Objectives: Focal cortical dysplasias (FCDs) are local malformations of the human neocortex and a leading cause of medically intractable epilepsy. FCDs are characterized by local architectural disturbances of the neocortex and often by a blurred gray-white matter boundary indicating abnormal white matter myelination. We have recently shown that myelination is also compromised in the gray matter of dysplastic areas, since transcripts encoding factors for oligodendrocyte differentiation and myelination are downregulated and myelin fibers appear fractured and disorganized. Methods: Here, we characterized the gray matter-associated myelination pathology in detail by in situ hybridization, immunohistochemistry, and electron microscopy with markers for myelin, mature oligodendrocytes, and oligodendrocyte precursor cells in tissue sections of FCD IIa and control cortices. In addition, we isolated oligodendrocyte precursor cells from resected dysplastic tissue and performed proliferation assays. Results: We show that the proportion of myelinated gray matter is similar in the dysplastic cortex to that in controls and myelinated fibers extend up to layer III.On the ultrastructural level, however, we found that the myelin sheaths of layer V axons are thinner in dysplastic specimens than in controls. In addition, the density of 172 | DONKELS Et aL. How to cite this article: Donkels C, Peters M, Fariña Núñez MT, et al. Oligodendrocyte lineage and myelination are compromised in the gray matter of focal cortical dysplasia type IIa. Epilepsia.

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