Myogenic Stem Cell-Dependent Regeneration in Fish Figure 5. Pax7-positive cells are localized in blastema regions that give rise to muscle, electric organ and dorsal spinal cord. Serial cross-sections taken from distal half of 14-day blastemas (A and B) immunolabeled with antibodies against Pax7 (A, green) and MF20 (B, green). MF20 labels all mammalian myosin heavy chain (MyHC) isoforms present in all muscle fibers and developing electrocytes that arise from the fusion of muscle fibers. Immunolabeling by Pax7 and MF20 was detected in peripheral regions of the blastema underneath the epithelium. Pax7 immunolabel was also detected in cells within the dorsal spinal cord of regenerating blastema (A, arrowhead). C, Pax7 immunolabeling was detected along the entire regenerating dorsal spinal cord in 14-day blastemas as shown in this longitudinal section. Abbreviations: ECs, electrocytes; mm, muscle fibers; SC, spinal cord. Scale bars: A, 300 mm; B, 300 mm; C, 150 mm. doi:10.1371/journal.pone.0036819.g005 Pax7 cDNA and whole tissue homogenates from muscle and electric organ of S. macrurus that correspond to the expected sizes of known Pax7 proteins in mammals and other teleosts (Figure 3B). More importantly, cells associated with muscle fibers and electrocytes with the morphology and location similar to those that define myogenic satellite cells were immunolabeled with the Figure 6. Degeneration of electrocytes injured by amputation. Portions of longitudinal cryosections (14–30 mm thick) from 7-day (A) and 10- day (B) blastemas stained with hematoxyline and eosin. (C) 7-day blastema co-labeled with anti-Pax7 (red) and anti-laminin (green) antibodies. (D) 7- day blastema co-labeled with anti-Pax7 (red) and anti-BrdU (green) antibodies. Dashed line in A, B and D show the sites of tail amputation. Arrows in A, B, C and D point to electrocytes that were damaged by the amputation. E, Enlargement of the region within the dashed area in D to more clearly visualize the colocalization of Pax7 (red) and BrdU (green) as yellow. F, Correlation of Pax7 and BrdU immunolabeling in cells from control tails (Con), and tails after amputation at 7 (7 d) and 14 (14 d) days. Abbreviations: EC, electrocyte; Ep, epithelium; mm, muscle fiber. Scale bars: A, 2 mm; B, 5 mm; C, 500 mm; D, 200 mm. F, Range of Pearson’s correlation values for Pax7/BrdU co-localization for images from 3 different tails taken from control fish, and 7-day and 14-day amputated tails. doi:10.1371/journal.pone.0036819.g006 PLoS ONE | www.plosone.org 9 May 2012 | Volume 7 | Issue 5 | e36819