Patterns of engraftment by marked progenitor cells in the regenerating vasculature of injured muscle. SP and non-SP cells from lacZ-expressing Rosa26 mouse muscles were isolated by FACS analysis (upper panel) and injected into the injured muscles of genetically matched recipient mice. Engraftment of injected cells was monitored by β-galactosidase staining of recipient tissues. The SP fraction engrafted into vascular endothelium (bottom left), whereas the non-SP fraction engrafted into the vascular smooth-muscle (bottom right) of new blood vessels in the regenerating muscle.