When fractures occur, the pain is carried to the brain by nerves running through the periosteum. Cells inside this cartilage grow and begin depositing minerals. The spongy bone forms, and osteoblasts attach and lay down the mineral portions of spongy bone. Osteoclasts remove material from the center of the bone, forming the central cavity of the long bones. The perichondrium , a connective tissue, forms around the cartilage and begins forming compact bone while the above changes are occurring.
Blood vessels form and grow into the perichondrium, transporting stem cells into the interior.
Two bands of cartilage remain as the bone develops, one at each end of the bone. During childhood, this cartilage allows for growth and changes in the shape of bones. Eventually the elongation of the bones stops and the cartilage is all converted into bone. Images from W.H. Freeman and Sinauer Associates, used by permission. Bones continue to change as adults, to adapt to the stresses generated by physical activity.