Midline glia of the tectum: a barrier for developing retinal axons.

A specialized group of radial glial cells constitutes a dorsal raphe in the mesencephalon and serves, at least, in part, to maintain the laterality of the retinotectal projection during development. Disruption of these cells is necessary, but may not be sufficient, for overriding their barrier function. Sulfated proteoglycans localized at the tectal midline form a putative molecular basis for the blockade; however, this blockade is selective: other fiber systems, such as the intertectal axons and under certain conditions the corticotectal axons, can cross the midline. Experimental studies of the tectal midline cells are advancing our understanding of the nature of boundaries in the developing brain.