Hippocampal mossy fibers in the regio superior of the European hedgehog.

The hippocampal mossy fiber zone of the European hedgehog has been studied at the light and electron microscopic levels. In contrast to the organization of this zone in other mammals, the mossy fiber zone of the hedgehog is not confined to the region of Ammon's horn that is characterized by large pyramidal cells (i.e. regio inferior) but extends into the region that contains small pyramidal cells (i.e. regio superior). The terminals, throughout the relatively extensive mossy fiber zone of the hedgehog, have the large multivesicular, multicontact morphology that characterizes the mossy fiber terminals observed in other species and can be demonstrated to be the axon terminals of granule cells located in the fascia dentata. In view of the primitive organization of the forebrain of the European hedgehog, which is believed to be representative of that of the first mammals, the boundaries of the mossy fiber zone of these animals suggest that the mossy fibers extended over a larger part of Ammon's horn during the first phases of mammalian evolution and became restricted to the regio inferior during the early stages of mammalian evolution.