Calcium metabolism and its control--a review.

It is now established that avians can only utilize the cholecalciferol form of vitamin D, which must be converted to the hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to perform normal calcium metabolism. Although 1,25(OH)2D3 is the final active form of vitamin D, hens fed only this form of vitamin D do not have normal hatchability of eggs. The problem appears to be caused by abnormal calcification and development of the embryonic beak. This appears to be caused by inadequate transport of 1,25(OH)2D3 into the egg. Although 1,25(OH)2D3 is not incorporated into the egg adequately, its precursor, 25-hydroxyvitamin D3 (25-OH-D3), is. The developing embryo however, can utilize 1,25(OH)2D and does so at least as early as Day 10 of incubation. During periods of maximal shell calcification and high circulating estradiol levels, the hen produces high levels of 1,25(OH)2D3. The kidney hydroxylase responsible for the final hydroxylation of the vitamin D hormone can be further stimulated by in vivo or in vitro administration of estradiol and, to a lesser extent, prolactin and parathyroid hormone. When eggs are not produced, as in the senescent or prepubertal stages of life, plasma 1,25(OH)2D3 concentrations are less than half that occurring during periods of active lay. Hens selected for their ability to produce thin or thick shells have 1,25(OH)2D3 concentrations in plasma that are positively correlated to their ability to produce egg shell.