Intracellular localization of basement membrane precursors in the endodermal cells of the rat parietal yolk sac. II. Immunostaining for type IV collagen and its precursors.

Antibodies to type IV collagen were linked with peroxidase and used for direct immunostaining of Reichert's membrane and the associated cells of the rat parietal yolk sac. Immunostaining was observed throughout the thickness of Reichert's membrane and within the endodermal cells arranged as a single layer on its inner side. The immunostaining of endodermal cells was mainly present in the cisternae of rough endoplasmic reticulum (rER) and in the Golgi apparatus, where it could occur in any saccule, but predominated in the GERL elements and associated prosecretory granule-like structures. Moreover, the secretory granule-like structures present in the ectoplasm next to Reichert's membrane were also immunostained. Finally, immunostaining was observed in multivesicular bodies and occasionally in secondary lysosomes. The antigenicity detected by immunostaining in Reichert's membrane is attributed to type IV collagen itself, whereas the antigenicity of endodermal cells is assigned to precursors of this collagen. It is proposed that initial precursors arise in rER cisternae, migrate to Golgi saccules, and pass to the GERL element, where they accumulate into prosecretory granules, which, perhaps by fusion with one another, become secretory granules. The secretory granules in turn migrate to the cell surface where they release their content, which becomes the type IV collagen of Reichert's membrane. Some diversion from this pathway may account for the immunostaining of multivesicular bodies and lysosomes.