The major components of Reichert's membrane (laminin, type IV procollagen, entactin and heparan sulphate proteoglycan) are all synthesized by the parietal endoderm cells of the mouse embryo. Fibronectin is found mainly on the trophoblast side of Reichert's membrane and does not appear to be a major structural component. Parietal endoderm cells are thought to differentiate and migrate as individual cells from the margins of the epithelial visceral endoderm layer. They may interact with the type IV collagen in Reichert's membrane via a surface-associated protein of Mr = 47000 known as 'colligin'. Parietal endoderm cells are a rich source of mRNAs for basement membrane components and have been used to prepare a cDNA library in the expression vectors pUC8 and pUC9 from which cDNAs for type IV collagen and laminin B chains have been isolated.