Fig. 2. Fate map of Tbx1-expressing cells. (A) The gene targeting strategy utilized to generate the Tbx1^mcm knock-in allele. (B) X-gal staining pattern generated by a Tbx1-lacZ knock-in allele (Tbx1^+/–) in an E10.5 embryo. (C) X-gal staining pattern in a Tbx1^mcm/+;R26R embryo at E10.5. (D) X-gal staining pattern in a Df1/Tbx1^mcm;R26R embryo at E10.5 (Tbx1 homozygous mutant background). Small arrows in B-D indicate the OFT, the red arrowheads in C point to a superficial, ectodermal domain. (E) Dissected heart from a Tbx1^+/– (lacZ knock-in) E10.5 embryo, and, F, from a Tbx1^mcm/+;R26R E10.5 embryo. (G,H) Histological sections through the OFT at the same stage in Tbx1^+/– (G) and Tbx1^mcm/+;R26R (F) embryos at E10.5. Black arrows show blue cells localized in similar position in the two mutants, red arrows indicate blue cells detected only by cell fate mapping. (I,J) X-gal staining pattern generated by a Tbx1-lacZ knock-in allele (Tbx1^+/–) in an E12.5 heart (I), compared with that in a Tbx1^mcm/+;R26R embryo (J) at the same stage. (K-M) Comparison of the lacZ knock-in X-gal staining pattern (K, 37 somites) with that obtained by cell fate mapping in a heterozygous (L, 39 somites) and homozygous (M, 36 somites) Tbx1 mutant. The area in the black box in M is magnified (5x) in the inset at the bottom-left of M. K'-M' show magnified details of panel K, L and M, respectively. Red arrowheads indicate endothelial cells, black arrows OFT myocytes, and black arrowheads SHF cells. Scale bars: 1 mm in B-D, E,F and I,J; 100 µm in G,H; 200 µm in K-M; 50 µm in K'-M'.