Rat intestinal basement membrane synthesis. Epithelial versus nonepithelial contributions.

Mesenchymal-epithelial interactions play an important role during tissue differentiation and morphogenesis. The basement membrane, which separates these compartments, appears to be critical to these interactions by providing a substratum for cell adhesion, promoting cell polarity and the differentiated phenotype. Unlike other epithelia, gut enterocytes adhere to, and migrate along a thin basement membrane as they differentiate along the crypt-villus axis with a turnover rate of 48 to 72 hours (rat). The relative importance of the enterocytes or of the mesenchymal cells of the lamina propria to the maintenance of the basement membrane is unknown. As indirect indicators of basement membrane biosynthesis, we have measured, by filter hybridization with labeled cDNA probes, the relative abundance of mRNAs for laminin and collagen IV chains in enterocyte fractions representing the crypt-villus gradient of differentiation and in cells of the underlying lamina propria. In confirmation of a gradient, mRNA for histone H2B was present as a decreasing gradient from crypt to villus, the crypt fraction containing the mitotically active enterocytes being most enriched for this transcript and, in contrast, the mRNA for beta-actin was present as an increasing gradient from crypt to villus, paralleling the abundance of microvillus core structures. The mRNAs for alpha 1(IV) and alpha 2(IV) collagen and laminin B1 and B2 chains were most abundant in the lamina propria. Little, if any, collagen IV mRNA was detectable in the enterocyte fractions. In contrast, laminin B1 and B2 mRNAs were enriched in crypt enterocytes but the steady-state level of these transcripts decreased in the superficial villus enterocyte fractions. These data suggest that the components of the intestinal basement membrane are synthesized by both mesenchymal and entodermal-derived cells. Alterations in the intestinal basement membrane structure and in cell adhesion during enterocyte differentiation may be partly mediated by changes in laminin synthesis by the enterocyte.