Projections of growth-cone-bearing fibers of retinal ganglion cells within co-cultured tectal explants: early branching depends on age of target tissue.

The projections of retinal ganglion cell axons within co-cultured tectal explants were analyzed in order to investigate some of the factors that determine the earliest responses of retinal axons to cues present in an isolated target tissue. Half retinas and superior colliculi (tecta) from the embryonal mouse were explanted, separated by a 0.5 mm gap. After 5 days in vitro retinal ganglion cells were labeled by extracellular ionophoresis of HRP into the optic nerve head region. Cleared co-cultures were studied as whole mounts. Growth-cone-bearing retinal fibers were studied in standard tectal co-cultures, and in cases where tectum had been explanted 2 weeks prior to retina. The heterochronously prepared cultures had a higher proportion of fibers with complex branching patterns than the synchronous explants. Cultures in which retinas were explanted 1 week after tecta exhibited intermediate proportions of such fibers. These observations suggest that older tecta facilitate branching of ingrowing retinal fibers, although other alterations during in vitro development must be evaluated. The growth patterns of axons originating in nasal and temporal hemi-retinas were analyzed in terms of possible positional cues provided by the target tecta. Axons originating in temporal hemi-retinas did not show evidence of preferential branching in, or growth toward, appropriate anterior regions of co-cultured tectal explants. In contrast, the majority of nasal retinal axons showed enhanced terminations and complex branching in, and bending towards, the posterior tectum.