Morphological abnormalities in the hippocampus of the weaver mutant mouse

The lamination of the hippocampus in the homozygous B6CBA weaver mouse (wv/wv) was compared with that in normal B6CBA littermates (+/+) and C57BL/6J mice using Nissl and Timm's staining. In Nissl-stained preparations, the normal littermates exhibit a compact, regular arrangement of pyramidal cells in area CA3 of the hippocampus. In contrast, in homozygous weaver mutant mice, the pyramidal cell layer of area CA3 frequently appears to be thicker than normal with an apparent increase of neuropil, as evidenced by the presence of cell-free spaces within the layer. Also, small ectopic clusters of pyramidal cells and sometimes the subdivision of the pyramidal cell layer into 2 or 3 layers were found throughout the dorsoventral extent of the hippocampus. In Timm's stained preparations of the normal mouse hippocampus, two clearly separated bundles of axons were seen emerging from the hilus: one bundle running above the pyramidal cell layer of area CA3 (i.e., the suprapyramidal mossy fiber layer, SPMFL), and the second bundle running below the pyramidal cell layer (i.e., the infrapyramidal mossy fiber layer, IPMFL). In contrast, in some homozygous weaver mice, the origin of the mossy fiber bundles is clearly different from normal; specifically, mossy fibers emerge in a diffuse fashion from the area between suprapyramidal and infrapyramidal mossy fiber layers. In other weaver mice, short, discontinuous bundles diverge from the infrapyramidal mossy fiber layer and invade the thickened pyramidal cell layer. In addition, ectopic pyramidal cells are situated below the IPMFL in area CA3. The morphological changes observed in hippocampus of weaver mutants are likely to be secondary to a more basic genetic defect.