Embryonic mouse kidney mesenchymes of various ages were tested for their abilities to initiate and support ureteric bud morphogenesis i n v i t ~o .
Nephrogenic cord mesenchyme taken from animaIs before metanephros initiation (10 days) was combined with ureteric buds. The buds showed no morphogenesis. This was true for presumptive inetanephric mesenchyme and for mesenchymes (pronephric and mesonephric) already undergoing nephrogenesis. These results indicate basic physiological differences between pronephric, mesonephric and metanephric regions of the cord. When entire nephric systems were cultured alone no metanephros developed. This, and the fact that presumptive metanephric mesenchyme had no influence on ureteric buds, indicates that mctanephros initiation (both bud and mesenchyme) results from some activity originating outside the nephrogenic cord.
When combined with metanephric mesenchyme at various stages of tubulogenesis (Ioose, condensed, or tubular), ureteric buds branched only in the presence of the loose mesenchyme. Branching occurred in loose mesenchyme taken from 11 to 15 day kidneys. Apparently once a given group of cells begins tubulogenesis they lose their ability to induce ureteric bud morphogenesis.
As tubulogenesis proceeds, the amount of loose mesenchymes per kidney decreases sharply. Ultimate size of the metanephros may be established by depletion of loose mesenchyme resulting in its ureteric bud-inducing potential falling below some critical level.