Growth and Development
Most of the skeleton is cartilaginous during prenatal development. The cartilage structures are shaped like the future bones and therefore provide "models" of the bones. Cartilaginous models are converted to bone when mineral salts are deposited in the matrix-first, by certain of the cartilage cells, and later, by bone-forming cells called osteoblasts (os'te-o-b lasts). Conversion of cartilaginous models to bones is called endochondral ossification. Another type of ossification, called intramembranous ossification, occurs without a previous cartilaginous model. Facial bones and certain cranial bones form in this way.
In the process of endochondral ossification in a long bone, at first, a primary ossification center is located in the middle of the bone. Later, a medullary cavity is surrounded by compact bone, and secondary ossification centers form in the epiphyses. A cartilaginous disk, called the epiphyseal (ep"i'-fiz'e-al) disk, remains in the epiphyses. Bone length is dependent on how long the cartilage cells within the disks continue to divide. Eventually, though, the disks disappear, and the bone stops growing as the individual attains adult height.
In the adult, bone is continually being broken down and built up again. After bone-absorbing cells, called osteoclasts (os'te-o-klasts"), break down bone, they remove worn cells and deposit calcium in the blood. Apparently, after a period of about three weeks, the osteoclasts disappear. Then, destruction caused by the osteoclasts is repaired by osteoblasts. As they form new bone, the osteoblasts take calcium from the blood. Eventually, some of these cells get caught in the matrix they secrete and are converted to osteocytes. Osteocytes are mature osteoblasts.
Thus, through a process of remodeling, old bone tissue is replaced with new bone tissue. Because of continual remodeling, the thicknesses of bones can change according to the amount of physical use or due to a change in certain hormonal balances. Strange as it may seem, adults seem to require more calcium in the diet than do children in order to promote the work of osteoblasts.
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