Defective regeneration of skeletal muscles and gene expression patterns in Pparδ -cKO mice after injury with cardiotoxin. (A) and (B) Tibialis anterior (TA) muscles from six-week-old wild-type and peroxisome proliferator-activated receptor δ conditional knockout (Pparδ-cKOmice were injected with cardiotoxin (CTX) to induce injury. The mice were allowed to recover for 14 days before their muscles were harvested and processed for staining. Small and large fibers (indicating regenerating and nonregenerating fibers, respectively) were counted. Scale bars = 50 μm for parts (A) and (B) and for parts (F) and (G). (C) Quantification of regenerating and nonregenerating fibers after injury with CTX (N = 3 pairs of mice, with five sections taken from each mouse). (D) Relative mRNA expression level showing reduced myosin heavy chain isoform 2b (MyHC-2b) in mutant muscle after injury with CTX (N = 4). (E) Relative mRNA expression levels of Pparδ target genes in the TA muscle after injury with CTX (N = 4). (F) and (G) Cryosections of regenerating wild-type (F) and mutant (G) mice at day 5 after injury with CTX. The sections are labeled with paired-box transcription factor 7 (Pax7) (red), myogenic differentiation antigen 1 (MyoD) (green) and 4', 6-diamidino-2-phenylindole (DAPI) (blue). Arrows, arrowheads and asterisks indicate examples of Pax7+/MyoD-, Pax7-/MyoD+ and Pax7+/MyoD+ myoblasts, respectively. (H) The number of total myoblasts labeled by Pax7 and/or MyoD per unit area counted from 8 to 13 areas. (I) Percentage distribution of Pax7+/MyoD-, Pax7+/MyoD+ and Pax7-/MyoD+ myoblasts in regenerating wild-type and mutant TA muscles at day 5 after injury with CTX. (J) Relative mRNA expression levels of Myf5 and Myogenin in TA muscles at day 3 after injury with CTX (N = 3). *P < 0.05 by single-tailed Student's t-test.