BROMODEOXYURIDINE LABELING REVEALS A CLASS OF SATELLITE-LIKE CELLS WITHIN THE ELECTRIC ORGAN

When the electric organ (EO) of weakly electric fish is amputated, a b lastema forms from which new EO and muscle cells arise. However, the p rogenitor cells that contribute to the blastema are unknown. We studie d regeneration of the electric organ in Sternopygus to answer this que stion. The EO of this species is composed of electrocyte cells surroun ded by peripheral bundles of muscle fibers. Fish were injected with 5' -bromodeoxyuridine (BrdU) 24 h after amputating the terminal portion o f the EO. At this time, a population of small cells were labeled in th e extracellular matrix between electrocytes and muscle fibers. These c ells did not label in control fish injected with saline or in nonamput ated BrdU-injected fish. For the first 6 days postamputation, increasi ng numbers of BrdU-labeled cells appeared at the wound margin. A blast ema formed 6 days after amputation and contained numerous BrdU-labeled cells. At 10 day's postamputation, clusters of BrdU-positive cells we re seen throughout the wound margin and proximal blastema. At 14 days, BrdU-labeled nuclei were present within developing electrocytes. Labe ling alternate sections with MF20 antimyosin and AE1 anticytokeratin a ntibodies confirmed that BrdU-positive multinucleate cells coexpress m yosin and cytokeratin epitopes, diagnostic of newly regenerated electr ocytes. Electron micrographs reveal that the small cells surrounding m uscles and electrocytes are similar; they contain an elongate nucleus, are largely devoid of cytoplasm, and possess few organelles. This mor phology and evidence of myogenic potential suggests that these cells a re satellite cells.