Figure 2. Six1 is expressed in cardiac progenitors.

(A–F) Six1 is expressed in cardiac/skeletal muscle progenitors and in cranial ectoderm and endoderm at E7.5 and E8.5. WT (A) and Six1^Cre/hpAP (B–F) embryos were stained for hpAP activity. Cross-sections of B are shown in C and D; cross-section of E is shown in F. CC, cardiac crescent; Ect, ectoderm; End, endoderm; NE, neural epithelium; SM, somite. (G) Whole-mount dorsal-right view of the looped heart from an E9.0 Six1^Cre/hpAPR26R^mTmG embryo. eGFP⁺ cells (Rosa/eGFP) in the Six1 lineage were found in inflow tract (IFT), OFT, and developing apex. (H and I) eGFP⁺ cells in the Six1 lineage and Isl1 colabeled SHF cardiac progenitor cells (arrowheads) in E9.0 sagittal sectioned tissue. Boxed region in H is enlarged in I. (J–N) Coimmunostaining of eGFP⁺ cells in the Six1 lineage with TNNT2 (J) and ACTN2 (K) in the RV and with SM22α (L) and CNN1 (M and N, arrows) in the proximal region of the great arteries from E17.5 hearts. Boxed region in M is enlarged in N. Original magnification, ×115 (A and B); ×100 (C, D, and F); ×63 (E); ×200 (G–N). (O) Summary of Six1⁺ progenitor cell lineage, based on results from Figure 2 and Supplemental Figure 5 (WT1, RALDH2; epicardial progenitor data).