The developmental morphology of Torpedo marmorata: the electric lobes

The development of the electric lobes of Torpedo marmorata has been investigated using light and electron microscopical techniques. The lobe Anlagen become visible in the rhombencephalon along the floor of the 4th ventricle at the 10-mm stage. Many of the neuroepithelial cells in the Anlagen differentiate, Becoming postmitotic and axonic by the 24 mm stage. Proliferative zones of neuroepithelial cells disappear from the electric lobes by the 30-mm stage. After their initial, early differentiation the electromotor neurons remain monopolar until the 40-mm stage when dendrite formation begins. The differentiation of the electromotor neuron from a mono- to an immature multi polar form occurs between the 40- and 55-mm stages and involves, in addition to dendrite formation, a change from a pear-shaped to a spherical cell body, a dramatic increase in cytoplasmic volume, a centralization of the nucleus, an enlargement of the nucleolus and its migration away from the nuclear membrane, and differentiation of the axon hillock. The electric lobes are invaded by sinusoids at the 24-mm stage but formation of the capillary network by sprouting cords of endothelial cells begins later at the 40-mm stage. Neuronal cell death (26-74-mm stages) appears to be mainly an autolytic process and the debris is removed by immature glial cells. Afferent fiber growth cones are first recognized in the lobes at the 60-mm stage but synapses are not observed until the 78-mm stage. Myelination begins in the electric lobes concomitantly with the onset of synaptogenesis. A twofold increase in dendrite length occurs over the period when synapses begin to form in the lobes but dendritic maturation is not complete until the neonatal (120-mm) stage. The results are discussed in relation to the development of the electric organs.