Development of midline glial populations at the corticoseptal boundary.

Three midline glial populations are found at the corticoseptal boundary: the glial wedge (GW), glia within the indusium griseum (IGG), and the midline zipper glia (MG). Two of these glial populations are involved in axonal guidance at the cortical midline, specifically development of the corpus callosum. Here we investigate the phenotypic and molecular characteristics of each population and determine whether they are generated at the same developmental stage. We find that the GW is derived from the radial glial scaffold of the cortex. GW cells initially have long radial processes that extend from the ventricular surface to the pial surface, but by E15 loose their pial attachment and extend only part of the way to the pial surface. Later in development the radial morphology of cells within the GW is replaced by multipolar astrocytes, providing supportive evidence that radial glia can transform into astrocytes. IGG and MG do not have a radial morphology and do not label with the radial glial markers, Nestin and RC2. We conclude that the GW and IGG have different morphological and molecular characteristics and are born at different stages of development. IGG and MG have many phenotypic and molecular characteristics in common, indicating that they may represent a common population of glia that becomes spatially distinct by the formation of the corpus callosum.