Incorporation of SP cells into cardiomyocytes. (a) Negative control: C57Bl/6 cardiac tissue stained for lacZ expression. (b) Positive control: C57Bl/6-Rosa26 cardiac tissue stained for lacZ expression. This typical section demonstrates both patterns of punctate and whole-fiber staining. (c) Cross-section of a heart from an SP cell–transplant recipient, which received an infarct. (d) Longitudinal section of an SP cell–transplant recipient, which received an infarct. (e–h) LacZ and α-actinin costaining of lacZ-positive fibers. (i) CD45 costaining of the section in g and h. (j) Anti-CD45 staining of spleen (positive control). Sections were stained with X-gal, and LacZ-positive sections were subsequently stained for α-actinin (f, h) and CD45 (i), and the sections were photographed.