Fig. 4. Tbx1 regulates cell proliferation in mesenchyme cells. (A,A') Tbx1 expression domain revealed by a Tbx1-lacZ knock-in allele (Tbx1^+/-) at E8.5. (A') Transverse section of A. The area where cell proliferation was scored is indicated by the white box in A. (B) Transverse sections of E8.5 Mesp1^Cre/+; Tbx1^+/flox embryos as controls. (B') Transverse sections of E8.5 Mesp1^Cre/+; Tbx1^E5/flox embryos. (B,B') Immunohistochemistry staining with an anti-phospho-Histone H3 antibody to evaluate cell proliferation. (C) Mitotic index (M.I.) in pharyngeal mesenchyme at E8.5 embryos. Cells in neural tube were counted as an internal control. P values were calculated using Student's t-test. da, dorsal aorta; se, surface ectoderm; pe, pharyngeal ectoderm; pm, pharyngeal mesenchyme; P, pharynx. Scale bars: 1 mm in A; 100 µm in A'-B'.