Electric organ development in Torpedo marmorata,chondrichthyes

The development of electric organs (EO) in the electric ray, Torpedo marmorata, is described. Four pairs of electric nerves contact the branchiomeric mesoderm layer, which produces four pairs of EO primordia (at about the stage of 21-mm total body length, 70-mg weight). These are the sites of column genesis, yielding the definitive column number at the stage of 25–27 mm (200–250 mg), and growing together on each side of the pharynx. Columns are composed of myotubes, each containing one myofibril and several nuclei. During the 35- to 55-mm stages (0.7 – 2.5 g), these “electroblasts” dedifferentiate, each giving rise to one electroplate derived only from its ventral half [“plate builder” (PB)]. Plate genesis appears as a perfectly synchronized process, involving coordinated changes of cell shape. The occurrence of cell fusions is not ruled out. At the same time, some individual contacts of PB with nerve fibers are seen. Reflex electric organ discharge (EOD) appears at the stages of 66–73 mm (6–7 g), when only a few nerve ending-like membrane enclosures are present and acetylcholinesterase (AChE) begins to accumulate. The parallel courses of EOD maturation, synaptogenesis, and AChE accumulation are described.