Fig. 2. A developmentally regulated signal can direct the fate of dividing embryonic cortical progenitor cells. Dissociated mouse E15 GFP cortical cells were cultured over rat E18, P0, or P15 cortical slices for 5 days in vitro in the presence of 10 µM BrdU to identify progenitors that were proliferating at the time of plating. (A-C) Examples of E15 GFP cells on top of E18 slices (A), P0 slices (B), and P15 slices (C) that were processed for double immunofluorescence with antibodies directed against GFP (green channel) and BrdU (red channel). E15 GFP+/BrdU+ neurons are indicated by the white arrowheads (A and B) and GFP+/BrdU+ glia by the yellow arrowhead (C). (D-F) Quantification of the differentiated fate of dividing cortical progenitor cells plated on top of E18, P0, and P15 cortical slices. Neurons, filled bars; glia, open bars. Bar heights represent mean ± s.e.m.