Most of the bones of a developing embryo originate as cartilage “models”. As the fetus grows, each model’s matrix hardens with calcium salts, cutting off diffusion to the cartilage cells. The cartilage matrix therefore degenerates, and capillaries penetrate the degenerating areas.
Bone cells migrate in via these new blood vessels and secrete bone matrix. These cells eventually become mature osteocytes. After birth, bone growth becomes concentrated near the ends of the long bones in thin disks of cartilage (“growth plates”).
The bones continue to elongate until the late teens, when bone tissue begins to replace the cartilage growth plates. By the early twenties, bone growth is complete. Even after a person stops growing, bone is continually being remodeled. Bones become thicker and stronger with strenuous exercise such as weight lifting. On the other hand, less-used bones lose mass, thanks to cells that slowly degrade the mineral matrix at the bone surface. For example, astronauts lose bone density if they are in a prolonged weightless environment because their bodies don’t have to work as hard as they do against Earth’s gravity.