Fig. 4. Neural crest cell migration and AP septum phenotype in conditional mutants (E10-E10.5). (A-F) Fourth pharyngeal arch and neural crest migration phenotype in controls (Tbx1^flox/+) (A,A',C,E) and conditional mutants (B,B',D,F). (A,B) Lateral view of wholemount immunohistochemistry with an anti-neurofilament M antibody (dorsal is left), note the abnormal branch of the glossopharyngeal nerve bundle (arrowheads in B,B'). The 4th pharyngeal arch is small with an apparently normal vagus nerve (arrows) but absent 4th pharyngeal arch artery (A',B'). A' and B' are coronal sections of the embryos shown in A and B, respectively. (C-D) Lateral view of wholemount RNA in situ hybridization with a neural crest marker (CrabpI) (dorsal is left), note the reduced staining at the 4th pharyngeal arch in the conditional mutant (compare with the region indicated by arrowhead in the wild-type embryo in C). (E,F) Radioactive in situ hybridization with the same probe on tissue sections of 32-somite embryos (E, wild type; F, conditional mutant). There is reduced expression of the neural crest marker and absence of the 4th PAA. (G,H) Partial overlap between Tbx1 expression (visualized by the Tbx1-lacZ knock-in pattern) (G) and Nkx2.5^cre-induced recombination (visualized by the R26R reporter) (H) in the endoderm of the 3rd and 4th pharyngeal pouches (arrows), sagittal sections. (I,J) Coronal sections show absence of the AP septum (arrow) in a conditional mutant (J), compared with a control (Nkx2.5^Cre/+;Tbx1^+/–) embryo (I). (K,L) Sagittal sections, stained with X-gal, showing overlap between Tbx1 expression (visualized by the Tbx1-lacZ knock-in pattern, K) and Nkx2.5^Cre-induced recombination (visualized by R26R reporter, L) in the endoderm lining the aortic sac adjacent to the AP septum (detail magnified in box, 5x). IV, fourth pharyngeal arch. P, pharynx. The developmental stages of embryos shown in G-L are in the range of 34-37 somites. Scale bars: 200 µm in A-D; 100 µm in the others.