The expression of laminin, a major glycoprotein constituent of basement membranes, was investigated in the rat developing intestine. The biosynthesis of laminin was studied after metabolic labeling of intestinal segments taken at various stages of development; the neosynthesized laminin was purified by affinity chromatography on heparin-Sepharose. Immunoblotting and immunoprecipitation experiments allowed us to analyze its constitutive chains. The data show that laminin is synthesized in very large amounts at 16–18 days of gestation concomitant with the onset of intestinal morphogenetic movements, i.e. villus emergence. Evaluation of the relative proportion of individual laminin polypeptides shows that laminin B1/B2 chains are produced in excess of A chains whatever the developmental stage considered. Interestingly at 17 days of gestation, levels of laminin A subunits are maximal. A second rise in the A/B chain ratio starts around birth and continues until adulthood. These quantitative data are corroborated by the immunocytochemical detection of laminin A and B chains, which revealed a specific spatiotemporal pattern. The finding that laminin A chains are located in the basement membrane of growing villi and of adult crypts raises the possibility that they may be involved in the process of cell growth and/or in the establishment of cell polarity by creating a specialized extracellular microenvironment.
The production and deposition of extracellular matrix proteins and the cellular origin of type-IV collagen have been analysed immunocytochemically in cocultured or transplanted intestinal epithelial-mesenchymal cell associations. In the first experimental model, rat intestinal endodermal cells were cultured on top of confluent monolayers of rat intestinal or skin fibroblastic cells. Under these conditions, interstitial matrix and basement membrane proteins were deposited within the fibroblastic layer over the whole culture period; interactions between the epithelial cells and the fibroblastic cell population, whatever their organ of origin, were required for the production of the basement membrane. In addition, its formation was progressive as assessed by the shift of a spot-like labelling to a continuous linear pattern at the epithelial-mesenchymal interface, and paralleled epithelial cell differentiation. In the second experimental model, chick-rat epithelial-mesenchymal recombinants developed as intracoelomic grafts were used, and the immunocytochemical detection of a basement membrane protein, type-IV collagen, was performed with species-specific antibodies. The major role of the mesenchyme in the deposition of type-IV collagen is supported by the fact that anti-chick but not anti-mammalian antibodies stained this antigen in chick mesenchyme-rat endoderm recombinants. These observations emphasize the role of tissue interactions in the formation of a basement membrane and show that the mesenchymal compartment is the principal endogenous source of type-IV collagen.