LacZ staining occurs primarily at the border of myocardial infarction. (a) Lower-power (×10) photograph of mouse myocardial infarction after 4 weeks. The arrowhead points to the location of lacZ staining shown in b and c. The lighter pink tissue to the left and above the arrowhead is primarily fibrotic and results from the infarction. (b) Higher-power (×20) photograph of the same section dual stained for lacZ and the antimacrophage Ab F480. The open arrowhead indicates a macrophage, the closed arrowhead indicates lacZ-positive cardiomyocytes (the same region shown in Figure 4, g and h). (c) Higher-power photograph of the same section (×40). (d) Macrophage density of a cardiac section after 1 hour of ischemia and 3 hours of reperfusion. The open arrowheads indicate two of the many macrophages present. The counterstain is eosin.