Spatial and temporal development of the gliovascular tissue in type II lissencephaly.

Type II lissencephaly is a complex cortical malformation in which mesenchymal and central nervous components are intermingled. It is generally believed that the histological pattern is created by migration of heterotopic neuroblasts into the leptomeninges through defects in the superficial basement membrane. Defects of the extracellular matrix have been suggested to be the primary cause of type II lissencephaly. To elucidate the underlying pathogenetic mechanisms, we immunostained extracellular matrix and basement membrane components of the cerebral cortex from six fetal and two infantile brains. We found that the pattern of collagen subtypes I, III and VI was not altered in type II lissencephaly brains when compared to normal controls. As to the pathogenesis of type II lissencephaly, a polymicrogyria-like pattern is created, which results in considerable cortical enlargement. The microgyri do not fuse but remain separated from each other by gliovascular tissue, i.e., leptomeninges which contain astrocytes. At the interface between the enlarged brain surface and the gliovascular tissue, neuronal migration takes place through gaps in the external basement membrane. Thus, the cortical dysplasia encountered in type II lissencephaly is only due to a limited amount to neuronal heterotopia in the leptomeninges.