(A) Glycerol-induced injuries result in impaired skeletal muscle regeneration of uPA- but not tPA-deficient mice. Frozen sections of muscles from wild-type (WT), uPA-deficient, and tPA-deficient mice were stained with H&E 2, 5, 7, 9, and 20 days after injury, as indicated. Contralateral control muscles were also stained with HE (0 days after injury). In WT and tPA-deficient mice, well-advanced regeneration is visible after 5 days, and regeneration is complete after 9 days. In uPA-deficient mice, a regeneration defect is already visible 5 days after injury but is most striking at 9 and 20 days. In WT and tPA-deficient mice, virtually no sign of the previous injury was detectable after 20 days, except for the centrally located myonuclei. Original magnification, 400 ×. (B) Persistence of developmental MHC-positive myofibers 9 days after glycerol injury in uPA-deficient mice. Cryostat frozen sections from WT and uPA-deficient mice were reacted with a monoclonal antibody against developmental MHC (MHCd) at 5 and 9 days after injury, as indicated in the figure (5 and 9 days after injury). Large numbers of MHCd-expressing myofibers are visible in regenerating muscle of uPA-deficient mice 5 and 9 days after intramuscular glycerol injection. In WT mice, fibers positive for MHCd are observed 5 days after injury, whereas no MHCd expression is detected 9 days after injury.