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.
Increasing calcium intake, reducing protein intake, exercise and low doses of estrogen are effective treatments for osteoporosis. Immovable joints, like those connecting the cranial bones, have edges that tightly interlock. The outer surface of the synovial joints contains ligaments that strengthen joints and hold bones in position. Bursae are small sacs filled with synovial fluid that reduce friction in the joint. The knee joint contains 13 bursae Images from W.H. Freeman and Sinauer Associates, used by permission.