(A, B, G, and H) Euploid and Ts65Dn P1 submucosal plexuses from small bowel were labeled with EdU ClickIT chemistry (green) plus antibodies against SOX10 (red) and TuJ1 (blue) antibodies (A and B) or with antibodies to Ki67 (green), SOX10 (red), and TuJ1 (blue) (G and H). In higher-magnification images (C and I), white arrowheads indicate TuJ1⁺EdU⁺ (C) or TuJ1⁺Ki67⁺ (I) neuron precursors; yellow arrowheads indicate SOX10⁺Ki67⁺ cells (I); and small white arrows indicate SOX10⁺Ki67^– glia (I). (D) TuJ1⁺ cell density was significantly reduced in P1 Ts65Dn submucosal plexus (P < 0.001, t test, n = 5 [euploid] and n = 8 [Ts65Dn]), while SOX10⁺ cell density was not significantly reduced in Ts65Dn animals (P = 0.267, t test; n = 5 [euploid] and n = 8 [Ts65Dn]). (E and F) The proportion TuJ1⁺EdU⁺ precursors relative to total TuJ1⁺ neurons (E), and the proportion SOX10⁺EdU⁺cells relative to total SOX10⁺ cells (F), was unchanged in P1 Ts65Dn submucosal plexus (TuJ1⁺EdU⁺ proportion: P = 0.747, t test, n = 5 [euploid] and n = 8 [Ts65Dn]; SOX10⁺EdU⁺ proportion: P = 0.719, t test; n = 5 [euploid] and n = 8 [Ts65Dn]). (J) P1 Ts65Dn mice had reduced SOX10⁺Ki67⁺ cell density but normal SOX10⁺Ki67^– cell density (SOX10⁺Ki67⁺: P = 0.0214, t test, n = 4 [euploid] and n = 7 [Ts65Dn]; SOX10⁺Ki67^–: P = 0.748, t test, n = 4 [euploid], n = 4 [Ts65Dn]). (K and L) The proportion TuJ1⁺Ki67⁺ precursors relative to total TuJ1⁺ neurons (K), and the proportion SOX10⁺Ki67⁺ cells relative to total SOX10⁺ cells (L), was unchanged in P1 Ts65Dn submucosal plexus (TuJ1⁺Ki67⁺ proportion: P = 0.782, t test, n = 4 [euploid] and n = 7 [Ts65Dn]; SOX10⁺Ki67⁺ proportion: P = 0.992, t test; n = 4 [euploid] and n = 7 [Ts65Dn]). Scale bars: 50 μm. All images are confocal Z-stacks. *P < 0.05, ***P < 0.001.