|Growth in long bones
Growth in flat bones
Repair after a fracture
During childhood, the long bones (in the arms, legs, and back) grow at the ends of the bones, whereas the flat bones (such as the skull) have a different pattern of growth. Adult bone actually continues to expand, although very slowly. Bone also continually undergoes remodeling, replacing old bone with new bone. Ordinary activity causes microscopic cracks in the bone, and these are dissolved and replaced with new bone. Remodeling also allows bone to respond to changes in mechanical forces. Thus, living bone is totally different from the skeleton in the closet.
Don't miss the "continue" and "start" buttons in these Flash animations.
This section updated 10/1/08.
|The growing skull, mandible (jaw) and clavicle (collar bone) form directly from osteoblasts, and do not have a cartilage stage. A layer of cells which can differentiate into the osteoblasts lines the bone, with a special concentration between the bones of the skull. These precursor cells turn into osteoblasts which secrete collagen in a haphazard pattern called woven bone. Later osteoclasts resorb (dissolve) the woven bone and new osteoblasts form the mature, lamellar (layered) bone.|
BMU means "Basic Multicellular Unit" and also has been called "Bone Remodeling Unit". Remodeling actually happens in 3-D, spreading over an area on the surface of the bone.
To get a better idea of bone turnover, you must imagine many BMUs forming and resorbing, as shown in this animation.
This animation demonstrates repair of a broken bone. The process is similar to intramembranous bone formation, and also resembles formation of a scar.
An adequate supply of blood is essential for fracture healing. Several growth factors, such as BMP (bone morphogenic protein) are involved in this process. |
This drawing is not to scale, in order to illustrate the process.
This section is slightly out of date and will soon be revised (8/4/08)
|The first x-ray was taken right after a fracture. The second was 2 months later, showing some callus. Notice that the leg is now in a cast, so the entire bone looks a little more dense and fuzzy. The last x-ray was 4 months after the fracture, showing a good callus. The bone now can bear weight, but it will take many months to remodel the area and complete the repair.|