Avian Parathyroid Physiology: Including a Special Comment on Calcitonin

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Abstract
During the reproductive cycle, the females of avian species metabolize large amounts of calcium, deposit large amounts of apatite in intramedullary bone, and develop hyperplastic and hypertrophied parathyroid glands. Secondary hyperparathyroidism develops when the amount of calcium in the diet is low. Hyperparathyroidism, either endogenous or from injections of parathyroid extract, produces resorption of endosteal bone and not mobilization of the deposits of intramedullary bone that normally store calcium for calcification of the eggshell.The specificity of the site of action of the hormone suggests that the process of mobilization of mineral from intramedullary bone and fluctuations in the level of calcium in the blood are attributable not to the action of the parathyroid hormone but indirectly to cyclical changes in the output of estrogen. Methods of measurement of calcium kinetics, parathyroid hormone-induced metabolic processes, and cellular reactions of bone cells to calcitonin should be investigated in vitro in explants of intramedullary bone to learn more about the specialized physiological characteristics of the tissue.

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