Myoblast accumulation after muscle injury in WT, uPA^−/−, and PAI-1^−/− mice. Muscles from WT, uPA^−/−, and PAI-1^−/− mice collected on days3 and 5 after cardiotoxin injury. (Top) Cryosections stained with antibody against the myoblast marker MyoD. Scale bar represents 50 μm. (Bottom) Number of nuclei staining positive for MyoD counted in 2 sections for each muscle, averaged, and expressed per square millimeter muscle area. Bars represent mean ± SE; n = 4 to 6 per time point. *Mean value significantly smaller than that for WT mice (P < .05). **Mean value significantly larger than that for WT mice (P < .05).