Carole Crimminger, age 12, of Charlotte, N.C., for her question:
HOW DO BONES GROW?
Probably the most common bone disorder is one that is brought on by old age. It is called senile osteoporosis, and it means that the bones have become brittle and porous. An old person's broken bone is slower to heal than a young person's. With the young, the bones contain a smaller amount of mineral matter than do adults and so are not easily broken or injured by falls.
If you were to break your arm it would be called a fracture. A fracture heals by the bones forming what is called a callus. This is a mass of new bone which grows after the break has been properly set by a doctor.
Your break will be held firmly in its correct place by a cast and then it will knit, or grow together, in four to 52 weeks, depending upon your age, the size of the broken bone and the location of fracture.
There are 206 bones in your body, not counting the many tiny ones embedded in the tendons over the bones of the knees, hands, feet and parts of the skull.
Bones are formed long before birth. The skull is formed by intramembranous bone formation, meaning that soft connective tissue changes directly into hard bone tissue. Long bones in the body develop by intracartilaginous bone formation. With this process, the soft connective tissue grows first into cartilage which later is absorbed and replaced by bone.
Long bones grow by means of a structure called the epiphyseal cartilage. The epiphyseal cartilage is a plate of soft tissue between the end of the bone and the knob¬like section next to it. In young people, the epiphyseal cartilage grows in thickness throughout the period of growth. Each new layer of cartilage is successively absorbed by a layer of new bone, by the process of intracartilaginous bone formation. In this way, the epiphyseal cartilage and the epiphysis grow away from the central portion of the bone and the shaft gradually grows longer.
Bones are among the most active tissues in your body. They store useful materials such as calcium, fluoride, phosphorus and sodium and also contain red bone marrow where new red blood cells are made.
If you don't have a sufficient supply of vitamin D in your body, it is possible that your bones will soften. This condition in children is called rickets.
About two thirds of the weight of the bony tissue in your body is mineral matter, chiefly calcium and phosphorus. The rest is organic matter consisting largely of a fibrous protein.