteoblasts become trapped in the matrix they have laid down and mature into osteocytes, the cells embedded throughout bone that sense mechanical strain. Those cells are the sensors that tell the system whether the skeleton is being asked to do anything, which is the physical basis of load driven bone adaptation.

That sensing responds to the size and rate of strain rather than to duration. High magnitude, rapidly applied loads, meaning heavy resistance work and impact, drive formation; long slow low load activity such as walking or swimming does many good things and does not reliably change density.

Alkaline phosphatase is an osteoblast product, which is why the enzyme rises in growing children, during fracture healing and in conditions with high bone turnover, and why a raised level is read alongside liver markers to work out where it came from.

Adequate calcium, vitamin D and protein are the permissive inputs. They allow osteoblasts to work rather than instructing them to, which is why correcting a deficiency helps and exceeding sufficiency does not.

In practice, the way to recruit osteoblasts is to give the skeleton something demanding to respond to, progressively and repeatedly, and to keep doing it as you age.