Fig. 6. Tbx1 conditional null embryos have abnormal musculature. (A-D) Transverse histological sections of E17.5 embryos. Tbx1 conditional null embryos have hypoplasia of the masseter (B) and pterygoid (D) muscles. Representative wild-type embryos are shown in A and C. mc, mandibular condyle; me, Meckel's cartilage; mt, masseter; pt, pterygoid. (E,F) Coronal sections through an E17.5 Tbx1 hygro/hygro embryo (H/H; F) reveal the presence of the masseter muscle (mt); a wild-type control embryo is shown in E. (G) ISH with Myod on an E10.5 wild-type embryo shows expression in the core mesenchyme of PAs (arrow). (H,I) Myod ISH reveals the lack of expression in the core mesenchyme of the first PA of homozygous conditional null (H) and Tbx1^-/- (I) embryos. (J) Tbx1 expression in the core mesenchyme of PA1 is maintained in the homozygous conditional null mutant at E10.5 (arrow). WT, wild type; PEKO/-, Tbx1 flox/null; Foxg1-Cre/+; -/-, Tbx1^-/-. Scale bar: 100 µm in B,D; 200 µm in F; 0.5 mm in I; 0.2 mm in J.