Astroglial differentiation in the opossum superior colliculus

Glial markers, namely, vimentin, glial fibrillary acidic protein (GFAP), and glycogen, as well as accumulation of axon-borne horseradish peroxidase (HRP), were used to visualize radial glial cells in the developing opossum superior colliculus (SC) and to follow changes in young astrocytes of the superficial layers. Vimentin, GFAP, and glycogen are relatively abundant in elements of the median ventricular formation (MVF), which persists at least as late as weaning time, i.e., postconception day 103, postnatal day 90 (PND90). Radial profiles and end-feet in the remaining collicular sectors (main radial system, MRS) are also vimentin-positive but show little or no glycogen or anti-GFAP staining. The numeric density of MRS profiles is very high at the final stages of neuronal migration (PND12) but falls to vestigial numbers by PND 56-60. Antivimentin staining and filling of MRS profiles by axon-borne HRP disappear in parallel. Before total regression of MRS profiles, young astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is not found in those of the subjacent layers. The results suggest that 1) radial glia at or near the collicular midline are well equipped for a mechanical supportive role, and their abundant glycogen accumulation may reflect their eventual transformation in cells with high glycolytic metabolism, including tanycytes; 2) in most collicular sectors, some radial glia cells persist for long periods after cessation of neuronal migration and may interact with afferent fibers coursing through the superficial neuropil; 3) radially oriented astrocytes of the superficial gray layer exhibit a transiently high GFAP expression that is temporally correlated with late transformations of the retinocollicular projections.