THE DEVELOPMENT OF THE RADIAL GLIAL SCAFFOLD OF THE CEREBELLAR CORTEXFROM GFAP-POSITIVE CELLS IN THE EXTERNAL GRANULAR LAYER

We have reinvestigated the origin and genesis of the radial glia of th e cerebellar cortex in the hamster using three astroglial markers, vim entin, GFAP, and S-100 protein antibodies. On embryonic day 12 (E12), before the emergence of the external granular layer, the cerebellar an lage is traversed from the ventricle to the pial surface by a primordi al radial glial scaffold which is vimentin-positive, but GFAP and S-10 0 negative. With the formation of the external granular layer on E13, a few GFAP positive cells appear among the unstained external granular layer cells. First seen within the germinal trigone and caudalmost pa rt of the external granular layer, they then develop rostrally, amongs t the cells of the expanding external granular layer, proliferating ad jacent to the basement membrane. Beginning on E15, cells that are posi tive for the S-100 protein also appear within the external granular la yer and the molecular zone. In later stages, S-100 is strongly express ed in Golgi epithelial cells, so we have considered it to be a marker for these cells. By contrast, the primordial radial glial cells were n ot stained with this marker. On the day of birth (E16/PO) many S-100 p ositive cells also appear at intermediate levels between the EGL and t he Purkinje cell plate. They are unipolar and bear a single radial pro cess that is directed towards the pial surface. The caudorostral appea rance of S-100-positive cells firstly in the external granular layer, then in the molecular zone and finally in the Purkinje cell plate is i dentical to the temporal sequence of development of these layers, and suggests that S-100-positive cells are at first integral constituents of the external granular layer, but later descend through the molecula r zone, to colonize the Purkinje cell plate. Here they proliferate and ultimately differentiate into Golgi epithelial cells, their numerous short radial glial processes traversing the molecular zone and the ext ernal granular layer to fill the interstices between the primordial ra dial glial fibres. At birth, S-100-positive Golgi epithelial cells hav e progressively colonized the Purkinje cell plate from the germinal tr igone rostrally, up to a region midway between primary fissure and ant erior medullary velum and, between P2 and P3, the rostralmost part of the cerebellum has become populated. GFAP- and S-100-positive cells:re main in the external granular layer up to the end of the first postnat al week. In the same interval, the number of Golgi epithelial cells an d Bergmann glial fibres increases rapidly in the expanding cerebellar cortex. Our results suggest that the majority of the Golgi epithelial cells are not translocated, morphologically transformed primordial rad ial glial cells, but derive from the external granular layer, transloc ate into the Purkinje cell layer and differentiate into the secondary radial glial cells which intercalate with the basal processes of primo rdial radial glia. The latter are thus supplemented by the former, pro viding a radially organized substrate allowing granule cells produced in the secondary proliferative zone of the EGL to migrate through the molecular zone into the IGL.