Postnatal neuron addition, if it occurred, would have profound implications both for the conceptualization of developmental processes and for efforts directed at replacing neurons that were lost to injury or disease. Although dorsal root ganglia (DRGs) offer the advantages of clear boundaries and functional homogeneity, studies comparing neuron number in the DRGs of animals of different ages or sizes have yielded conflicting results. In the present study, neuron number in DRGs L3-L6 was compared in neonatal (approximately 11 days old, mean weight of 24.5 g, mean volume of 25 cm3) and adult (approximately 80 days old, mean weight of 373.5 g, mean volume of 346 cm3) male Sprague-Dawley rats. Estimates of neuron number were derived by using both stereological (physical disector) and profile-counting (one or more nucleoli within a nucleus) methods. The reliability and validity of the two methods were evaluated by comparing estimates of neuron number with those derived from three-dimensional reconstruction of a subset of neurons. The recommended protocol for using the physical disector was found to give accurate estimates of neuron number, but the heterogeneous distribution of neurons in the ganglion led to sampling errors of up to 50%. Reliability was improved by increasing the number of disector pairs examined. Counts of nuclear/nucleolar profiles were more reliable, but introduced a bias that worked against the experimental hypothesis in that estimates of neuron number in neonates exceeded actual values. Nonetheless, both methods indicated that adult rats had more DRG neurons than did neonates. Profile counts were 19% higher in adults (P < .01, two-tailed t-test); and data obtained by using the physical disector showed that adult rats had 28% more neurons than did neonates (P < .05). The difference in neuron number between adults and neonates could be due either to neuron proliferation or to late differentiation of neurons that do not assume a typical appearance until adulthood.

The spinal cord of the bullfrog (Rana catesbeiana) tadpole contains primary neurons, born during embryonic stages, and secondary neurons born for the most part during larval stages. Electrophysiological and anatomical characteristics of these two categories of neurons were examined during larval development to trace the development of secondary neurons and to determine whether primary neurons persist into adult life or are replaced by secondary neurons. Five classes of primary neuron were identified on the basis of their distinctive locations, morphologies, cytoplasmic melanin content, and presence at the earliest larval stages examined: primary motoneurons, Rohon-Beard cells, commissural cells, dorsal marginal cells, and anterolateral marginal cells. Secondary neurons of the lateral motor column and dorsal root ganglia underwent extensive developmental changes during larval life manifested both in anatomical studies with horseradish peroxidase and electrophysiological experiments on the isolated spinal cord. Primary motoneurons that innervate the tadpole tail were not found in the adult, although those innervating thoracic musculature persisted, as did at least some primary neurons projecting to other spinal segments or brainstem. Primary neurons are thus replaced or maintained through metamorphosis depending on their class and location.

Motoneuron number in the lumbar lateral motor column of the bullfrog, Rana catesbeiana, was investigated through the course of premetamorphic development and in postmetamorphic frogs. Motoneurons were distinguished on the basis of histological characteristics into two classes, type L (less differentiated) and type M (more differentiated). The number of type L motoneurons on each side showed a precipitous decline between stages V and VI (6,300 to 2,500) and a slower rate of loss until stage XI (to 550). Type M motoneurons increased in number between stages V and VII (560 to 2,775) and declined precipitously between stages VII and VIII to a value similar to that of juvenile frogs (1,100). These changes in motoneuron number do not correspond to the formation of myotubes or to the appearance of contractile properties in hindlimb muscles. The development of myotubes in the hindlimb occurs only after total motoneuron number has declined by 35%. Similarly, hindlimb muscle contraction develops after the early decline in type L motoneuron number and is restricted to proximal thigh at the peak of type M motoneuron number. In postmetamorphic frogs, a weak (r = 0.44) but statistically significant correlation was found between type M motoneuron number and body length. In the largest frogs (greater than 15 cm body length), 1262 +/- 157 (mean +/- s.d.) motoneurons were present, whereas the smallest frogs (less than 5 cm body length) had 1099 +/- 98 motoneurons. These results are not consistent with previous findings that the variance of motoneuron number among small frogs is greater than that among larger frogs. The present results are thus inconsistent with explanations of size-related differences in motoneuron number that are based on selection of small frogs with greater number of motoneurons for survival. The increase in motoneuron number may be due to a slow addition of newly born motoneurons to the LMC or to the differentiation of existing motoneurons. The latter possibility is supported by the finding that the number of presumptive type L profiles is less in larger frogs.

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