Influence of olfactory innervation on neurogenesis in the developing olfactory bulb of the frog, Xenopus laevis

Previous research on development of the Xenopus olfactory bulb from our laboratory has shown that mitral cells begin to differentiate after olfactory axons make contact with the olfactory bulb, and the number of olfactory axons is correlated with the number of mitral cells throughout development. In embryos, removal of all afferent innervation before the mitral cells begin to differentiate results in a failure of the bulb to form; removal of half the olfactory axons, results in development of half the normal number of mitral cells. At larval stages, transection of the olfactory nerve results in a decrease in the number of neurons in the olfactory bulb. Thus, the olfactory axons play a major role in stimulating or maintaining development of the olfactory bulb neurons. Since we have found that neurogenesis in the bulb continues through metamorphosis, the goal of the current study was to determine whether olfactory axons influence cell genesis and/or neuronal maturation in the larval olfactory bulb. To study cell genesis, we cut the olfactory nerves, and 6 days later, injected the animals with [3H]thymidine. After 6 hr, the animals were killed and the tissue was processed for autoradiography. The number of labeled cells in the ventricular zone of the olfactory bulb was determined in every fifth section through the bulb in control and experimental animals. There was no statistical difference (Mann-Whitney rank sum test) in the number of labeled ventricular cells between controls and experimentals. Thus, olfactory axon innervation does not appear to play a role in precursor cell division during larval development. To study the influence of olfactory axon innervation on the ability of newly generated neurons to mature, we followed the same procedures. However, the animals were killed 21 days after the [3H]thymidine injection. The results from this experiment showed that there are many fewer labeled mitral cells in the experimental animals at 21 days. Together these results suggest that sensory deafferentation influences mitral cell differentiation or survival even during late stages of larval development.