Fig. 6. Tbx1 regulates endodermal cell proliferation. (A,B) Examples of immunohistochemistry with an anti-phospho H3 antibody used to evaluate endodermal cell proliferation in E10.0 TgCAGG-CreER^TM;Tbx1^flox/+ (control, A) and TgCAGG-CreER^TM;Tbx1^flox/- (test, B) embryos exposed to TM at E8.5. ec, ectoderm; en, endoderm. (C) Mitotic index (M.I.) count from at least 2000 cells per tissue per embryo; data refer to the average of three embryos per genotype; P values were calculated using the Student's t-test. Brain cells were counted as an internal control. Error bars indicate standard deviation. (D) The cartoon illustrates a model for Tbx1 role during pharyngeal segmentation. A cranial-to-caudal `wave' of Tbx1 gene expression causes cranial-to-caudal expansion of the pharyngeal endoderm. This is followed by invasion of neural crest-derived cells (NCC) and formation of a new pharyngeal arch. Tbx1 expression in pharyngeal mesoderm may also contribute to pharyngeal segmentation. PAA, pharyngeal arch artery. Scale bar: 50 µm in B.