Postmetamorphic changes in the lumbar lateral motor column in relation to muscle growth in the toad, Bufo americanus

Motoneuron number and size (nuclear cross-sectional area) were measured from serially sectioned spinal cords of Bufo americanus to investigate the relation between changes in the lumbar lateral motor column (L-LMC) and postmetamorphic increases in hindlimb muscle fiber number. Previous studies of neuron number in a variety of anuran species reported a correlation between number and body size, suggesting the possible addition of neurons during growth. Our results show a poor correlation between motoneuron number and body size with at most a 25% increase in neuron number occurring over the body size range where previous work had shown a hindlimb muscle fiber increase of ten to 20-fold. Thus, most new muscle fibers must be incorporated into motor units that exist at metamorphosis. Motoneurons, but not ependymal cells, showed a significant size increase with increasing body size; this is perhaps related to an increased motor unit size that results from axonal sprouting. The range of variation of L-LMC cell numbers in newly metamorphosed toads was nearly equal to that of all other toads examined. This suggests that the weak correlations between motoneuron number and size observed in this and previous studies may be due to differential survival of individuals with larger neuron populations rather than to postmetamorphic addition of motoneurons. Our findings also show a strong bilateral correlation of motoneuron numbers, a finding suggesting that factors other than peripheral size may be important in regulating motoneuron number.