Endogenous Epicardial Regenerative Response Figure 1. Epicardial activation in response to myocardial ischemia. Panels A–J show the pattern of expression of cTnI (A,F), Raldh1 (B,G), Raldh2 (C,H), Tbx18 (D,I) and Wt1 (E,J) mRNA in the normal healthy heart. Raldh1 (B), Raldh2 (C), Tbx18 (D) and Wt1 (E) are expressed in the epicardium and the adjacent subepicardial mesenchyme of the atrioventricular sulcus to various extends. Raldh1 (G) is expressed throughout the epicardium overlaying the atria. Raldh2 (H), Tbx18 (I) and Wt1 (J) are expressed in a small subset of the epicardial cells overlaying the atria. Panel K shows a representative four-chamber view of an adult heart one day post-MI; the absence of expression of cTnI mRNA demarcates the infarcted area. The boxes indicate the atrium (1), atrio-ventricular sulcus (2), infarcted area of left ventricular free wall (3), the apex (4), and the right ventricular free wall (5). In the subsequent panels we have indicated this number at the left top site to indicate the position the picture was taken. Wt1 mRNA is expressed in the epicardium covering the ischemic region three days post-MI (L), to a level comparable to the one in the epicardium covering the atria (M). At one week post-MI, Wt1 is expressed in the epicardium covering the apex and left ventricle (N). Tbx18 (O–R) is expressed throughout the epicardium of the atria (O), within the atrioventricular sulcus (P), and the epicardium overlaying the infarct (Q) and it border zone (R). Raldh1 (S–U) is expressed throughout the epicardium covering the ventricles after a myocardial infarction (S,T), and in the subepicardial mesenchyme that is found in the wide subepicardial space overlaying the border zone of the infarction (U). Raldh2 (V) is expressed throughout the epicardium at one week post- MI. To validate these data we performed a qPCR analysis (W). As expected cTnI mRNA is down-regulated immediately after the induction of the infarction. Wt1, Tbx18 and Raldh1 mRNA become transiently up-regulated, being most highly expressed at 3d post-MI. Abbreviations: LA; left atrium, LV: left ventricle, RV: right ventricle, AVS: atrioventricular sulcus, Epi: epicardium, Subepi: subepicardium. The bar is 100 mm. doi:10.1371/journal.pone.0044692.g001 PLOS ONE | www.plosone.org 4 September 2012 | Volume 7 | Issue 9 | e44692