Ex utero culture of viviparous embryos of the lizard, Zootoca vivipara, provides insights into calcium homeostasis during development

The chorioallantoic membrane resides adjacent to either the inner surface of the egg shell or uterine epithelium in oviparous and viviparous reptiles, respectively. Chorionic cells face the shell or uterine epithelium and transport calcium to underlying embryonic capillaries. Calcium transport activity of the chorioallantois increases in the final stages of development coincident with rapid embryonic growth and skeletal ossification. We excised embryos from viviparous Zootoca vivipara females at a stage prior to significant calcium accumulation and incubated them ex utero with and without calcium to test the hypothesis that chorioallantois calcium transport activity depends on developmental stage and not calcium availability. We measured calcium uptake by monitoring incubation media calcium content and chorioallantois expression of calbindin-D28K, a marker for transcellular calcium transport. The pattern of calcium flux to the media differed by incubation condition. Eggs in 0 mM calcium exhibited little variation in calcium gain or loss. For eggs in 2 mM calcium, calcium flux to the media was highly variable and was directed inward during the last 3 days of the experiment such that embryos gained calcium. Calbindin-D28K expression increased under both incubation conditions but was significantly higher in embryos incubated with 2 mM calcium. We conclude that embryos respond to calcium availability, yet significant calcium accumulation is developmental stage dependent. These observations suggest the chorioallantois exhibits a degree of functional plasticity that facilitates response to metabolic or environmental fluctuations.