Development of spinocerebellar afferents in the clawed toad, Xenopus laevis

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

Morphological evidence for a monosynaptic connection between cerebellar Purkinje cells and vestibulospinal tract neurons in the larval clawed toad, Xenopus laevis

A double-labeling technique was used to trace synaptic connections between the efferent neurons of the cerebellum (Purkinje cells) and vestibulospinal tract neurons in larvae of the clawed toad, Xenopus laevis. The efferent cerebellar projection was labeled with horseradish peroxidase (HRP) while in the same preparations the brainstem neurons projecting to the spinal cord were labeled with cobaltous lysine. It was found that the distribution of the Purkinje cell terminal boutons overlaps significantly with the location of vestibulospinal neurons in the brainstem. Moreover, several close appositions were seen between Purkinje cell boutons and the dendrites and somata of these latter neurons. The close appositions seen in light microscopy were confirmed by subsequent electron microscopy. This study shows that early in development (at least well before metamorphosis) a cerebello-vestibulospinal connection exists in Xenopus laevis. This connection is likely to persist throughout metamorphosis to the adult state.