When the skeletal system is formed most of the bones are not actually bones, they begin as cartilage which slowly ossifies into hard bone with the help of osteoblasts, osteoclasts and the deposit of minerals such as calcium. For example, when a baby is born their skeleton is not completely ossified, all their bones are still slightly 'soft' especially the fontanels of the skull.
The bones grow by the laying down of cartilage in the basic mold of what the bone will be, then osteoblasts continuously form bone cells. The osteoclasts basically mold and sculpt the bone into correct shape by reabsorbing cells, which not only aids in the initial formation of bone formation, but also allows the bones to respond to constaint stress and injury by changing shape, size and density. When bones are broken osteoblasts and osteoclasts aid in the reformation of bone to reconnect the fracture.
Explanation of Diagram:
A. Bone formation begins with cartilage model.
B. & C. Invasion of the diaphysis by blood vessels and the combined action of osteoblast and osteoclast cells result in cavit formation, calcification, and the appearance of bone tissue.
D. & E. Centers of ossification also appear in the epiphyses of the bone
F. The epiphyseal plate is visible, indications that this bone is not yet mature and has additional growth possible.
G. In a mature bone the epiphyseal line replaces the plate where the cartilage has disapeared and the centers of ossification have fused together.