Sites of Serum α-Fetoprotein Synthesis in the Human and in the Rat*

Selected tissues from human embryos of 6 to 9 weeks' gestation, from rat fetuses of 15 days' gestation, and from rats 2 days of age were incubated with (14)C-labeled amino acids. Immunoelectrophoresis of the culture fluid after incubation, using rabbit antisera against human and rat fetal serum proteins, followed by radioautography revealed that: 1) Radioactive alpha-fetoprotein was present in cultures of human liver, rat liver, and rat yolk sac, but not in cultures of human or rat brain, lung, heart, kidney, intestines, skeletal muscle, skin, or placenta; human yolk sac was not studied. 2) Radioactive transferrin was also present in rat yolk sac cultures, and the same protein was found in rat liver cultures as well. 3) Rat liver and rat placenta cultures both produced radioactive serum Ralpha(2)-globulin. Serum alpha-fetoprotein concentrations in the rat declined abruptly after birth to approximately half of the prenatal level by 2 to 3 days of age, in accord with the loss of the fetal membranes at delivery; the alpha-fetoprotein level then remained relatively constant until the rat was 6 to 8 days of age, after which synthesis of the protein was increasingly suppressed.