Neuroanatomy of Spastic, a behavior mutant of the mexican axoloti: Purkinje cell distribution in the adult cerebellum.

The spastic mutant, found in the Mexican axolotl, shows swimming coordination and equilibrium deficiencies. Histological analyses of wild-type and spastic mutant cerebella previously characterized in physiological studies revealed changes in Purkinje cell location in the mutant auricle or vestibulo-cerebellum. Purkinje cells are "translocated" ventrally correlated with a similar translocation of vestibular single units described previuosly (Ide, '77). Where wild-type Purkinje cells are distributed from the surface to a depth of 250 micrometers, mutant Purkinje cells are "crowded" between 250 and 350 micrometers. Although mutant granule cells are present, boundaries between granule cell and Purkinje cell zones are less precise in mutants. Cerebellar nucleus cells are translocated medially, failing to organize into the discrete cell group appearing in wild-type. Cerebellar white matter tracts and fibers show changes, both in orientation with respect to the underlying tegmentum, and in fascicular organization. Obvious changes in the gross anatomy of the cerebellum are confirmed in reconstructions which define cell and fiber translocation. Thus, the spastic gene is compatible with differentiation of all cerebellar elements, but appears to alter interactions between cells, or between cells and the external milieu. Although all cell types are present in the mutant cerebellum, they fail to attain their proper positions along all three body axes.