Dy. Wu et al., A ROLE FOR TECTAL MIDLINE GLIA IN THE UNILATERAL CONTAINMENT OF RETINOCOLLICULAR AXONS, The Journal of neuroscience, 18(20), 1998, pp. 8344-8355
Retinal fibers approach close to the tectal midline but do not encroac
h on the other side. Just before the entry of retinal axons into the s
uperior colliculus (SC), a group of radial glia differentiates at the
tectal midline; the spatiotemporal deployment of these cells points to
their involvement in the unilateral containment of retinotectal axons
. To test for such a barrier function of the tectal midline cells, we
used two lesion paradigms for disrupting their radial processes in the
neonatal hamster: (1) a heat lesion was used to destroy the superfici
al layers of the right SC, including the midline region, and (2) a hor
izontally oriented hooked wire was inserted from the lateral edge of t
he left SC toward the midline and was used to undercut the midline cel
ls, leaving intact the retinorecipient layers in the right SC. In both
cases, the left SC was denervated by removing its contralateral retin
al input. Animals were killed 12 hr to 2 weeks later, after intraocula
r injections of anterograde tracers to label the axons from the remain
ing eye. Both lesions resulted in degeneration of the distal processes
of the tectal raphe glia and in an abnormal crossing of the tectal mi
dline by retinal axons, leading to an innervation of the opposite (''w
rong'') tectum. The crossover occurred only where glial cell attachmen
ts were disrupted. These results document that during normal developme
nt, the integrity of the midline septum is critical in compartmentaliz
ing retinal axons and in retaining the laterality of the retinotectal
projection.