Synaptic reorganization by mossy fibers in human epileptic fascia dentata

Abstract

This study was designed to identify whether synaptic reorganizations occur in epileptic human hippocampus which might contribute to feedback excitation. In epileptic hippocampi, (n = 21) reactive synaptogenesis of mossy fibers into the inner molecular layer of the granule cell dendrites was demonstrated at the light microscopic and electron microscopic levels. There was no inner molecular layer staining for mossy fibers in autopsy controls (n = 4) or in controls with neocortex epilepsy having no hippocampal sclerosis (n = 2). Comparing epileptics to controls, there were statistically significant correlations between Timm stain density and hilar cell loss. Since hilar neurons are the origin of ipsilateral projections to the inner molecular layer, this suggests that hilar deafferentation of this dendritic zone precedes mossy fiber reafferentation. Quantitative Timm-stained electron microscopy revealed large, zinc-labelled vesicles in terminals with asymmetric synapses on dendrites in the inner molecular and granule cell layers. Terminals in the middle and outer molecular layers did not contain zinc, were smaller and had smaller vesicles. These histochemical and ultrastructural data suggest that in damaged human epileptic hippocampus, mossy fiber reactive synaptogenesis may result in monosynaptic recurrent excitation of granule cells that could contribute to focal seizure onsets.