Abstract
Spinal motoneurons of the axial muscles and dorsal root ganglionic neurons in glass eels of the Japanese eel Anguilla japonica were visualized by means of retrograde labeling with horseradish peroxidase (HRP). HRP-labeled spinal motoneurons were divided into two classes, “large” and “small”, based on their soma size and location in the spinal cord. The “large” motoneurons were located near the central canal in the dorsomedial part of the motor column and had extensive dendritic arborizations in the lateral neuropil layer of the cord. On the other hand, the “small” motoneurons were distributed over the whole area of the motor column, although relatively small cells in the “small” motoneuron group tended to be located in the ventrolateral part of the motor column. Presumptive somatic sensory neurons were labeled in the dorsal root ganglia which were situated ventrolateral to the spinal cord. The dorsal root ganglion was located just peripheral to the meeting of the dorsal root and the ventral root, and hence the motor axons traveled through the ganglion. The present observations revealed that no substantial anatomical changes in the spinal motoneurons of the axial muscles and the dorsal root ganglionic neurons of the Japanese eels occur during the metamorphic period, from the leptocephalus to the glass eel, during which time the eels undergo drastic changes of the somatic structures.
