Development of the olfactory bulb in the clawed frog, Xenopus laevis: a morphological and quantitative analysis

The relationship between olfactory axons and the cells of the olfactory bulb during normal development was analyzed to determine whether olfactory afferent axons could play a role in the induction of olfactory bulb formation. The morphology of the olfactory bulb in Xenopus larvae from stages 26 to 58 and in adult frogs was analyzed with light and electron microscopy. Axons were first observed beneath the basal lamina of the neural tube at stages 30 and 32; at stage 32, neurons in this area of the neural tube began to differentiate. Synapses of olfactory axons onto young neuronal processes were observed as early as stages 36 and 38. By stage 44, all of the layers of the olfactory bulb were present. The basic structure of the mature form of the olfactory bulb was apparent as early as stage 48/49 and remained constant throughout late larval life and even into adulthood, with only the size increasing. To determine the numerical relationship between olfactory axons from both main and vomeronasal epithelia and mitral/tufted cells in the main and accessory olfactory bulbs, a quantitative study was also performed in which the number of olfactory axons and the number of mitral/tufted cells were estimated for larval stages (stages 50 to 58) and adults. The number of axons increased with stage, with a 16-fold increase between stage 58 and adulthood. The number of mitral/tufted cells increased with stage, with only a 2.3-fold increase between stage 58 and adults. There is a correlation between axon number and mitral/tufted cell number during larval development that is consistent with the hypothesis that olfactory axons influence olfactory bulb development. The convergence ratio of olfactory axons onto mitral/tufted cells was 5:1 in larvae and increased to 34:1 in adults; this increase probably results in increased olfactory sensitivity in adult frogs.