Balloon cells are histopathological hallmarks of various cortical malformations, 
i.e., focal cortical dysplasia (Taylor's type, FCD IIb), hemimegalencephaly 
(HME) or cortical tubers (tuberous sclerosis, TSC). Whether this intriguing cell 
type results from similar pathogenetic pathways remains to be shown. Here, we 
analyzed the immunohistochemical distribution pattern of the CD34 epitope in 
surgical specimens from 34 patients with FCD IIb, compared to that of 6 patients 
with TSC and 3 patients with HME. In normal brain, CD34 occurs only transiently 
during neurulation, but cannot be detected in mature neuroectodermal cell 
progenies. In contrast, 58% of our patients showed CD34 immunoreactivity within 
a subpopulation of balloon cells. Interestingly, CD34-positive balloon cells 
were confined to the white matter, but never observed in neocortical layers. 
Furthermore, balloon cells expressing neurofilament protein were also restricted 
to white matter, whereas GFAP-positive balloon cells were observed either in 
white or gray matter location. Clinical characteristics did not significantly 
differ between patients with CD34-positive versus CD34-negative lesions. No 
significant correlation was found between CD34 expression and genetic 
alterations of the TSC1 gene, which is affected in many FCD and TSC patients and 
which plays a role in the regulation of cell size. Further studies are warranted 
to clarify the restricted expression of CD34 in balloon cells of the white 
matter.