Defects of neuronal migration and the pathogenesis of cortical malformations are associated with Small eye (Sey) in the mouse, a point mutation at the Pax-6-locus.

The mouse Small eye (Sey) locus is situated on chromosome 2. Molecular analyses have shown that SeyNeu represents a point mutation leading to a splice site error and loss of the functional gene product. The Sey locus has been shown to be identical with the paired box (Pax)-6 gene, which contains paired-like and homoeobox domains and is a developmental control gene. Pax-6 expression occurs in many parts of the central nervous system during embryogenesis. Therefore, we may expect the Sey mutation to result in abnormal development of the central nervous system. The present study shows that Pax-6 mutation has a bimodal effect upon neurogenesis in mouse: it causes a delay of premigratory neurons in a stage-, region-, and gene-dose-dependent manner. Additionally, Sey mutation impairs axonal growth and differentiation. Neurons of the cortical plate cease differentiation on approximately day 16 of gestation and appear to have increased cohesion: their cytoplasm is swollen and vacuolated. These changes coincide both with reduced formation of axons and with the onset of vacuolar degeneration in existing axons, glial cells and radial glial fibers. Consequently, there is an impairment of the peripheral migration of putative neurons so that the neonatal lesion pattern of the neocortical roof becomes dominated by a broad spectrum of neuronal migration disorders.