Interference with the development of early generated neocortex results in disruption of radial glia and abnormal formation of neocortical layers

Early generated layers of neocortex are important factors in forming the subsequent architecture of the cerebral cortex. To further explore the role of early generated cortex, we disrupted formation of an early generated cohort of cells by intraperitoneal injections of the mitotic inhibitor methylazoxymethanol (MAM) into pregnant ferrets timed to coincide with generation of subplate neurons in the ventricular zone. Our studies demonstrate that if early development of the neocortex is interrupted by injection of MAM during embryogenesis (on embryonic day 24 or 28; E24 or E28), a distinct laminar pattern fails to form properly in the parietal cortex. A reduced number of MAP2-positive cells were observed in the region of the subplate when compared with the number of MAP2-positive cells found in normal animals. Interference with the superficial neocortical layers that form later during development (on embryonic day 33) by appropriately timed MAM injections does not result in a severely disrupted laminar pattern. The interrupted laminar pattern that arises after early MAM injections coincides with distorted radial glial cells (identified by immunoreactivity to the intermediate filament protein, vimentin), which occur after early, but not late, MAM injections. Further analysis suggests that interference with early development of neocortex leads to premature differentiation of radial glial cells into astrocytes, as demonstrated by the presence of glial fibrillary acidic protein (GFAP). Experiments involving injections of the thymidine analog, bromodeoxyuridine (BRDU), demonstrated that 4 days after E24 MAM injection cells are generated and migrate into the thin cortical plate. By E38, however, cells continue to be generated in animals treated with MAM on E24 but do not reach their normal positions in the cortical plate. In addition, immunoreactivity using the CR50 antibody, which identifies presumptive Cajal-Retzius cells present in layer 1, demonstrates that the CR50-positive cells, normally precisely located in the outer portion of layer 1, are distributed in disarray throughout the thickness of the neocortex and intermediate zone in early MAM-treated animals, but not in those treated with MAM injections later during gestation. These findings are consistent with the idea that early generated layers are important in providing factors that maintain the environment necessary for subsequent neuronal migration and formation of neocortical layers.