Evidence for a two-step mechanism operating during in vitro mouse kidney tubulogenesis.

The process of kidney tubulogenesis was investigated in mouse metanephrogenic mesenchyme, differentiating in tissue culture under the influence of an inductive stimulus from embryonic mouse brain. The metanephrogenic mesenchyme was separated from the brain by a membrane filter. The time of exposure to the inductive stimulus was controlled by removing the brain from the filter. Restricting the period of transfilter association of metanephrogenic mesenchyme and brain resulted in incomplete tubulogenesis. A 30 h interaction time with brain led to the formation of small tubules. These small tubules were at an unstable stage of differentiation, and regressed during the 6-day culture period. Stabilization and elongation was achieved by the addition of mesenchymal tissues or chick embryo extract. Induction was clearly not a one-time triggering event, and as the degree of differentiation increased so the specificity for mesenchymal requirement decreased. Embryo extract did not mimick the inductive event that initiated tubulogenesis, but supported the later stages of tubulogenesis. This was interpreted to indicate a certain specificity in the induction reaction. The incidence of tubule elongation was related to both the initial mass of metanephrogenic mesenchyme and to the total contact time with brain. The greater the initial mass of mesenchymal tissue, the less contact time with brain needed for complete tubulogenesis. The morphology of the elongated tubules depended on the nature of the mesenchyme with which the small tubules were associated. This suggested that the expression of tubule morphology may be under the control of the surrounding mesenchymal cells. Possible mechanisms operating during tubule elongation were discussed. It was concluded that an integrated two-step mechanism was operating during kidney tubulogenesis. The first step, cell condensation, was induced by contact with brain. The second step, tubule elongation, was dependent upon the association of the condensates with the surrounding non-induced mesenchymal cells.