Distribution of heterotopic neurons in normal hemispheric white matter: A morphometric analysis

One of the frequent abnormalities described in the context of surgically resected temporal lobe (TL) specimens is the presence of heterotopic neurons within white matter (WM). We have attempted to morphometrically define the distribution of these heterotopic neurons in normal subjects, comparing the incidence of heterotopic neurons in TL WM with that in occipital (OL) and frontal lobe (FL) sections. Using a combination of routine and special stains combined with immunohistochemical confirmation 20 adult autopsy cases were examined. WM from TL, FL, and OL sections was outlined and the area was measured by image analysis. Using defined criteria, heterotopic neurons within these areas were counted. Results confirm our hypothesis that normal adult TL WM contains a significantly higher population of residual/heterotopic neurons than OL and FL WM groups. It is felt that these neurons represent interstitial remnants of the subplate which have Failed to undergo programmed cell death. The significance of these findings with regard to assessment of similar findings in temporal lobectomy specimens is addressed. A second intriguing association of this TL WM heterotopia concerns its possible relationship to the more frequent occurrence of ''malformative neoplasms'' with neuronal elements (such as ganglioglioma and dysembryoplastic neuroepithelial tumor) in the temporal lobe.