© 1978 by The Society for Integrative and Comparative Biology
Genetic Dissection of Cerebellar Development: Mutations Affecting Cell Position
Jenkins Dept. of Biophysics, The Johns Hopkins University Baltimore, Maryland 21218
SYNOPSIS. The spastic mutation induces swimming coordination and equilibrium deficiencies in the Mexican axolotl (Ambystoma mexicanum). Behavioral ontogeny studies determined that spaslks fail to develop behavior trains of sinusoidal flexures necessary to mediate escape swimming at the time of onset of cerebellar function. Behavior analysis, after lesioning different cranial nerve roots and CNS areas in wild-type animals, confirmed the "behavioral focus" of the mutation to lie in the auricle or vestibulo-cerebellum. Single unit recordings in the cerebellar auricle and adjacent brainstem vestibular zone (area acoustico-lateralis) of mutants revealed a full complement of vestibular unit types found in wild-type. However, the gene appears to alter the physical location of vestibular units in both areas, including a ventral "translocation" of cerebellar units responding to sustained ipsilateral tilt. Correlated with this unit translocation, mutant Purkinje cells and allied afferent tracts are malpositioned ventrally, i.e., "crowded" into an ectopic zone in the ventro-posterior cerebellum. Studies on cerebellar structure at the time of onset of spasticity (early feeding stage) confirmed the ventral malpositioning of cerebellar cells and fiber tracts seen in adults. In conjunction with these larval studies, mutant larvae injected with tritiated thymidine during early cerebellogenesis and assayed at the early feeding stage revealed a medio-ventral malpositioning of labelled cells; in wild-type, labelled cells were positioned laterally. Interestingly, the neuropathology of the reeler mutation (rev., Mariani et al., 1977) found in the mouse is remarkably similar to that of the spastic axolotl. Both cerebella are reduced in size, misshapen, and lack fissures. Purkinje cells appear ectopically in the granular layer, white matter and deep cerebellar nuclei. Although both cerebella lack structural integrity, no individual cell type shows marked or progressive degeneration, and cellular untrastructure appears intact; thus, both genes appear to act independently of the genesis of cerebellar elements. Instead, they appear to influence morphogenetic movements by which presumptive cerebellar cells attain their proper positions during the neurogenetic sequence.