a-c, Immunocytochemical analysis of p-Marcks positive epicardial cells with sm α-actin (b) and p-Cytokeratin (c) primary antibodies indicate endothelial and/or smooth muscle cell fate seven days after culturing. (a) endothelial marker positive cells highlighted by green arrowheads, smooth muscle marker positive cells highlighted by red arrowheads. d-i, sm α-actin, Cytokeratin, Pecam-1 (red) and p-Marcks (green) co-immunostaining with DAPI (blue) reveals cellular heterogeneity in the TB4 activated (d,f,h) or in the single layered epicardium (e,g,i) of adult control hearts. White arrowheads in d-i indicate p-Marcks positive cells with endothelial or smooth muscle cell fate in the developing capillaries. j-l, Immunohistochemical analysis of the mature coronary vessels suggest smooth muscle and endothelial cell fate for p-Marcks positive cells. ec, epicardium; e, endothel; sm, smooth muscle; m, mesenchyme