Motoneurons of the tail of young Xenopus tadpoles.

The purpose of this study was to identify, characterize, and follow the morphogenesis of motoneurons innervating tail myotomes of Xenopus during early and middle larval stages (33-48). Application of horseradish peroxidase to a single myotome results in a column of labeled motor cells extending over four spinal segments. By midlarval stages two morphologically distinct cell types were recognizable in this column-primary and secondary motoneurons. Primary motoneurons occupied the more rostral and secondary motoneurons the more caudal portions of the column with some overlap of the two populations. Primary neurons developed earlier, were larger, and displayed thicker axons and proximal dendrites. Secondary neurons, in contrast, were smaller, more variable in configuration and dendritic distribution, and displayed thinner axons and dendrites. Development of the labeled motor column during the period observed involved the addition of more labeled neurons, principally secondary cells, and the enlargement of individual cell somata with the progressive elaboration of more and longer dendrites into the developing lateral fasciculus.