Specificity of motoneuron projection patterns during development of the bullfrog tadpole (Rana catesbeiana)

The pattern of connectivity between motoneurons of the lumbar lateral motor column (LMC) and hindlimb regions was examined in bullfrog tadpoles (Rana catesbeiana) over the course of larval development. The purpose of this study was to determine if a period of relatively imprecise connectivity, such has been found in the toad Xenopus laevis (Lamb, '76), could be identified. Patterns of connectivity were assessed by placing small amounts of the retrogradely transported enzyme horseradish peroxidase (HRP) into discrete hindlimb regions and mapping the locations of labeled motoneuron somata along the transverse and longitudinal axes of the LMC. The distribution of labeled motoneurons was as circumscribed in the youngest animals studied (st. IV of Taylor and Kollros, '46), before mesenchymal condensation into distinct myotubes, as in metamorphic or adult animals. This finding that the pattern of neuromuscular connectivity is as precise early in development as in mature animals is consistent with previous studies of chick hindlimb (Landmesser, '78). The relevance of these results to the hypothesis that naturally occurring cell death plays a substantial part in molding the mature pattern of neuromuscular connectivity (Lamb, '77) is discussed.