Variation during development in the response of chicken embryos to calcitriol administered via slow-release pellets

Embryonic chickens were exposed to 0, 30, or 300 pg of calcitriol per day via slow-release pellets implanted adjacent to the chorioallantoic membrane. Pellets were placed in eggs on Days 10 and 15, and eggs were sampled on Days 12 and 17, respectively. The hormone induced high mortality among embryos receiving pellets on Day 10, but not among those whose treatment was begun on Day 15. Embryos receiving hormone were hypercalcemic and hypophosphatemic on both Day 12 and Day 17, but the concentration of magnesium in plasma was not affected. Size of embryos sampled on Day 12 was not affected by hormone treatment, but embryos sampled on Day 17 showed a dose-related reduction in size. Yolk-free carcasses of the embryos sampled on Day 17 also showed dose-dependent reductions in phosphorus and magnesium, but calcium content of carcasses on Day 17 was unaffected by treatment. These results indicate that both younger and older embryos respond to very small quantities of calcitriol administered via slow-release pellets. The absence of sustained hypercalcemia in earlier studies that used this protocol to dispense hormone was not caused by exposing older embryos to subthreshold quantities of calcitriol. The fact that reduced body size in late embryos receiving calcitriol was not accompanied by a reduction also in calcium content may mean that embryos deposit calcium in the carcass in an effort to deal with the extreme hypercalcemia induced by calcitriol.