Spatial arrangement of radial glia and ingrowing retinal axons in the chick optic tectum during development

Neuroanatomical tracing of retinal axons and axonal terminals with the fluorescent dye, DiI, was combined with immunohistochemical characterization of radial glial cells in the developing chick retinotectal system. Emphasis was placed on the mode of the tectal innervation by individual retinal axons and on the distribution and fate of the tectal radial glial cells and their spatial relation to retinal axons. It was obvious from fluorescent images obtained from anterogradely filled axons that these axons deserted the superficial stratum opticum (SO) to penetrate the stratum griseum et fibrosum superficiale (SGFS) by making right-angled turns within the SO. Frequently, axons which had invaded the SGFS were bifurcated and had a superficial branch which remained within the SO. Terminal axonal arborization occurred at various depths within the SGFS. Characterization of the tectal glial cells and their radial fibers by means of the anti-filament antibody, R5, and post-mortem staining with the fluorescent dye, DiI, revealed the following. (a) At least from day E8 to P1, tectal glial fibers traversed all tectal layers from the periventricular location of their somata to the superficial interface between SO and pia mater. In this interface they enlarged and formed characteristic endfeet. (b) Glial endfeet covered the whole tectal surface. They showed at early ages anterior-posterior differences having a higher density in the posterior tectum. These differences disappeared at embryonic day E13. (c) After innervation, glial endfeet of the anterior tectal third were arranged in rows parallel to the retinal fibers within the SO. This arrangement was not observed in eyeless embryos. (d) Radial glial fibers could be stained with R5 from day E8 to late embryonic stages throughout their entire length. (e) At the first posthatching days, only the segments of the radial glial fibers restricted to the thickness of the SO were R5-positive, although the fibers still traversed throughout the depth of the tectum. The results are discussed in context to the genesis of the retinotectal projection.