Cell fate specification in an in vitro model of neural development.

We have studied in an in vitro model of neural development the effect of neighboring cells on the fate of single fluorescently labeled precursor cells. In one line of experiments, PCC7-Mz1 embryonal carcinoma cells were transiently transfected with "green fluorescent protein" (GFP) and, following incubation with 0.1 microM all-trans retinoic acid (RA), the number and morphology of derivatives (neuronal or non-neuronal) was determined that form groups of GFP-expressing cells in a surrounding of unlabeled cells. Because single PCC7-Mz1 cells can produce single-lineage and mixed-lineage derivatives, they are individually pluripotent. In another line of experiments, we have analyzed the fate of GFP-expressing PCC7-MzN cells in different cellular environments. Whereas in the absence of other cells, PCC7-MzN cells exclusively differentiated to neuronal derivatives following RA induction (Lang, E., M. L. Mazauric-Stüker, A. Maelicke, J. Cell Biol. 109, 2481-2493 (1989)), they differentiated also to non-neuronal phenotypes (astrocytes and fibroblasts) when co-cultured with either PCC7-Mz1 stem cells or freshly RA-induced cells. The fate of PCC7-MzN cells could also be shifted in the absence of other cells when the cells were grown on laminin-coated surfaces. These results suggest that a putative fate-shifting activity (FSA) is released by PCC7-Mz1 and PCC7-MzN cells which requires, at least in the case of MzN cells, presentation by extracellular matrix-like structures in order to function in cell fate specification. Very few other cell types, in particular primary cultures of mouse forebrain cells of embryonic day 13, were capable of shifting the developmental potential of PCC7-MzN cells in a similar manner as PCC7-Mz1 cells do. We conclude that cell type specification in this model of neural development may occur by similar mechanisms as have been established in Drosophila neurogenesis. A default pathway (neuronal) is modulated by lateral signaling between neighboring cells so that cellular diversity can arise from initially homogeneous populations of progenitor cells.