Figure 3. Sca-1⁺ cells in the adult heart have minimal myogenic potential.

(A) Diagram of the Sca-1^MerCreMer/+ allele. Sca-1^MerCreMer/+ mice were crossed with the ROSA26R^tdTomato reporter to obtain Sca-1^MerCreMer/+;ROSA26R^tdTomato/+ double heterozygous mice. Tamoxifen was administered 9 times in one month (days 1, 3, 5, 7, 11, 15, 19, 23 and 27) to induce Sca-1^MerCreMer expression. (B) A representative Sca-1^MerCreMer/+;ROSA26R^tdTomato/+ heart showed substantial ROSA26R^tdTomato-positive cells present after tamoxifen treatment (red, arrows in B2). (C-E) Immunostaining with an anti-PECAM antibody (green) of Sca-1^MerCreMer/+;ROSA26R^tdTomato/+ hearts at P30, P60 and P90 after 1 month of tamoxifen treatment. ROSA26R^tdTomato cells co-localized with PECAM staining in these hearts (yellow, arrows in C2, D2 and E2). (F) Diagram of the Sca-1^MerCreMer/+;cTnTn^{LacZ-H2B-GFP/+} double heterozygous alleles. cTnT2^H2B-GFP was expressed when Cre activity was specifically induced in cardiomyocytes. Hearts were collected after 1 month of tamoxifen treatment. (G) All cryosections (10 μm) of Sca-1^MerCreMer/+;cTnTn^{LacZ-H2B-GFP/+} mouse hearts at P60, P90, P120 and P180 were examined under a microscope. Very few GFP cells were detected (arrows in G), and the total number was determined and is shown in the corner. LV, left ventricle; RV, right ventricle. Black scale bar, 1 mm. White scale bar, 100 μm.