Biosynthesis of basement membrane by parietal yolk sac cells.

The biosynthesis of basement membrane (Reichert's membrane) by parietal yolk sac cells of the mouse embryo was studied using immunohistochemistry and autoradiography with the electron microscope. Fab' fragments of characterized antisera to either a mixture of type IV collagen and laminin, or to type IV collagen alone, were conjugated to horseradish peroxidase for the immunohistochemical study; radiolabeled proline, leucine, and glucosamine were utilized either in vivo or in isolated yolk sac membranes in vitro for autoradiography. Whereas neither of the two antisera employed localized in either the Golgi apparatus or in the cytosol, both localized in the rough endoplasmic reticulum (RER) and in the extracellular basement membrane. Furthermore, no evidence of passage of isotopically labeled precursors of basement membrane from the RER to the Golgi apparatus was observed. Consequently, it is concluded that the basement membrane precursors are synthesized in the RER of parietal yolk sac cells and neither traverse the Golgi apparatus nor diffuse across the cytosol of the cell in the process of secretion. By elimination, secretion must be via direct communications between RER and plasma membrane or via small vesicles observed in the cytosol between RER and plasma membrane. These data are not in accord with the concept of the universality of the Golgi apparatus in the biosynthesis of glycoprotein. Parietal yolk sac cells do not use the Golgi apparatus in the biosynthesis of basement membrane, but do use the Golgi apparatus for the biosynthesis of other proteins.