The morphology of basement membrane formation.

A transplantable, basement-membrane (BM)-producing murine tumor was investigated using immunofluorescence and electron microscopy. Collagen type IV, fibronectin, laminin, and a BM proteoglycan (BM-1) could be discriminated in the intercellular substance (ICS). The occurrence of these substances and the electron microscopic findings speak for a comparability of the ICS of the tumor with the BM of normal tissues. Hence, a model has become available to study the morphology of BM-formation. The BM-producing cells are characterized by numerous, often enlarged cavities of the rough endoplasmic reticulum (RER) and a large Golgi complex. The BM-material is released into the ICS via prosecretion and electron-dense secretion granules. The morphology of the extracellular BM-material depends upon the fixation technique. After addition of tannic acid, typical BM-lamellae are formed, which in most cases show a granular structure. This is due to shrinkage and attachment of glycoproteins to the collagen skeleton. After ruthenium red or after glutaraldehyde fixation, the extracellular matrix consists of homogeneously distributed filamentous material with only few granular structures. This situation corresponds to the small amount of acid proteoglycans in the BM. The present findings facilitate the evaluation of BM-formation in other cell types.