Interactions between the olfactory placode and the olfactory bulb during development in Xenopus laevis.

Abstract

The relationship between olfactory receptor-cell axons and the output cells of the olfactory bulb (the mitral/tufted cells) was analyzed during normal development and following experimental manipulation to investigate the role of olfactory afferent axons in the induction of olfactory-bulb formation. A light- and electron-microscopic study of Xenopus embryos, larvae, and adult frogs outlined the major events in olfactory-bulb formation. Axons were observed in the neural tube immediately before the future olfactory-bulb neurons began to differentiate. Quantitative analysis during normal development indicated that both olfactory-axon number and mitral/tufted-cell number increased and were positively correlated throughout larval development. By adulthood, however, the number of axons increased significantly more than the number of mitral/tufted cells. The correlation of axons to mitral/tufted cells in larvae and the time course of normal, larval development are consistent with the hypothesis that olfactory axons influence olfactory-bulb development. To examine further the relationship between the cons and output cells, experimental manipulations to alter the number of incoming cons were performed. The effects of partial deafferentation of the olfactory bulb, by removal of one olfactory placode before differentiation of the neural tube, were analyzed. This manipulation had a major effect on the morphology of the olfactory bulb and resulted in a significant reduction in olfactory-axon number (to about one-half of control values) and a corresponding decrease in mitral/tufted-cell number. The effects of transplanting an extra olfactory placode on the development of the larval olfactory bulb were also examined. The transplanted tissue retained the normal appearance and cell types of olfactory epithelium. The volume of the olfactory bulbs in animals with a transplant was not significantly different from control values. While there was initially an increase in total con number, by late-larval stages, the number of olfactory cons was not different from control values. This suggests that the olfactory bulb regulates the number of afferent fibers. The number of mitral/tufted cells at early- and late-larval stages was not affected by this manipulation. Therefore, it appears that there are reciprocal interactions between the olfactory epithelium and the olfactory bulb during development.