The quantitative relationship between olfactory axons and mitral/tufted cells in developing Xenopus with partially deafferented olfactory bulbs.

Partial deafferentation of the olfactory bulb in Xenopus embryos was performed to analyze the effects of afferent innervation on the development of the central olfactory structure. In an attempt to analyze a possible early inductive role of the olfactory axons, one olfactory placode was removed before differentiation of the neural tube began (stages 26-31). A morphological and quantitative analysis was performed on larvae at the onset of metamorphic climax (stage 58). When the single olfactory nerve innervated one side of the rostral telencephalon, a single olfactory bulb developed on that side and no olfactory bulb formed on the contralateral side. When the nerve innervated the midline of the rostral telencephalon, a smaller-than-normal, fused olfactory bulb developed. Partial deafferentation at these early stages resulted in a significant reduction in the number of olfactory axons (to approximately one-half of control values) and a corresponding decrease in the number of mitral/tufted cells (output neurons of the olfactory bulb). To control for possible damage to the neural tube during olfactory-placode removal, a portion of the neural tube directly beneath one of the olfactory placodes was removed in embryos. In these animals, the neural tube regenerated within 24 h and formed a normal olfactory bulb; olfactory axon and mitral/tufted-cell numbers were not significantly different from controls. In conclusion, olfactory-afferent innervation was critical for differentiation of the olfactory bulb, and decreasing the number of olfactory axons resulted in a reduction in the number of output neurons of the olfactory bulb.