EARLY GENERATION OF GLIA IN THE INTERMEDIATE ZONE OF THE DEVELOPING CEREBRAL-CORTEX

Radial glia are present at the earliest stage of cerebral cortical dev elopment, and later they transform into astrocytes. Other glial cells including astrocytes and oligodendrocytes are thought to appear only a fter neuron generation is complete and the cortical layers are formed. Little is known of when and where microglia enter the central nervous system and proliferate. We addressed the question of the origin of th ese three glial cell types in the developing ferret cerebral cortex. W e assessed the temporal pattern of glial cell division by administerin g [H-3]thymidine to label cells in S phase, and by using survival peri ods of 1-2 h to label dividing cells in situ. Labeled cells were ident ified in the developing intermediate zone of the ferret cerebral wall. These cells were present at E28, and reached a maximum number at pi. Double labeling experiments identified these cells as astrocytes, olig odendrocytes or microglia. None of the dividing cells expressed neuron al markers. These data show that all three types of glia are generated in the developing subcortical white matter, and that glial progenitor s are present in the intermediate zone as soon as it becomes a recogni zable structure. These data also show that the period of glial generat ion overlaps extensively with the period of neuron generation, since n euron generation is not complete until the end of the second postnatal week in the ferret. (C) 1997 Elsevier Science B.V.