Robust Regeneration of Adult Electric Pulse Fish and the Formation of Its Electric Organ from Fast Muscle Fibers

Abstract

In most weakly electric fish species, the cells that make up the electric organ (EO) are called electrocytes (EC) and these derive from a subpopulation of striated muscle fibers that undergo an extreme phenotypic conversion to replace their contractile function for an electrogenic one that fish use to navigate, communicate, mate, and detect prey/predators. Our previous findings using the knifefish Sternopygus macrurus revealed that ECs form from the fusion of fast muscle fiber types during adult tail regeneration. Whether or not this EC formation process is conserved in other highly regenerative electric fish species is the focus of this study. Specifically, we used immunolabeling to begin characterizing the morphology and muscle-protein expression of mature electrocytes in the species Brachyhypopomus pinnicaudatus and the muscle fiber type composition in the distal-most tail surrounding ECs prior to tail amputation. Regeneration of skeletal muscle and EO tissues was studied at different time points post tail cut – 1, 2, 3, 4, and 6 weeks. Our analyses to date show a distinct sexual dimorphism in EC length and cross-sectional area (CSA), with a mean (±SE) CSA of 4718µm2±543 in males (n= 3; triplicates per fish) and 1763µm2±181 in females (n=3; triplicates per fish) and a mean EC length of 19µm±2 in males (n= 3; triplicates per fish) and 13µm±1 in females (n= 3; triplicates per fish). Our immunolabeling studies detected the presence of the “muscle” protein desmin in all adult ECs. In addition, some but not all, ECs contained alpha-actinin, tropomyosin, titin, and sarcomeric myosin heavy chain (MHC) IIb. In B. pinnicaudatus, cell differentiation progressed from proximal to distal and peripheral to central, with muscle fibers being detected prior to ECs at each stage of regeneration studied. Myogenesis was evident by Day 7 with presence of fibers positively labeled with mouse antibody (MF20) against all MHC isoforms (n= 2). In addition, by Day 7, regenerating ECs were also evident in 1-week blastemas (n=2) fish based on the clustering of myofibers stained by cresyl violet and these early ECs were immunolabeled with MF20, cell shape transformation, and medial location relative to the muscle fibers. These data suggest that there are sexual dimorphic differences in the CSAs and lengths of ECs in the EOs of male and female B. pinnicaudatus. In addition, similar to S. macrurus, developing ECs in B. pinnicaudatus are given rise to by their type II fast muscle fiber precursors.