Role of transferrin in branching morphogenesis, growth and differentiation of the embryonic kidney

Our previous work has suggested that transferrin is an important serum component for differentiation of the kidney. In this study we have analysed more closely the response of cultured mouse embryonic kidney to exogenous transferrin and the dependence of kidney tubule induction on transferrin. Our results show that transferrin causes a dose-dependent increase in cell proliferation in the differentiating kidney mesenchyme, but no stimulation of cell proliferation in the inductortissue used, the embryonic spinal cord. In cultures of whole kidney rudiments a remarkable increase in the amounts of DNA and protein are caused by transferrin but not by other serum components present in a transferrin-depleted serum. The morphology of the explants was similar when culturedin the presence of human serum and in the transferrin-depleted serum supplemented with transferrin. In transferrincontaining chemically-defined medium the explants flattened and spread out, but the morphology of the kidney tubules was similar as in explants cultured in the presence of serum. Examination of the cultured explants by electron microscopy showed that in all transferrincontaining culture media the mesenchymal cells had differentiated into kidney tubules consisting of epithelial cells lined by a basement membrane. The experiments with the transferrin-depleted serum demonstrate that the main mitogen for kidney development is transferrin, and that other serum factors are mainly required for maintenance of tissue compactness. Our earlier studies have shown that exogenous transferrin is not needed for certain changes preceding overt tubule formation in the kidney mesenchyme, and we suggested that transferrin responsiveness is acquired during the induction of kidney mesenchyme. Our present results do not contradict the postulate, although they demonstrate that the acquisition of the responsiveness is more complicated than previously thought. When the mesenchyme is exposed to inductor tissue for 24 h without transferrin, and then subcultured without the inductor in the presence of transferrin, morphogenesis fails and there is no proliferation of the mesenchyme. The experiment shows that the inductor, the mesenchyme and transferrin must all three be simultaneously present for the acquisition of the transferrin responsiveness. Other experiments show that the induced mesenchyme can be a direct target tissue, since it can proliferate in response to transferrin also in the absence of the inductor. It is evident that the inductor is required for the acquisition of the responsiveness, as suggested. However, there is apparently a large overlap between the transferrin-independent and transferrin-dependent proliferation. The mesenchyme is not a synchronous cell population and cells do not become induced and transferrin-responsive at the same time. Therefore, in the organ culture, it is necessary to have transferrin present also during induction. Although this explanation seems most likely, we cannot exclude that transferrin has two actions, one measurable direct effect on the proliferation of induced mesenchymes, and another yet unidentified effect on the induction process.