/. Embryol. exp. Morph. 92, 33-41 (1986) 33 Printed in Great Britain © The Company of Biologists Limited 1986 Albumin and transferrin synthesis in whole rat embryo cultures CAROLYN L. WILLIAMS, PAUL K. PRISCOTT, I. T. OLIVER AND GEORGE C. T. YEOH The Raine Centre for the Study of Developmental and Perinatal Biology, Departments of Biochemistry and Physiology, University of Western Australia, Nedlands, Western Australia 6009 SUMMARY The uptake of [ 3 H]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, [ 3 H]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 2h 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. INTRODUCTION At about 10-5 days gestation in the rat, the liver diverticulum emerges from the embryonic gut. The rat liver diverticulum in organ culture, will, after 3 days, release plasma proteins into the medium (Parsa & Flancbaum, 1975). This suggests that the ability to synthesize plasma proteins is a property which is acquired by the liver at a very early stage in its development. It is of interest to determine when plasma-protein-synthesizing cells emerge from the primitive gut and to learn whether the ability to synthesize the different plasma proteins such as albumin and transferrin is acquired collectively or sequentially. Furthermore, it is of interest to determine whether extrahepatic sites for plasma protein synthesis exist and to assess the relative importance of these sites during embryonic development. In a previous study using minced material from rat embryos and membranes, Yeoh & Morgan (1974) reported that incorporation of [ 14 C]leucine into transferrin could be detected in foetal membranes but not the foetus at 13 days gestation. Albumin synthesis was present in preparations from both tissues. These results Key words: rat embryo, albumin, transferrin, protein.