Embryonic neurons as in vitro inducers of differentiation of nephrogenic mesenchyme.

Nephrogenic mesenchyme differentiates into epithelium as a result of morphogenetic tissue interactions. In vivo, the ureter bud is thought to induce tubular differentiation of the mesenchyme. In vitro recombination experiments have shown that various embryonic tissues can act as inducers when put in close proximity to nephrogenic mesenchyme. Induction also occurs across a porous filter. In the present study we show that only a few embryonic tissues are potent inducers in transfilter cultures in which mesenchyme and inducing tissue are separated by a membrane filter. Of the tissues tested, only embryonic spinal cord and brain were effective, whereas the ureter bud did not induce. All tissues tested sent processes through the filter. Weak inducing capacity of embryonic tissues is thus not due to a failure of the cells to make contact with the mesenchyme. To analyze which cell type within the embryonic brain possesses inducing capacity, neurons were selectively removed from primary cultures of chick tectal cells by antibody and complement-mediated cell lysis. These cultures, consisting of glial and undifferentiated cells, were then recombined with nephrogenic mesenchyme. They proved to be ineffective in inducing tubulogenesis, whereas cell populations containing neurons retained their inducing capacity. In transfilter cultures, ingrowth of neuronal processes deep into the mesenchyme, as assayed by anti-neurofilament staining, occurred within the first 24 hr of culture. Thus, it is not the time needed for processes to grow through the filter, but the time needed to grow into the mesenchyme that corresponds to the minimal induction time. These studies suggest that embryonic neurons are the most effective inducers of nephrogenic mesenchyme in vitro. Differentiation may be triggered by neuronal processes that establish cell contacts deep within the mesenchyme. Neurons might be important for nephrogenesis in vivo as well, although we can present no direct evidence to support this idea, since we failed to detect neurons at early stages of kidney development when the first tubules are induced.