Characterization of skeletal muscle fiber types and electrocytes in the tail of the electric fish Apteronotus albifrons before and after regeneration following tail amputation.

In most groups of electric fish, cells that make up the electric organ (EO), or electrocytes, derive from skeletal muscle cells that suppress many muscle properties. This muscle‐to‐electrocyte conversion is repeated during tail regeneration in adult fish after tail amputation. One of few exceptions to the adult muscle‐derived EO is the Apteronotus albifrons . In A. albifrons , the first electrocytes that form during the larval stage do derive from muscle. However, an EO that is solely composed of the terminal endings of electromotor neurons subsequently replaces this larval EO. We hypothesize that following tail amputation, regeneration of the adult electric organ includes the process of formation of the myogenically derived EO as it occurs during larval stage of development. Adult fish were obtained commercially and housed individually in 15 to 20‐gallon aerated aquaria maintained at 25–28°C and fed three times weekly. Ten fish were anesthetized using 2‐phenoxy ethanol (1:1,500 in tank water), and the distal segment of the tail was transected at the caudal end of the anal fin, then returned to their tanks and monitored until they fully recovered from anesthesia. The cut control tail segments were mounted on cork, frozen in liquid nitrogen‐cooled isopentane, and stored at −80°C. These fish were re‐anesthetized 2 weeks after amputation and their regeneration blastema were removed and frozen as done with control tails. Longitudinal and cross‐sections (20μm‐thick) were mounted on glass slides and processed histologically for analysis of general cell type morphology. Immunolabeling studies used antibodies against markers known to detect different muscle fiber types, axon projections, myogenic stem cells, and nerve endings. These approaches will allow us to determine the muscle fiber type composition of the tail region that contains the adult EO in A. albifrons and the fiber type transition that occurs during regeneration. Our preliminary data shows that 1) type I and type II adult muscle fibers surround the EO with type II being more predominant in number, and 2) muscle proliferation is abundant along the entire periphery of the regeration blastema. Further immunolabeling studies are underway to complete testing our hypothesis. Support or Funding Information Funded in part by the NIH MARC Program Grant #GM07667‐41