Hindlimb sensory neuron number increases with body size

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Abstract
As an animal grows, its sensory systems face the task of maintaining sensitivity and discrimination in peripheral fields that are continually enlarging. Without the addition of neurons, existing cells would have to innervate a wider skin area, leading to a decrease in the precision with which stimuli are localized. Neurons were counted in the three dorsal root ganglia (DRGs) that innervate the hindlimb of the bullfrog (Rana catesbeiana). Profiles of neuronal nuclei containing the single nucleolus found in these cells were counted in every third section of serially cut ganglia. This means of assessing neuron number was validated by comparing these profile counts with three-dimensional reconstructions of sensory neurons. Large frogs (10–17 cm) had more than twice as many DRG neurons as small frogs (3.3–5 cm). The rate of increase was greatest between 3 and 8 cm, when over 1,300 hindlimb sensory neurons were added for each 1 cm increase in body length. The possibility that selective survival of frogs with many neurons biases estimates of mean neuron number was ruled out by the finding that frogs drawn from the same closed population, half of which were sacrificed immediately and half of which were sacrificed after 1 year's survival, showed expected differences in neuron number. Horseradish peroxidase applied to particular hindlimb nerves retrogradely labeled more neurons in large frogs than small frogs, supporting the hypothesis that added neurons extend their axons to the periphery. No evidence of tritium labeling was found following injection of 3H-thymidine (a marker of DNA synthesis) every third day for 20–22 weeks, a period over which, on the basis of changes in body length, substantial numbers of neurons were likely added. These results, combined with previous studies of motoneuron addition in the lumbar lateral motor column, are consistent with the hypothesis that the population of sensory neurons is augmented by late differentiation of existing precursor cells.