Muscle morphology and cell proliferation after muscle injury in WT, uPA^−/−, and PAI-1^−/− mice. Muscles from WT, uPA^−/−, and PAI-1^−/− mice collected from uninjured control mice (Con) and on days 1, 3, 5, and 10 after cardiotoxin injury. BrdU (30 mg/kg) injected intraperitoneally into mice 1 hour before muscle harvest. (Top) Cryosections stained with either hematoxylin and eosin for morphologic analysis or a BrdU antibody to detect incorporation of BrdU into newly synthesized DNA. Scale bar represents 50 μm. (Bottom) Number of nuclei staining positive for BrdU 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).