Balloon cells in type II focal cortical dysplasia: developmental origins, molecular signatures, and epileptogenic mechanisms.

Type II focal cortical dysplasia (FCD II) is a major cause of drug-resistant epilepsy in children, with balloon cells representing a defining histopathological feature of FCD IIb. Although diagnostically important, the biological identity and epileptogenic contribution of balloon cells remain incompletely understood. Current evidence links balloon cells to disrupted cortical development, often driven by somatic mosaic mutations in the PI3K-AKT-mTOR pathway, resulting in impaired differentiation, cellular enlargement, and hybrid neuroglial phenotypes. This review examines current evidence on the developmental origins and lineage states, molecular signatures, electrophysiological features, and network effects of balloon cells in FCD II. Persistent mTOR hyperactivation appears to interact with cytoskeletal abnormalities, oxidative stress, and local inflammatory signaling, thereby creating a microenvironment that favors epileptogenic remodeling. Despite being largely electrically silent, BCs are embedded within lesion microenvironments characterized by disrupted inhibitory circuitry, altered neurotransmitter homeostasis, and complement-associated synaptic remodeling. BCs may participate indirectly in these processes, although their cell-specific causal contribution remains unresolved. Recent single-cell and spatial transcriptomic studies further distinguish balloon cells from dysmorphic neurons and implicate them in local immune activation. Understanding balloon cell biology may reveal cell-specific therapeutic targets and support precision treatment strategies beyond surgical resection.