Experiments reveal the cell cycle time and lack of self-renewal capacity of the 1stgeneration neuronal precursors. A. Graph of average numbers of BrdU-labeled cells in the niche after BrdU incubation for 24, 72 or 120 hours, followed by immediate sacrifice. The mean number of BrdU-labeled cells is between 2 and 3 for all groups, and there is no statistical difference between groups. B. Graph of average numbers of BrdU-labeled cells in the streams after BrdU incubation times of 24, 72 and 120 hrs. Mean counts are shown for the streams between the niche and Cluster 9, between the niche and Cluster 10, and the combined counts in the streams. C. Live crayfish were incubated in BrdU for 24 hours and then maintained in fresh pond water for 7 days. Just before sacrifice, they were treated with EdU (green) for 6 hours. Fixed brains also were labeled immunocytochemically for glutamine synthetase (blue) to reveal the niche and streams (C, D). S phase cells in the niche and streams are labeled only with EdU, demonstrating that migration is uni-directional (away from the niche) and that niche cell divisions are not self-renewing. D. Higher magnification image of the LPZ. The BrdU-labeled cells, which were labeled first in the sequence, are found only in the proliferation zones (MPZ and LPZ) and not in the niche. Arrow indicates direction of migration. Scale bars: C, 100 μm; D, 50 μm.