Microanatomy of the dysplastic neocortex from epileptic patients

Focal cortical dysplasia (FCD) is a pathology that is characterized by the abnormal development of the neocortex. Indeed, a wide range of abnormalities in the cortical mantle have been associated with this pathology, including cytoarchitectonic alterations and the presence of dysmorphic neurons, balloon cells and ectopic neurons in the white matter. FCD is commonly associated with epilepsy, and hence we have studied the ultrastructure of cortical tissue resected from three subjects with intractable epilepsy secondary to cortical dysplasia to identify possible alterations in synaptic circuitry, using correlative light and electron microscopic methods. While the balloon cells found in this tissue do not appear to receive synaptic contacts, the ectopic neurons in the white matter were abnormally large and were surrounded by hypertrophic basket formations immunoreactive for the calcium-binding protein parvalbumin. Furthermore, these basket formations formed symmetrical (inhibitory) synapses with both the somata and the proximal portion of the dendrites of these giant ectopic neurons. A quantitative analysis revealed that in the dysplastic tissue, the density of excitatory and inhibitory synapses was different from that of the normal adjacent cortex. Both increases and decreases in synaptic density were observed, as well as changes in the proportion of excitatory and inhibitory synapses. However, we could not establish a common pattern of changes, either in the same patients or between different patients. These results suggest that cortical dysplasia leads to multiple changes in excitatory and inhibitory synaptic circuits. We discuss the possible relationship between these alterations and epilepsy, bearing in mind the possible limitations that preclude the extrapolation of the results to the whole population of epileptic patients with dysplastic neocortex.