The FHF CPs marked by BAC <i>Tbx5</i>^CreERT2 transgene are unipotent cells that contribute only to the cardiomyocyte lineage.

<p>(A-G). Sections of the heart of E15.5 BAC <i>Tbx5</i>^CreERT2/<i>ROSA26</i>^eYFP/eYFP mouse embryos treated with tamoxifen at E7.5 <i>in vivo</i> were subjected to confocal immunofluorescence analysis with antibodies to the indicated proteins. Nuclei were stained with DAPI. The boxed region in (A) is shown at higher magnification in (D), and those in (B) are shown at higher magnification in (F) and (G). Data are representative of three embryos. Asterisks indicate the sinoatrial node (F) or the bundle of His (G). The white arrow in (G) indicates the AV node. (H) Horizontal section stained with indicated antibodies at the atrial level of a BAC <i>Tbx5</i>^CreERT2/<i>ROSA26</i>^eYFP/eYFP mouse embryo at E10.5 after tamoxifen treatment at E7.5 <i>in vivo</i>. TBX5⁺ cells in the DMP (white arrows) were negative for eYFP. LA; left atrium, RA; right atrium, RAVC; right anterior vena cava. Scale bars: 100 μm (A and B), 20 μm (C–G) and 200 μm (H).