Harmones influence these processes. The parathyroid hormone in conjunction ,vith the vitamin D hor:25-(OH)2D, controls the resorption of calcium e. and a thyroid hormone-calcitonin-inhibits withdrawal from bone.
Bone, like other tissue, is in a state of dynamic equilibrium with the constituents of the plasma and other tissues. Calcium and phosphorus, when the food supply is abundant, can be stored in the trabeculae at the ends of bone. From this storehouse these minerals are readily avilable to meet the needs of other tissues of the body when the dietary intake of calcium or phosphorus is inadequate. However, if calcium has not been stored in the trabecule, calcium is withdrawn from the bone structure itself. Prolonged removal of calcium from bone naturally results in bones that are more easily bent or broken. when phosphate is removed from bone, the remaining tissue as flexible as cartilage; in fact, it is essentially the sme as cartilage. Cartilage precedes bone in the development of the fetus and the young animal, and normally the phosphate is deposited in it as growth and strain The amount of calcium needed by the bones depends on the rate of skeletal development. The calcium content of the body increases from approximately 28 g at to about 1200 g at maturity. This is an average in body calcium of about 165 mg per day. However, variation of calcium deposition throughout the growth period is extremely great, ,vith maximum needs of 300 mg to 400 mg per day occurring in conjunction with the growth spurt in parly adolescence. When nature's plan is thwarted by an inadequate supply of these minerals in food or by the body's inability to use them, growth may be retarded, or, as more often happens in young children, growth in size continues, but the new bone is abnormal in structure and poorly calcified. This may result in the bowed legs, enlarged ankles and wrists, prolapsed thorax, and other bone deformities characteristic of rickets. Tooth structures, particularly dentin and enamel, are metabolically more stable than the bones. The calcium phosphate in the teeth occurs in the same form, hydroxyapatite crystals, as in the bones. The protein matrix in the enamel is kera tin Clnd in the dentin collagen. There is little turnover of calcium in the teeth.
The deciduous teeth begin to calcify in the fetus around the 20th week of pregnancy and continue almost until they erupt into the mouth. Calcification of the permanent teeth may commence anywhere from 3 months to 3 years. Wisdom teeth begin to erupt at around 10 years of age. Once formed, teeth do not require additional calcium because they cannot repair themselves after they have erupted.
However, poor tooth structure, reflected by increased susceptibility to dental caries, mav be the result of inadequate calcium intake during the period of tooth formation.
