Abnormal cells are generally observed in some malformations of cortical development that include focal cortical dysplasia type IIB (CDIIB) and tuberous sclerosis complex (TSC). Balloon and giant cells can be found in CDIIB and TSC respectively. They express both neuronal and glial markers and appear as a result a hyperactive mTOR pathway. Electrophysiologically, balloon and giant cells share several similarities. They both demonstrate the inability to generate action potentials and the absence of synaptic inputs.

In the preset study, we examined and compared the morphology and electrophysiology of balloon and giant cells in pediatric cases of CDIIB (n=23) and TSC (n=25), undergoing surgical removal of the epileptogenic zone. Tissue samples were sliced (350 µm) and allowed to recover for at least 1 hr. Single-cell electrophysiological recordings were obtained using the whole-cell patch clamp recording technique in current (K-gluconate as the internal solution) or voltage (Cs-methanesulfonate as the internal solution) clamp modes.

Regardless of pathology and based on morphological and electrophysiological assessments, some cells appeared more glial-like (low membrane input resistance and linear current-voltage relationships), while others appeared more neuronal-like (high membrane input resistance and rectifying current-voltage relationships, but no action potentials). The morphology also differed in terms of somatic shape and dendritic elaboration (i.e., multipolar vs. circumferential). We found that categorizing balloon cells and giant cells by these morphological and electrophysiological assessments (neuronal-like vs. glial-like) instead of pathology demonstrated significant differences in the cell membrane input resistance, i.e., higher in neuronal-like vs. glial-like, but not in cell capacitance and decay time constant.