It is proposed to investigate the roles of two organs, the yolk sac and the kidney in hormonal regulations of calcium and phosphate in the chicken embryo. The kidney functions during embryogenesis but its role in calcium and phosphate regulation during this time is largely unknown. During the first half of incubation the mesonephros develops; however during the latter half of incubation both meso-and metanephros (the definitive adult kidney) are functional. The mesonephros gradually regresses and ceases functioning about day 18. Dr. Clark proposes to measure the glomerular filtration rate (GFR) and clearance of calcium and phosphate in embryos of different ages and in response to the presence or absence of calcium-regulating hormones ?parathyroid hormone (PTH), calcium (CT) or 1,25-dihydroxyvitamin D3!. The yolk sac is an embryonic structure that the first provides calcium and phosphate from the yolk to the embryo; later in development excess calcium derived from the shell is deposited in the yolk. Dr. Clark will study the structure and function of the yolk sac in normal and hormone-treated embryos in regard to uptake and transport of 45Ca and 32p, presence of calcium binding protein, and calcium and phosphate content of the yolk. This work should provide new basic information regarding the function of yolk sac and embryonic kidney, and add much to our understanding of the ontogeny of calcium-regulating hormones and targets in birds.
