Production of 1,25-Dihydroxyvitamin D3and Formation of Medullary Bone in the Egg-Laying Hen*

Renal 25-hydroxyvitamin D-1-hydroxylase, the percentage of medullary bone in the femur, plasma calcium, and plasma phosphorus were measured in female chickens reaching maturity. These parameters and plasma 1,25-dihydroxyvitamin D levels were also measured during the daily egg-laying cycle in mature hens. The renal 25-hydroxyvitamin D3-l-hydroxylase becomes elevated in maturing hens before and at the time of ovulation. This elevation in the 1-hydroxylase correlates with the elevation in total plasma calcium concentration but lags and at the time of ovulation and is followed by a further elevation of plasma 1,25-dihydroxyvitamin D levels. The plasma 1,25-dihydroxyvitamin D level remains high until 12 h postovulation. At this time, it falls to the preovulation level. No relationship could be found between the plasma 1,25-dihydroxyvitamin D levels and the changes in medullary bone found during the egg-laying cycle. However, plasma 1,25-dihydroxyvitamin D levels are highest immediately before and during the egg shell calcification phase of the egg-laying cycle.