Aspects of Calcium Regulation in Embryonic Lepidosaurians and Chelonians and a Review of Calcium Regulation in Embryonic Archosaurians

Abstract

Embryos of oviparous and viviparous lepidosaurians, chelonians, and archosaurians must maintain calcium homeostasis while mobilizing large quantities of calcium from the yolk, eggshell, or maternal circulation. The 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 embryos 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 eggshell and the transport of shell calcium to the vasculature in oviparous species, and the chorioallantoic placenta, which mediates the maternal-fetal transfer of calcium in viviparous species, are potential targets for calcium-regulating hormones (calcitonin, parathyroid hormone, calcitriol) during embryogenesis. However, the role of these hormones in mediating the calcium status of embryos has been examined only in avian species, and the most detailed information available in this regard is for calcitriol (the vitamin D hormone). The presence of receptors for calcitriol in the yolk sac and chorioallantoic membranes of avian 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 parathyroid hormone also is required for normal development, but the target organ(s)for this hormone have not been identified. The role of calcitonin in calcium regulation by embryonic birds is poorly understood, and the potential for hormonal control of calcium homeostasis in embryonic reptiles has not been examined.