Fig. 4. Fate mapping Pitx2 descendants in Pitx2 mutant embryos. (A,B) Lineage tracing with the Wnt1^cre transgenic line that outlines core mesoderm of the first branchial arch. (C,D) Lineage tracing with the Pitx2^cre allele showing Pitx2 lineage contributes to the core mesoderm in both control (arrow in C) and Pitx2^null embryos (arrow in D). (E,F) Lineage tracing with the Pitx2^cre allele in control (E) and Pitx2 hypomorphic embryos (F) at 16.5 doc. Pitx2 descendents contribute to branchiomeric muscle in the control, but in the mutant branchiomeric muscle is absent (arrow). (G,H) Lineage tracing and TUNEL double labeling in control and Pitx2^null mutant embryos. The LacZ-positive cells are Pitx2 descendents that show upregulated TUNEL-positive cells in the mutant (arrow). mo, molar tooth; to, tongue.