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Abstract. The expression of type IV collagen mRNA during mouse intestinal morphogenesis was examined by in situ hybridization using a cDNA probe corresponding to mRNA for ul (IV) chain. Type IV collagen mRNA is detected in the embryonic mesenchymal cells at early stages of development (12 d of gestation). A segregation of mesenchymal cells expressing high levels of type IV collagen mRNA in close vicinity of the epithelium occurs just before villus formation. During villus outgrowth, type IV collagen mRNA, still confined to mesenchyme-derived tissues, is progressively restricted to the mucosal connective tissue (the lamina propria) and to a lesser extent to the muscular layers. In the adult, the amount of messenger is quite low as compared to the level found in the developing intestine and the in situ hybridization signal, indistinguishable from the background, is uniform throughout the whole intestinal wall. At all developmental stages no detectable specific hybridization signal is virtually observed over the epithelium cell layer.These results show that high amounts of the type IV collagen messenger are detected during phases of intensive morphogenetic events. Furthermore, they reinforce the notion already gained previously (SimonAssmann et al. 1988) that the mesenchymal compartment is the principal endogenous source of type IV collagen. They also indicate that the continuous migration of epithelial cells along the basement membrane of intestinal villi in the mature organ is not accompanied by a significant remodeling of the collagen IV network.