(A) Mammary gland sections of WT, PlexinA4 KO, BDNF Het, and PlexinA4 KO/BDNF Het male embryos at different embryonic stages were stained with Tuj1 (red) and DAPI (blue) to visualize sensory neuron innervations. Scale bar: 50 µm. (B–D) Quantification of mammary gland innervation, means ±SEM. Late E12: n = 11, 9, 9, 10; Late E13: n = 10, 7, 7, 8; Late E14: n = 6, 11, 6, 6 for WT, PlexinA4 KO, BDNF Het and PlexinA4 KO/BDNF Het, respectively. (B) WT vs PlexinA4 KO: Late E12 p<0.0001, Late E13 p<0.0001, Late E14 p<0.0001. PlexinA4 KO: Late E12 vs Late E13 p=0.7803, Late E13 vs Late E14 p<0.0001. (C) WT vs BDNF Het: Late E12 p=0.0013, Late E13 p=0.1156, Late E14 p=0.3739. WT: Late E12 vs late E13 p=0.0473, Late E13 vs late E14 p=0.3282. BDNF Het: Late E12 vs Late E13 p=0.6117, Late E13 vs Late E14 p=0.5833 (D) WT vs PlexinA4 KO/BDNF Het: Late E12 p<0.0001, Late E13 p<0.0001, Late E14 p=0.00011. PlexinA4 KO vs PlexinA4 KO/BDNF Het: Late E12 p=0.0010, Late E13 p=0.0003, Late E14 p=0.1049. *p<0.05, **p<0.01, ***p<0.001, ns – non-significant, two-way ANOVA followed by separate t-tests per stage. P values compared to WT are marked on top of each bar.