The developmental morphology of Torpedo marmorata: electric lobe-electromotoneuron proliferation and cell death.

Electromotoneuron proliferation and cell death have been quantitatively studied in the electric lobe of Torpedo marmorata from an embryonic body-length stage of 26-mm to adult animals. These neurons project to the electric organ and form synapses with electrocytes which possess a remarkably large postsynaptic target surface. For this reason cell death would not be predicted to occur if synaptic competition were to be hypothesized as the cause. Isolated observations at the ultrastructural level suggested, however, that cell death was indeed taking place and therefore it seemed appropriate to examine this question in detail. Our findings show first that neuron production appears to be a continuous process throughout the period studied, generating totals of over 70,000 electromotoneurons per lobe by adulthood. Second, two waves of cell death were identified, one occurring early in embryogenesis (stage 30 mm), well before the onset of synaptogenesis, and a second coincident with the onset of synaptogenesis (stages 55--74 mm). It is difficult to reconcile this latter wave with the hypothesis of synaptic competition as the postsynaptic surface at this time of development is largely devoid of synaptic contacts. We conclude that in the electromotor system of Torpedo, synaptic competition is probably not the mechanism of cell death.