Do primary afferent cell numbers change in relation to increasing weight and surface area in adult rats?

Adult mammalian primary afferent neurogenesis implies considerably more plasticity for the adult nervous system than is presently envisioned. One hypothesis is that such neurogenesis does not occur, because no tritiated-thymidine-labeled or mitosing neurons are observed in adults, and no increase in cells can be found in young as compared to old adults. The other hypothesis is that adult primary afferent neurogenesis does occur, but that it has not been observed because it is correlated with changes in the size of the animals. This has not been tested, and because of the implications of adult neurogenesis, it is important to see whether dorsal root ganglion (DRG) cell numbers increase with animal size. This is particularly pertinent because of recent advances in the technology for counting neurons and improved methods for resolving the cells in question. The present results indicate that DRG cell numbers are approximately the same in animals whose sizes are different by a factor of approximately 5. Thus our data are consistent with the hypothesis that numbers of adult DRG cells are stable, and we cannot demonstrate a change in relation to age or body size in our animals. Therefore, we do not find evidence of adult neurogenesis in our animals.