Fig. 2. Tbx1-lacZ expression and Cre-induced recombination in the pharyngeal region of embryos at E10.5. (A) Tbx1-lacZ expression in a whole-mount Tbx1^+/– embryo (A), and on sagittal sections of the same embryo (Aa-Ad) showing expression in the core arch mesoderm of pharyngeal arches I and II (arrowheads in Aa), in the pharyngeal mesoderm (secondary heart field, arrow in Ab) and in the third and fourth pharyngeal pouches (red arrowheads in Ac). The absent fourth PAA (asterisk in Ad, Dd, Ed, Fd) at E10.5 is revealed by intracardiac ink injection. I-IV, pharyngeal arches I-IV; 3p, 4p, third and fourth pharyngeal pouches; da, dorsal aorta; 3, 6, third and sixth pharyngeal arch artery; A, anterior; P, posterior; D, dorsal; V, ventral. (B-F) X-gal-stained wholemounts and sagittal sections of Cre-driver; R26R embryos. (B) Nkx2.5^Cre/+; R26R; (C) Mesp1^Cre; R26R (D) Foxg1^Cre/+; R26R; (E) TgFgf15Cre; R26R; (F) Hoxa3^Cre/+; R26R. Sagittal sections in B-F are similar to those in A, and arrows and arrowheads show the same tissues and structures. In all histological sections, cranial is up and dorsal is right. In the lower panel of ink injected embryos, Ad, Bd and Dd are oriented as indicated in Ad; Cd, Ed and Fd are in the opposite orientation, as indicated in Cd. Scale bars: 100 µm.