Figure 8. *P2rx3*−/− SGN axonal terminal branching phenotype is largely normal at P6, and *P2rx3*−/− cochleae show normal presynaptic Ribeye and postsynaptic Shank1a structures. *A*, *B*, Representative images of WT and *P2rx3*−/− Type I SGN terminals at P6. *C*, Quantification of branch number at P6 and all of the following quantifications are at P6. Each datapoint represents one SGN terminal arborization (WT apex, *n* = 176, *N* = 8, KO apex, *n* = 99, *N* = 7, WT base, *n* = 106, *N* = 8, KO base, *n* = 113, *N* = 7; four WT animals from two litters and four KO animals from three litters; WT apex: 15.0 ± 0.5, KO apex: 16.4 ± 0.8, WT base: 14 ± 0.5, KO base: 13.7 ± 0.7; comparisons between WT and KO apex, *t*_{(181)} = 1.48, *p* = 0.1408, between WT and KO base, *t*_{(206)} = 0.39, *p* = 0.6991, between WT apex and base, *t*_{(268)} = 1.31, *p* = 0.19, between KO apex and base, *t*_{(196)} = 2.59, *p* = 0.0102). *D*, Quantification of average branch length (in μm, WT apex: 2.5 ± 0.06, KO apex: 2.5 ± 0.09, WT base: 2.2 ± 0.06, KO base: 2.3 ± 0.07; comparisons between WT and KO apex, *t*_{(185)} = 0.48, *p* = 0.6351, between WT and KO base, *t*_{(215)} = 0.56, *p* = 0.5756, between WT apex and base, *t*_{(263)} = 2.77, *p* = 0.0059, between KO apex and base, *t*_{(188)} = 2.1, *p* = 0.0371). *E*, Quantification of average branch depth (WT apex: 3.1 ± 0.08, KO apex: 3.2 ± 0.1, WT base: 3.1 ± 0.1, KO base: 2.9 ± 0.1; comparisons between WT and KO apex, *t*_{(180)} = 0.84, *p* = 0.4047, between WT and KO base, *t*_{(217)} = 1.32, *p* = 0.1899, between WT apex and base, *t*_{(225)} = 0.07, *p* = 0.9439, between KO apex and base, *t*_{(202)} = 1.99, *p* = 0.0479). *F*, Regression plot of average branch length and branch number (WT: Y = −0.03*X + 2.9, KO: Y = −0.04*X + 3.0). *G*, Quantification of average branch diameter (in μm, WT apex: 1.4 ± 0.01, KO apex: 1.4 ± 0.02, WT base: 1.4 ± 0.02, KO base: 1.5 ± 0.02; comparisons between WT and KO apex: *t*_{(218)} = 1.77, *p* = 0.0774, between WT and KO base, *t*_{(217)} = 3.59, *p* = 0.0004, between WT apex and base, *t*_{(208)} = 1.84, *p* = 0.0674, between KO apex and base, *t*_{(210)} = 3.99, *p* < 0.0001). *H*, Quantification of total branch volume (in μm^{3}, WT apex: 47.9 ± 1.5, KO apex: 54.6 ± 2.0, WT base: 42.9 ± 1.4, KO base: 48.7 ± 1.7; comparisons between WT and KO apex, *t*_{(209)} = 2.68, *p* = 0.0079, between WT and KO base, *t*_{(209)} = 2.64, *p* = 0.0089, between WT apex and base, *t*_{(274)} = 2.44, *p* = 0.0152, between KO apex and base, *t*_{(202)} = 2.26, *p* = 0.025). *I*, *J*, WT and *P2rx3*−/− cochleae at P6 were stained with Shank1a (green) to show postsynaptic scaffold structures of ribbon synapse. *K*, *L*, WT and *P2rx3*−/− cochleae at P6 were stained with Ribeye (magenta) to show presynaptic ribbon structures. *M*, Shank1a volume (in μm^{3}, WT apex: 144 ± 3.6, *N* = 8; KO apex: 150.1 ± 6.4, *N* = 8; WT mid: 156 ± 3.8, *N* = 8; KO mid: 164.2 ± 7.0, *N* = 8; WT base: 173.4 ± 6.7, *N* = 7; KO base: 169 ± 8.9, *N* = 8; comparison in apex: *t*_{(11)} = 0.83, in mid: *t*_{(11)} = 1.03, in base: *t*_{(13)} = 0.40, see *p* values in the figure; from eight animals, three litters, in each genotype group). Comparisons between two genotypes at apex, mid, base indicate a normal postsynaptic morphology. *N*, *O*, Number of Ribeye puncta per inner hair cell (WT apex: 24.4 ± 1.7, *N* = 8; KO apex: 28.1 ± 0.6, *N* = 8; WT mid: 23.3 ± 1.3, *N* = 8; KO mid: 25.6 ± 1.3, *N* = 8; WT base: 20.0 ± 1.0, *N* = 8; KO base: 23.1 ± 1.0, *N* = 8; comparison in apex: *t*_{(9)} = 2.12, in mid: *t*_{(14)} = 1.19, in base: *t*_{(14)} = 2.14, see *p* values in the figure; from eight animals, three litters, in each genotype group). Average Ribeye volume (in μm^{3}, WT apex: 0.17 ± 0.008, KO apex: 0.19 ± 0.017, WT mid: 0.19 ± 0.01, KO mid: 0.19 ± 0.018, WT base: 0.21 ± 0.01, KO base: 0.21 ± 0.015; comparison in apex: *t*_{(10)} = 1.07, in mid: *t*_{(11)} = 0.015, in base: *t*_{(12)} = 0.17). Comparisons between two genotypes at apex, mid, base indicate a normal presynaptic morphology. n.s., not significant. **p* < 0.05; ***p* < 0.01; ****p* < 0.001; *****p* < 0.0001.