Distribution of mossy fibres in the hippocampus of two closely related species of mice.

The relative volumes of different parts of the mossy fibre projection system- the suprapyramidal mossy fibres (SP-MF) and the intra- and infrapyramidal mossy fibres (IIP-MF)- were determined in the hippocampi of Apodemus sylvaticus and A. flavicollis. The mean contribution of the IIP-MF to the mossy fibre system differed significantly between the two species. An increased relative size of the IIP-MF as compared to the SP-MF in A. sylvaticus was linked with a reduction in the relative size of the hilus found in a previous study. In addition, the two species differed significantly in the amount of intraspecific variability observed. Despite size differences, the distribution of volume along the dorsoventral axis of the hippocampus was remarkably similar for the two species. We also compared measures from five mid-septotemporal sections with results for the whole structure. Mid-septotemporal measures correctly predicted the direction of difference in relative size but overestimated its magnitude in the two species investigated.