Neuronal turnover in the Xenopus laevis olfactory epithelium during metamorphosis

Metamorphic changes in the amphibian olfactory system present many interesting questions concerning the competing possibilities of neuronal respecification versus replacement. For example, are olfactory neurons retained during this transition with their presumed sensitivity to waterborne versus airborne stimuli respecified, or are olfactory neurons completely replaced? We address this question using the African clawed frog (Xenopus laevis) as a model. The water-sensing nose (principal cavity; PC) of larval X. laevis is respecified into an air-sensing cavity in adults, with changes in odorant receptor gene expression, ultrastructure, and site of innervation of the receptor neurons. The vomeronasal organ (VNO) does not appear to change function, structure, or innervation during metamorphosis. We labeled PC and VNO olfactory receptor neurons with injections of retrogradely transported fluorescent microspheres into the main and accessory olfactory bulbs. Injections were performed in larvae, and animals were allowed to survive through metamorphosis. After metamorphosis, few labeled cells were observed in the PC, whereas the VNO and the olfactory bulbs remained heavily labeled. Animals that were killed before metamorphosis always had extensive label in the PC epithelium regardless of how long the beads were present. This suggests that changes in the PC olfactory epithelium that are seen during metamorphosis are due primarily to turnover of the neurons in this epithelium rather than to respecification of existing neurons. These results also are discussed in terms of natural turnover time of olfactory receptor neurons.