Fig. 5. Splanchnic mesoderm-derived Mef2cAHFcre-marked muscle precursors are reduced in the Pitx2^null mutant branchial arch. (A,B) Lineage tracing with the Mef2c AHF cre transgenic driver and the R26R allele indicated that the Mef2c-marked splanchnic mesoderm lineage contributes to first branchial arch branchiomeric muscle (denoted by arrows). (C-H) Lineage tracing with the Mef2c AHF cre in the control and Pitx2^null mutant embryos indicated a deficiency in the splanchnic mesoderm component of the first branchial arch muscle precursors (denoted by arrows) in Pitx2^null mutant embryos at 9.5 dpc (C,D), 10.5 dpc (E,F) and 11.5 dpc (E,F). It is notable that in the 11.5 dpc Pitx2 mutant embryo, Mef2c AHF cre-marked cells are found diffusely scattered in the caudal branchial arches (arrowhead). e, eye; md, mandibular process; mx, maxillary process.