SYNTHESIS, SECRETION AND IMMUNOELECTRON MICROSCOPIC DEMONSTRATION OF APOLIPOPROTEIN-B-CONTAINING LIPOPROTEIN PARTICLES IN THE VISCERAL RAT YOLK-SAC

Electron microscopic investigations on the involvement of the fetal me mbranes of the rat (visceral yolk sac) in the lipid metabolism reveale d the occurrence of lipoprotein-sized particles located in cisternal G olgi stacks, Golgi vesicles and secretory vesicles of the cells of the visceral yolk sac epithelium as well as in distended areas of the int ercellular space between adjacent epithelial cells. Application of the protein A-gold technique with specific anti-apoB antiserum resulted i n a specific location of immunogold both over the different compartmen ts of the lipoprotein pathway (RER, Golgi complex, secretory vesicles) as well as over the distended intercellular spaces, thus confirming t hese particles to be lipoproteins in nature. Isolated visceral epithel ial cells prepared by a tryptic digestion method exhibited some ultras tructural alterations, such as a loss of apical brush border, a change from columnar to spherical cell shape, a decrease in phagolysosomes, but an increase in autophagosomal structures after 6 h incubation at a vitality rate of at least 85%. Within this period the epithelial cell s secreted measurable amounts of apoB-containing lipoproteins into the medium floating in the density classes d < 1.006 g/ml, d = 1.006-1.02 0 g/ml and d = 1.020-1.064 g/ml. The production of the lipoproteins wa s partly inhibited by cycloheximide indicating the secretion of partic les with preformed as well as newly synthesized apoB. Negative stainin g of the particles revealed an average diameter of 34 nm of VLDL, 31 n m of IDL and 24 nm of LDL. In summary, our studies demonstrate that in the feto-placental unit of the rat the fetal membranes are capable of synthesizing and secreting lipoproteins. The cells of the visceral yo lk sac epithelium were shown to be the producers of apoB-containing pa rticles.