Morphogenesis of neuronal microdysgenesis in hippocampal formation of Ihara's genetically epileptic rat (IGER)

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Experiment J. Structural property of microdysgenesis in IGER(Ihara's genetically epileptic rat) hippocampus was investigated. In both two and twelve month-old IGERs and in both sexes, the ectopic neuronal clusters were found in the region of stratum radiatum (St. rad) in the CA1 of the anterior dorsal hippocampus. The disarrangement of pyramidal neurons or small foci of interruption in the pyramidal cell layer were also found in the CA1 and sometimes in the CA3 regions. In control rats in any age and in both sexes, those microdysgenesis in the hippocampal formation was not found. This study demonstrates that the ectopic neuronal clusters and the disarrangement of the pyramidal neurons are the hereditary lesion, i. e. programmed neuronal microdysgenesis.Experiment II and III. BrdU labeling method traced neuronal migration in the hippocampal neurons of IGER. Labeled neurons in the ventricular layer (VL) of CA1region migrated into the superficial portion, the intermediate zone (IMZ), and then reached to the presumptive pyramidal layer. On ED16, one hour after the third injection, labeled cells abounded in the subventricular zone and a few were located in IMZ in both IER and control hippocampus. On ED17, there were significant more labeled cells in IMZ of IGER than that of control. On ED20 and ED21, greater part of labeled cells was located in presumptive pyramidal layer and St. rad. of control hippocampus. On the contrary, significant more labeled cells were located in IMZ of IER. These results indicated that in IER, primitive neurons migrate earlier from SVZ to IMZ, then remained longer in IMZ than those of control, respectively. It was suggested that a failure of gene expression, not yet defined, responsible for regulation of neuronal migration in embryonal developing phase might cause MDG in