Fig. 1. Pitx2 is required for survival of branchiomeric muscle precursors. (A,B) Right and left views of 9.5 dpc mouse embryos after whole-mount in situ hybridization with a Pitx2 probe (arrows denote hybridization signal). (C-H) Expression in chick embryos of Pitx2 (C,E,F,H) and MyoD (D,G) analysed by whole-mount in situ hybridization (C,F) and radioactive in situ hybridization to tissue sections (D,E,G,H) of stage 10 (C,F), 24 (D,E) and 26 (G,H) chick embryos. C is a dorsal view and D,E,G,H are frontal sections through the facial primordial. F is a transverse vibratome section through the head of the embryo in C. Arrows in D,G indicate developing muscles, in E the ectodermal and mesodermal expression of Pitx2, in H the ectomesenchymal Pitx2 expression. do, dorsal oblique muscle; e, eye; f, frontonasal mass; hy, hyoid arch; md, mandibular primordia.