ASPECTS OF CALCIUM REGULATION IN EMBRYONIC LEPIDOSAURIANS AND CHELONIANS AND A REVIEW OF CALCIUM REGULATION IN EMBRYONIC ARCHOSAURIANS

Embryos of oviparous and viviparous lepidosaurians, chelonians, and ar chosaurians must maintain calcium homeostasis while mobilizing large q uantities of calcium from the yolk eggshell, or maternal circulation T he same calcium-regulating hormones that control the calcium status of adults are assumed to control the calcium status of embryos as well, but the target organs for calcium regulation may be different in embry os and adults. The yolk sac, which mediates the uptake of calcium from the yolk (and the deposition of calcium into yolk in archosaurians), the chorioallantois, which mediates the release of calcium from the eg gshell and the transport of shell calcium to the vasculature in ovipar ous species, and the chorioallantoic placenta, which mediates the mate rnal-fetal transfer of calcium in viviparous species, are potential ta rgets for calcium-regulating hormones (calcitonin, parathyroid hormone , calcitriol) during embryogenesis. However, the role of these hormone s in mediating the calcium status of embryos has been examined only in avian species, and the most detailed information available in this re gard is for calcitriol (the vitamin D hormone). The presence of recept ors for calcitriol in the yolk sac and chorioallantoic membranes of av ian species and the extreme calcium deficiency of vitamin-D-deficient embryos indicate that calcitriol plays an important role in regulating calcium metabolism during avian embryogenesis. A source of Parathyroi d hormone also is required for normal development but the target organ (s) for this hormone have not been identified The role of calcitonin i n calcium regulation by embryonic birds is poorly understood, and the potential for hormonal control of calcium homeostasis in embryonic rep tiles has not been examined.