Phylogenetically consistent features of cerebellar climbing fibers present in the tadpole

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Chemical identification and morphological characterization of the inferior olive in the frog

Tritiated D-aspartate was injected into the cerebellar cortex of grassfrogs, Rana temporaria. Retrograde labeling was observed in a cell column of the contralateral caudal medulla, but not in other areas known to give rise to cerebellar mossy fibers. The aspartate-positive neurons are therefore considered to represent the origin of cerebellar climbing fibers in the inferior olive, as reported earlier for rat and turtle by other investigators. Contrary to earlier reports we found no extraolivary climbing fibers in the VIIIth nerve and in Scarpa's ganglion. Our results support the view that the climbing fiber system of vertebrates is anatomically, physiologically and chemically very distinct and phylogenetically very conservative.

Purkinje cell maturation in the frog cerebellum during thyroxine-induced metamorphosis

Purkinje cell maturation during thyroxine-induced metamorphosis in premetamorphic bullfrog tadpoles was studied using electron microscopy and Golgi (silver-impregnated) preparations. Cerebella from tadpoles were examined following 1, 2, or 3 weeks of thyroxine treatment. Particular attention was paid to possible differences between the two populations of Purkinje cells previously described, i.e. (i) the smaller population located in the dorsal part of the cerebellum, where the Purkinje cells show dendritic arborization long before the appearance of the external granular layer, and (ii) the larger population located in the middle and ventral regions of the cerebellum, where the Purkinje cells begin to undergo maturation during metamorphosis when the external granular layer is established. Following thyroxine treatment, both populations of Purkinje cells showed rapid maturational change. In the mature (dorsal) group, dendritic growth resumed in the presence of an external granular layer increasing the complexity of their dendritic arbors. Moreover, climbing fiber synapses translocated from contacts on the soma to the thorns of growing dendrites, and somatic processes often disappeared. The immature (ventral) group showed dramatic differentiation of the perikaryon including polarization of cytoplasm with subsequent dendritic outgrowth and formation of somatic processes in the presence of climbing fibers. Stellate cell contacts appeared on the smooth portion of the soma of many Purkinje cells. Dendritic growth during thyroxine-induced metamorphosis was characterized by growth (elongation) with minimal branching, which is initially observed during spontaneous metamorphosis. Typically, these growing dendrites ended in growth cones, some with one or several filopodia. Developing Purkinje cell dendritic spines formed synapses with parallel fibers. The present study has provided an example of the dramatic nature of thyroxine's action in inducing the complex series of detailed maturational changes in the cerebellum 1-2 yr ahead of schedule. In addition, the results show that thyroxine-induced Purkinje cell maturation is more rapid and synchronous than that seen during spontaneous metamorphosis. It is concluded that Purkinje cell maturation during metamorphosis is largely dependent on thyroid hormone.

Afferent connections of the cerebellum in various types of reptiles

The origin of cerebellar afferents was studied in various types of reptiles, viz., the turtles Pseudemys scripta elegans and Testudo hermanni, the lizard Varanus exanthematicus, and the snake Python regius, with retrograde tracers (the enzyme horseradish peroxidase and the fluorescent tracer "Fast Blue"). Projections to the cerebellum were demonstrated from the nucleus of the basal optic root, the interstitial nucleus of the fasciculus longitudinalis medialis, the vestibular ganglion, and the vestibular nuclear complex, two somatosensory nuclei, viz., the descending nucleus of the trigeminal nerve and the nucleus of the dorsal funiculus, the nucleus of the solitary tract, the reticular formation, and throughout the spinal cord. A distinct bilateral projection to the cerebellum was found to arise in a nucleus previously called nucleus parvocellularis medialis (Ebbesson, '67). In the present study this cell mass is termed the perihypoglossal nuclear complex, considering its comparable position and fiber connections to the perihypoglossal nuclei in mammals. In all reptilian species studied a contralateral cerebellar projection of a cell mass located in the caudal brainstem adjacent to the nucleus raphes inferior was observed. It seems likely that this cell mass represents the reptilian homologue of the mammalian inferior olive. Most of the spinocerebellar fibers appeared to arise in neurons located in area VII-VIII of the gray matter. In this respect the origin of the spinocerebellar projection in reptiles resembles the origin of the rostral and ventral spinocerebellar tracts in mammals. No indications for the existence of a column of Clarke or a central cervical nucleus in the reptilian spinal cord were obtained. On comparison of the cerebellum afferents in reptiles with the known connections of the cerebellum in amphibians, birds, and mammals, a basic pattern of cerebellar afferent projections appears to exist in these vertebrate classes, including retinal, vestibular, precerebellar, somatosensory, and spinal afferents.