Fig. 7. Conditional ablation of Pitx2 in mesoderm-derived cells reveals an autonomous role for Pitx2 in branchiomeric muscle development. Pitx2 deletion in mesoderm using the Mesp1^cre and Pitx2^flox alleles. (A,B) Lineage tracing indicates a reduction in the number of LacZ-marked Pitx2 mutant descendents in the masseter muscle (outlined, arrow). (C,D) Myogenin expression (arrow) is greatly reduced in the Pitx2 mutant compared with control. Sections through the masseter muscle of control (E,F) and Mesp1^cre; Pitx2 n/f mutant (G,H) indicate a muscle deficiency in the Pitx2 conditional mutant (arrows). (I-L) Myosin immunostaining with the MF20 antibody in 14.5 control (I,J) and Pitx2 conditional null mutant (K,L) to mark branchiomeric muscle in control and show deficiency in mutant (arrows). Note that in the mutant (L) there is residual subcutaneous muscle that shows myosin reactivity (arrowhead). e, eye; mc, Meckel's cartilage; md, mandible; mx, maxilla.