Synthesis of serum proteins by the posthaematopoietic feline yolk sac.

The feline yolk sac persists even after the end of its haematopoietic phase with prominent ER-cisternae in the endoderm suggesting biosynthetic capacity. Therefore, yolk sac explants from the 54th day and 57th day were incubated with [3H]-L-leucine in order to study its protein biosynthesis. Newly synthesized proteins were discovered in sliced SDS-polyacrylamide gels by the use of scintillation technique and identified by molecular weight determination and isoelectrofocusing, also using stained electropherograms of unlabeled tissue, serum, and marker proteins. The highest radioactive incorporations were found in 69,000--70,000 dalton proteins and interpreted as serum albumin and alpha-fetoprotein. The autoradiography revealed that the cytoplasm of the endoderm is the site of the most active biosynthesis of proteins which were obviously stored in the ER-cisternae for some time. The yolk sac fluid proteins are almost exclusively serum proteins, although in a very low concentration. We regard a large-scale formation of serum proteins in the yolk sac endoderm as the cause of this organ's very late regression in the cat.