In vivo exposure to serotonin increases total niche cell numbers, but does not alter the number of dividing niche cells. A. Images of niches stained with propidium iodide from control (A) and 24-hr 10-9 M serotonin-treated (B) crayfish. C. Graph of mean numbers of BrdU+ cells per niche in controls and those exposed to 10-9 M serotonin for 18 and 24 hours. 5-HT does not alter the number of BrdU-labeled cells in the controls (ANOVA: p = 0.960). The number of BrdU-labeled niche cells is lower in this experiment than the one graphed in Figure 2A because these crayfish were slightly larger/older; the cell cycle of niche precursors slows as the animals age . D. However, there are significant increases in the number of propidium iodide stained cells in the niche in 5-HT-treated crayfish compared with untreated controls (ANOVA: p < 0.0001 followed by Tukey multiple comparisons). Different letters above the histograms indicate significant differences among the groups. Error bars = standard error of the means. n = numbers of niches per group. Scale bar refers to A and B, 50 μm.