Intracellular localization of basement membrane precursors in the endodermal cells of the rat parietal yolk sac. III. Immunostaining for laminin and its precursors.

Reichert's membrane and the endodermal cells of the parietal yolk sac were examined for the presence of laminin antigenicity using anti-laminin antibodies and the peroxidase-antiperoxidase sequence. Immunostaining was observed through the full width of Reichert's membrane and within endodermal cells. In these cells immunostaining was observed in rough endoplasmic reticulum (rER) cisternae and Golgi apparatus. The Golgi staining could occur in any saccule, but predominated in components interpreted as the last saccule of the stack, the GERL element, and associated prosecretory granules. The secretory granules found in the ectoplasm were also immunostained. Finally, multivesicular bodies showed some staining. The immunostaining of Reichert's membrane indicates the presence of laminin itself, while that of rER cisternae and the Golgi apparatus is attributed to laminin precursors. Presumably the biosynthesis of laminin occurs along the usual protein pathway, that is, from rER through Golgi saccules and the GERL element to secretory granules, which release their content into Reichert's membrane. The laminin immunostaining of Reichert's membrane and endodermal cells is similar to that of type IV collagen. It is, therefore, likely that the two substances are processed and secreted simultaneously.