Role of vitamin D in the development of the chick embryo.

Chick embryos of various ages and 1-day-old chicks were injected with different doses of 1,25(OH)2D3 and the concentrations of Ca and Pi in their blood were determined 24 hr after. It was confirmed that the dose required to induce hypercalcemia and hypophosphatemia was much lower in the case of 15-day-old embryos than in younger ones. The age of maximal response corresponds to the period when the chorionic epithelium is fully differentiated. Newly hatched chicks did not respond even with doses 20 times higher than those used in 15-day-old embryos. These findings support the idea that the humoral changes produced by 1,25(OH)2D3 result from increased resorption of calcium from the shell and that the chorionic epithelium is instrumental in this phenomenon. The autoradiographic study of tissues from embryos injected with tritiated 1,25(OH)2D3 showed that the exposure time required to detect nuclear concentration of radioactivity was much shorter in the case of "villus-cavity" cells than in any other tissue (kidney, intestine, bone, parathyroid glands) in which target cells were recognized. The above results are interpreted as indicating that the chorionic epithelium is the main target for 1,25(OH)2D3. The histological study of various organs from embryos injected with large doses of 1,25(OH)2D3 showed generalized infiltration of spleen, liver, and bursa of Fabricius with eosinophilic granulocytes. It has been suggested that 1,25(OH)2D3 influences the proliferation and differentiation of leukocytes; the chick embryo might be a good model for further exploration of this problem.