Abstracts

Rationale: Hypothalamic hamartomas (HH) are an uncommon congenital mass lesion, associated with refractory epilepsy, and are now known to be intrinsically epileptogenic for the gelastic seizures that occur in these patients. The basic cellular mechanisms responsible for seizure onset within HH tissue are unknown. Prior neuropathological studies have suggested that HH has two predominant neuronal populations, with small neurons (8-10 microns diameter) that occur in clusters, interspersed with larger neurons (>16 microns diameter) (Coons, et al, 2007). Additionally, we have shown that small HH neurons have a distinctive functional phenotype with intrinsic pacemaker-like firing activity (Wu, et al, 2005). Here we have studied the ultrastructural features of neurons in HH tissue to further characterize the phenotype of these cells.Methods: Surgically resected HH tissue was obtained from 6 patients with refractory epilepsy (age range 1.6 to 24.2 years; 5 males). Specimens were fixed, sectioned, and stained for electron microscopy for qualitative study of ultrastructural features. In addition, a total of 62 neurons from 6 patients were selected for morphometric analysis. Based upon morphometric features, we statistically compared small neurons (<10 microns) and large neurons (>16 microns).Results: The fine structure of HH tissue consists of well-differentiated neurons interspersed with glial and supporting cells. Most neurons are small with a relatively thin rim of cytoplasm. The neuropil has a spongy character, with large numbers of synapse-containing neuronal processes. Axodendritic and axosomatic synapses with either asymmetric or symmetric features are identified. Pre-synaptic profiles contained abundant clear vesicles, and many also contained smaller numbers of dense-cored vesicles. Neurites, including thinly myelinated axons, were haphazardly arranged. The presence of glycogen granules, and the absence of Nissl substance, was predictive of the small cell phenotype. The following ultrastructural features differed significantly between small and large neurons: Nissl body density (p<.0001), glycogen density (p<.005), polyribosome density (p<.01), and ratio of nuclear area to cytoplasm area (p<.05). Aberrant ultrastructural profiles were not seen.Conclusions: There are predictable ultrastructural differences between the otherwise well-differentiated phenotype of small and large neurons in HH tissue. These findings are consistent with our current model for HH intrinsic epileptogenicity, which includes two predominant neuronal phenotypes, consisting of small, clustered, spontaneously-firing, GABAergic neurons, and smaller numbers of larger neurons that may be projection cells (Fenoglio, et al, 2007).