Abstract
The yolk is an important calcium source for the developing chick embryo. The epithelial yolk sac endodermal cells lie in direct contact with the yolk and are the principal nutrient-transporting cell type. We previously reported that vitamin D treatment stimulated yolk calcium mobilization and that the vitamin D-dependent Ca2+-binding protein, calbindin-D28K, is present in the yolk sac. We report here the developmental expression and regulation of calbindin-D28K in the yolk sac. Calbindin-D28K is expressed as early as incubation d 3 and is found exclusively within the cytoplasm of endodermal cells. Comparative protein and mRNA analyses of yolk sac and dissociated yolk sac endodermal cells as a function of development and treatment with calcitriol (1,25-dihydroxyvitamin D3) in vitro and in vivo showed a development-specific and vitamin D-inducible expression of calbindin-D28K. Northern analysis revealed the expression of vitamin D receptor mRNA in the yolk sac, beginning as early as d 3, strongly indicating that the extraembryonic yolk sac is an early vitamin D target tissue. Cultured yolk sac endodermal cells should serve as a useful in vitro cell model for analyzing the cellular and molecular mechanisms of vitamin D action.
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