Fig. 7.

Expanded activation of Hedgehog signaling causes aberrant atrial septum development. (A-H) Constitutive activation of the Hh pathway using the Cre-activated smoothened-M2 allele (R26-smoM2) in cells that normally receive Hh signal has no effect on atrial septation, whereas activating the Hh pathway in a broader domain results in a hypercellular atrial septum. Whole-mount (A,E) and cross-section (B-D,F-G) histology. (A,B) Gli1-CreER^T2 expression domain demonstrated with the R26R reporter. (C,D) Constitutive activation of the Hh pathway in the Gli1-CreER^T2 expression domain has no effect on the size or overall structure of the atrial septum compared with littermate controls (arrows). (E,F) Nkx2-5^Cre expression domain demonstrated with the R26R reporter. Inset in F depicts Nkx2-5^Cre expression in pulmonary endoderm. (G,H) Constitutive activation of the Hh pathway in the broader Nkx2-5^Cre domain results in an enlarged atrial septum when compared with wild-type littermates (arrows).