(A–B) Phospho-histone H3 (PHH3) immunostaining within the first pharyngeal arch in Wnt1;1Het2Het;R26R-LacZ−/+ (Ai, Ai′) and Wnt1;1KO2KO;R26R-LacZ−/+ (B, B′) at E9.5 (Black, PHH3+). (C) Quantification of PHH3 positive nuclei per 100 nuclei (proliferative index; presented as mean (SD); *, p=0.022). (D–E) PHH3 immunostaining within the neural tube (NT) in Wnt1;1Het2Het;R26R-LacZ−/+ (D, D′) and Wnt1;1KO2KO;R26R-LacZ−/+ (E, E′) embryos at E9.5 (Black, PHH3+). (F) Quantification of PHH3 positive nuclei per 100 nuclei (proliferative index; presented as mean(SD); *, p=0.043). (G–H) Cleaved caspase 3 (CC3) immunostaining within the first pharyngeal arch in Wnt1;1Het2Het;R26R-LacZ−/+ (G, G′) and Wnt1;1KO2KO;R26R-LacZ−/+ (H, H′) at E9.5 (Black, CC3+). (I) Quantification of CC3 positive nuclei per 100 nuclei (apoptotic index; presented as mean (SD); **, p=0.0038). (J–K) CC3 immunostaining within the NT in Wnt1;1Het2Het;R26R-LacZ−/+ (J, J′) and Wnt1;1KO2KO;R26R-LacZ−/+ (K, K′) at E9.5 (Black, CC3+). (L) Quantification of CC3 positive nuclei per 100 nuclei (apoptotic index; presented as mean (SD); *, p=0.028). (M–R) Relative mRNA levels of in E9.5 Wnt1;1Het2Het and Wnt1;1KO2KO microdissected first pharyngeal arches of Cdkn2b (M, *, p=0.042), Cdkn2c (N, *, p=0.032), Cdkn1a (O, ***, p=0.0008), Cdkn1b (P, *, p=0.035), Tp53 (Q, ***, p=0.0001), and Cdkn1c (R, **, p=0.0073). NT, neural tube.