Albumin and transferrin synthesis in whole rat embryo cultures

The uptake of [3H]leucine by the rat yolk sac and embryo and the subsequent synthesis of albumin and transferrin have been studied in whole embryo culture. Rat embryos of 12 days gestation were used in all experiments. Isotopically labelled transferrin was detectable in yolk-sac and embryo tissue extracts. In contrast, [3H]albumin could not be found in either tissue extract. Levels of radioactive transferrin in the yolk sac of cultured whole conceptuses decreased during 12 h in cold media. Embryonic transferrin showed an opposite trend in that it increased over 12 h by nearly 30-fold. In view of these results experiments were conducted in embryos and yolk sacs cultured in separate bottles. Radioimmunoprecipitation for transferrin revealed that there was synthesized protein in the yolk sac which then decreased by approximately 30% after 2 h in normal cultured medium. There was no evidence of transferrin synthesis in embryo extracts over a 12 h period. These results present evidence that the visceral yolk sac is the primary site of transferrin synthesis in the rat and that the protein is thereafter transported, intact, to the embryo.