THE ABNORMAL DISTRIBUTION OF MOSSY FIBER-BUNDLES AND MORPHOLOGICAL ABNORMALITIES IN HIPPOCAMPAL-FORMATION OF DREHER(J) (DR(J) DR(J)) MOUSE/

The organization of pyramidal cells and messy fibers in the hippocampa l formation of homozygous dreher(J) mutant mice was investigated using Timm's and Golgi methods. Five clear abnormalities were found: (1) so me pyramidal cells were located below the infrapyramidal messy fiber l ayer, (2) messy fibers emerged in diffuse fashion from between the sup rapyramidal and infrapyramidal messy fiber layers, and their fibers in vaded within the pyramidal cell layer, where they traveled as 3-6 smal l, usually quite short, bundles, (3) some normally situated pyramidal cells had unusual contacts with messy fibers at two or three places on their apical and/or basal dendrites, (4) some normally situated pyram idal cells had abnormal dendritic trees typified by the occurrence of fine-caliber dendritic branches extending out of the apical dendrite o r the apical portion of the soma, and (5) a few Timm positive fibers e xtending from the dentate hilus to the dentate molecular layer in both dreher(J) and control mice were observed. These abnormalities indicat e that in the hippocampal formation a variety of cell populations and neuronal circuits can be indirectly modified by the dreher mutation.