Department of Morphological and Biomedical Sciences, University of Verona, Verona, Italy. Regional Center of Epilepsy Surgery C. Munari‘, Hospital Niguarda Ca‘ Granda, Milan, Italy 3. Division of Experimental Neurophysiology and Neuroanatomy, National Neurological Institute C. Besta‘, Milan, Italy
A brief survey of cortical development is presented, focusing on neuronal migration and its alterations. Corticogenesis is achieved through ordered temporospatial steps, via the formation of transient structures, and successive waves of cell proliferation and migration (followed by cell differentiation and maturation), and apoptotic cell death. The appearance of the proliferative ventricular zone and marginal zone, and of the superficial primordial plexiform layer, is followed by the formation of the prospective layer I, of the subplate, whose neurons are destined to die, and of the cortical plate that will give rise to layers II‐VI. Cells arising in the ventricular zone migrate radially using radial glia as a scaffold, and are destined to form pyramidal cells. Cortical interneurons are mainly generated in the ganglionic eminence and migrate along axonal substrates following tangential routes. Disorders of this complex process lead to a wide range of alterations, and focal derangements of cortical organization have been grouped under the term focal cortical dysplasia (FCD). As the result of a neuropathological revision of FCD cases with intractable epilepsy, a novel classification comprising three subgroups of FCD has been introduced, and is supported by electroclinical and neuroimaging data, as well as by the postsurgical outcome of patients: i) architectural dysplasia, characterized by altered cortical lamination; ii) cytoarchitectural dysplasia, with the occurrence of giant neurons besides cortical dyslamination; iii) Taylor‐type cortical dysplasia, in which altered cortical lamination is consistently associated with the occurrence of giant, dysmorphic and ectopic neurons, and frequently with the so‐called balloon cells.