Figure 3. Epicardial HC clusters respond dynamically to myocardial injury.
Following MI, there is an initial proliferative response as determined by BrdU+ pulse labelling of cluster cells 1 day post injury (1 d.p.i.) in both the left (LV) and right (RV) ventricles (white arrowheads highlight BrdU+ cells; a,b). CD45+ cells located within the epicardial layer were confirmed as undergoing cell cycle activity via Ki67 co-staining in both the LV (c,d) and RV (e,f; white arrowheads highlight CD45+/Ki67+ cells). As early as day 2 post MI (2 d.p.i.) in the LV, the clusters begin to break down (g). Disassembly of ECM-encapsulated cell clusters persists through day 4 (4 d.p.i.; i) and progresses to day 7 (7 d.p.i.; k) to release CD45+ cells (g,i,k) This is not the case in the RV where the clusters remain intact over the same time course of injury constraining the resident cell types (h,j,l); white arrowhead in l highlights intact cluster 7 d.p.i. By day 21 post MI (21 d.p.i.), the clusters have reformed (white arrowheads in higher magnification view highlight Fn at the apical surface) in the LV (m) to resemble the persistent intact clusters of the RV (n) and pre-injury state (Fig. 1); the reinstated ‘niche’ is evident 42 days post MI (42 d.p.i.; o) and is comparable to that in the RV as highlighted by white arrowheads (p). White inset boxes in the left panels are highlighted at higher magnification in neighbouring right panels. ep, epicardium; my, myocardium. Scale bars, 50 μm (a–p, left panels); 20 μm (g,i,j,l,n); 10 μm (a,b,h,k,m–p, right panels).