Development of olivocerebellar fibers in the clawed toad, Xenopus laevis: a light and electron microscopical HRP...

An anterograde tracer study was undertaken to provide a light- and electron microscopical description of climbing fiber development in the clawed toad, Xenopus laevis, ranging from premetamorphic stages to the adult state. The inferior olive was unilaterally labeled with horseradish peroxidase and the contralateral climbing fiber morphology investigated. At early stages of development, only undifferentiated fibers were observed in the rostral alar plate. At later stages, these fibers form large varicosities, which contact presumed cerebellar Purkinje cells. Finger-like protrusions arising from the Purkinje cell somata penetrate the climbing fiber varicosities and form synaptic specializations at these contact sites. In older tadpoles, a large variety of climbing fiber morphologies was found displaying a mediolateral gradient. At dorsolateral cerebellar areas long and straight climbing fibers follow the Purkinje cell primary dendrites. However, in ventromedial areas pericellular baskets or nests were found on presumed Purkinje cell somata. These pericellular nests were found throughout development but were not observed in adult animals. Both pericellular nests and real climbing fibers make synaptic contacts on spiny protrusions of the Purkinje cell's somatic or dendritic surface. In several cases, labeled as well as unlabeled climbing fiber profiles were observed on the same Purkinje cell, indicating multiple, convergent innervation. Also, divergent Purkinje cell innervation was found. In conclusion, this study shows that anuran climbing fiber development encompasses stages and processes similar to those observed in mammals. The only principal difference with climbing fiber development in mammals is the low degree of synchrony observed in anurans.