Uptake and distribution of exogenous serum proteins in the cultured rat embryo

Head fold stage rat embryos were cultured on a medium consisting of adult rat serum for 48 hr. During the final 10 or 20 hr of culture [35S]-methionine was added to the medium. Sera from visceral yolk sac blood vessels, exocoelomic cavity fluid, and amniotic sac fluid were analyzed by SDS-polyacrylamide gel electrophoresis followed by Amido Black staining and fluorography. Amido Black staining showed bands in all three embryonic fluid compartments which migrated at rates comparable to transferrin, alpha-fetoprotein, albumin, IgG, and hemoglobin. Fluorography, however, showed that radioactivity was restricted to alpha-fetoprotein, IgG, and hemoglobin. This suggested that at least some transferrin and albumin were transferred intact from the exogenous medium to the embryonic fluid compartments.