Fig. 1. Tbx5 is expressed in the murine cardiac conduction system. (A) Schematic representation of the atrioventricular canal conduction system, showing the plane of dissection (dashed line) and specimen orientation (arrow) for images in (B,C). Atria were removed and the atrioventricular canal was viewed from the superior/posterior. The atrioventricular canal from newborn minK^lacZ/+ (B) and wild-type (C) mouse hearts. X-gal staining of minK^lacZ/+ hearts (B) showed two rings of specialized conduction cells, surrounding the tricuspid and mitral annulus. Scale bar: 200 µm. Whole-mount in-situ hybridization with a Tbx5 probe of wild-type hearts (C) demonstrated rings of Tbx5 expression that overlap minK expression. (D) Schematic representation of dissection of the interventricular septum for images in (E-H). Sagital sections of the muscular interventricular septum of newborn minK^lacZ/+ (E) and wild-type (F-H) mouse hearts. A minK^lacZ/+ heart (E) revealed ß-galactosidase activity in the atrioventricular bundle (arrow), and the ventricular bundle branches (arrowheads). Scale bar: 100 µm. In-situ hybridization with a Tbx5 probe in wild-type hearts (F) demonstrated expression in the atrioventricular bundle (arrow) and bundle branch (arrowheads) conduction system. In-situ hybridization with a connexin 40 probe (G) and Tbx5 probe (H) in sequential sections from the same wild-type heart demonstrates overlapping expression in the atrioventricular bundle (arrow) and bundle branch (arrowheads) conduction system.