Figure EV2. Localization and quantification of mG‐labeled cells in Isl1 ^MerCreMer/+; R26 ^mTmG/+ and Wt1 ^{Cre ERT 2/+};R26 ^mTmG/+ mouse embryos at E9.5.

1. Temporal restriction of Cre‐mediated labeling of Isl1⁺ and Wt1⁺ progenitors and their derivatives using tamoxifen‐inducible Isl1 ^MerCreMer/+;R26 ^mTmG/+ and Wt1 ^CreERT2/+;R26 ^mTmG/+ mouse lines. Labeling was induced by tamoxifen treatment at E7.5, and embryos were analyzed at E9.5. Immunostaining of cTNT (red), Wt1 (magenta), Isl1 (magenta), and mG (green) in Isl1 ^MerCreMer/+;R26 ^mTmG/+ (upper panels) and Wt1 ^CreERT2/+;R26 ^mTmG/+ (lower panels). The boxed regions in the left panels are shown in higher magnification (in consecutive sections) in the four right panels a–d and a'–d' for Isl1 ^MerCreMer/+;R26 ^mTmG/+ and Wt1 ^CreERT2/+;R26 ^mTmG/+, respectively. Isl1 ^MerCreMer/+‐mediated mG labeling was absent in the PEO (a), but it was observed in epicardial cells (b), Isl1⁺ SHF progenitors (c), and CMs (d), as indicated by arrows. Wt1 ^CreERT2/+‐mediated mG labeling was detected in Wt1⁺ cells of the PEO (a') and in epicardial cells (b'), as indicated by arrows; mG expression was absent in Isl1⁺ SHF progenitors (c') and CMs (d'). Scale bars, 100 μm (left panels), 25 μm (right panels).
2. Table summarizing the regional distribution and number of mG‐labeled cells in Isl1 ^MerCreMer/+;R26 ^mTmG/+ and Wt1 ^CreERT2/+;R26 ^mTmG/+ mouse embryos at E9.5 as determined in (A). Epi, epicardium; LV, left ventricle; RV, right ventricle.
3. Bar graph depicting the percentage of the regional distribution (PEO, epicardium, SHF and myocardium) of mG⁺ cells in Isl1 ^MerCreMer/+;R26 ^mTmG/+ and Wt1 ^CreERT2/+;R26 ^mTmG/+ mouse embryos at E9.5. Major contribution of Isl1‐derivatives was detected in the SHF and myocardium, while most Wt1‐derivatives were found in the PEO. Both Isl1‐ and Wt1‐expressing progenitors contributed to the epicardium.