Shift in Collagen Type as an Early Response to Induction of the Metanephric Mesenchyme 276

Conversion of the nephrogenic mesenchyme into epithelial tubules requires an inductive stimulus from the ureter bud . Here we show with immunofluorescence techniques that the undifferentiated mesenchyme before induction expresses uniformly type I and type III collagens. Induction both in vivo and in vitro leads to a loss of these proteins and to the appearance of basement membrane components including type IV collagen . This change correlates both spatially and temporally with the determination of the mesenchyme and precedes any morphological events . During morphogenesis, type IV collagen concentrates at the borders of the developing tubular structures where, by electron microscopy, a thin, often discontinuous basal lamina was seen to cover the first pretubular cell aggregates . Subsequently, the differentiating tubules were surrounded by a well-developed basal lamina . No loss of the interstitial collagens was seen in the metanephric mesenchyme when brought into contact with noninducing tissues or when cultured alone. Similar observations were made with nonnephrogenic mesenchyme (salivary, lung) when exposed to various heterotypic tissues known to induce tubules in the nephrogenic mesenchyme . The sequential shift in the composition of the extracellular matrix from an interstitial, mesenchymal type to a differentiated, epithelial type is so far the first detectable response of the nephrogenic mesenchyme to the tubule-inducing signal . In the embryonic kidney rudiment, an inductive interaction between the epithelial component and the mesenchymal blastema is required for differentiation (9) . This reciprocal interaction leads to the orderly branching of the ureter bud and to the determination of the mesenchyme . The mesenchymal cells then aggregate into condensates, and these convert into epithelial tubules (22) . The induction of the mesenchyme can be precisely timed and correlated with the morphological and biological events by culturing the interacting components on opposite sides of a filter membrane (10) . In this particular system with a heterotypic inductor (embryonic spinal cord), a 24-h transfilter contact is necessary for the full response in the metanephric mesenchyme (23) . The first pretubular cell aggregates develop in the mesenchyme -12 h later, and tubular structures are seen after another 12 h, on day 2 . Antigens specific for the luminal side of the tubules appear on day 4 of culture (6) . In the kidney model system, the kinetics of the induction and the morphogenetic events following determination are thus well known. However, changes during and after induction before cell aggregation are poorly understood at the molecular level. Advances in the characterization of extracellular matrix proteins have made immunohistological localization of these proteins possible (18, 27, 29, 30) . By this technique we recently showed the appearance of one basement membrane protein, laminin, in the kidney mesenchyme during the 24-h induction period (5, 29) . This suggests that the response of the mesenchyme to induction involves a change in the matrix composition, which may also include several of the known isotypes of collagen (2) . Previous studies (reviewed in references 2 and 30) have established that type I and III collagens are mainly found in interstitial connective tissue where they often show extensive codistribution and that type II collagen is a major component of hyaline cartilages, whereas type IV and perhaps type V collagens are unique for basement membranes . We have therefore studied the expression of the different collagen types THE JOURNAL OF CELL BIOLOGY " VOLUME 89 MAY 1981 276-283 © The Rockefeller University Press 0021-9525/81/05/0276/08 $1 .00 on Jne 3, 2017 D ow nladed fom Published May 1, 1981