Medullary bone formation and resorption in avian calcium metabolism
2005
Sugiyama, T.(Niigata Univ. (Japan). Faculty of Agriculture)
Avian calcium metabolism in an egg-laying period is extraordinary when compared with all other classes of vertebrates, because hens lay an egg with a hard-eggshell that consists of calcium carbonate. Medullary bone is a specific tissue to egg-laying hens and reticularly developed in marrow cavities of long bones, such as femurs and tibiae. The onset of medullary bone formation requires the combined influence of both estrogen and androgen, coinciding with sexual maturation. On the medullary bone surface, osteoblastic bone formation and osteoclastic bone resorption alternatively occur in relation to the position of the egg in the oviduct (an egg-laying cycle) and consequently medullary bone provides a ready supply of calcium to form eggshells. The cyclic sequences of osteoblasts and osteoclasts are regulated by calcium-regulating hormones such as estrogen, parathyroid hormone, and calcitonin. There is controversy about whether calcitonin regulates avian physiology. Therefore, the present article reviewed the calcitonin structure and secretion, the effects of calcitonin on calcium metabolism, and the calcitonin receptors, and simultaneously discussed the future research about the physiological role of calcitonin in avian calcium metabolism.
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