Transferrin as a fetal growth factor: acquisition of responsiveness related to embryonic induction.

Differentiation of the metanephric mesenchyme, which is triggered by an inductive tissue interaction, has been shown to proceed in a chemically defined medium containing transferrin. Here, we report that neither transferrin-depleted serum nor a chemically defined medium devoid of transferrin promote differentiation and that activity can be restored by the addition of transferrin. It thus appears that we have identified the serum factor required for kidney differentiation. Transferrin seems to affect differentiation by stimulating cell proliferation. We show by using an organ-culture model system that only mesenchymes induced to differentiate by the 24-hr tissue interaction respond to transferrin by proliferation and differentiation, whereas uninduced mesenchymes remain unresponsive. The inductor tissue used is not responsive to transferrin. Thus, the data suggest that the short-range cell-mediated tissue interaction acts by making the nephrogenic mesenchyme responsive to the long-range mediator, which is transferrin. Transferrin is suggested to be an important circulating growth factor required for proliferation during embryogenesis.