On the development of the adult electric organ in the mormyrid fish Pollimyrus isidori (with special focus on the innervation).

The adult electric organ of the mormyrid Pollimyrus isidori consists of four longitudinal tubes, two dorsal and two ventral ones, entirely filling the space in the caudal peduncle, as in other mormyrids. Each tube comprises about 100 electrocytes possessing a large innervated stalk just behind their posterior faces. The numerous nerve terminals are deeply embedded into the surface of the stalk. The electrocytes begin to differentiate in 10-11 mm long fish (about 20 days old); they are initially arranged myotomically. The electromotoneurons are seen for the first time in 9.5 mm long fish (about 16 days old). In 12 mm long fish many putative nerve fibres are found surrounding the stalk, but it is only in 15-15.5 mm fish that nerve terminals establishing typical synapses (synaptic vesicles, synaptic cleft about 80 nm wide, and active zones) are first found. This coincides with the recording of the first electric discharges of the adult electric organ. At this stage the electrocytes are well developed, kidney-like and highly compressed, arranged in parallel in the caudal peduncle. Subsequently the number of nerve terminals increases, the terminals become more deeply invaginated into the membrane of the stalk, the myelin sheath terminating just at its surface. In 19 mm fish the first indication of penetrating stalks at the periphery of the electrocytes can be detected. With increasing length of the larvae the amplitude of the electric discharge also continuously rises. The electrocytes are acetylcholinesterase-positive, the activity is more pronounced over the stalk and the anterior face shows higher activity than the posterior face, mainly those at the periphery, where the penetrating projections are seen.