Characterization of embryonic antigens in the plasma of developing chick embryos.

Two prominent alpha-fetoproteins are found in chick embryo plasma up to and slightly beyond hatching. Only antisera against chick embryo plasma resolve them. alpha3-fetoprotein is a lipoprotein, the first fetoprotein thus described. It probably functions to transport lipid from the yolk to the embryo. The other fetoprotein (alpha4) is a major glycoprotein. Both are made by the liver and yolk sac. From 9 days onward, albumin is the principal export protein of the liver; at 7 days, when it first appears in the plasma, the yolk sac, but not the liver, makes it. The reverse situation occurs at 9 days. These facts suggest a humoral relationship between the yolk sac and liver. The embryo synthesizes every plasma protein including fetoproteins and "adult" proteins (prealbumin, albumin, and transferrin). No "cold" or unlabeled proteins are seen.