Differential reaction of crossing and non-crossing rat retinal axons on cell membrane preparations from the chiasm midline: an in vitro study.

In the rat, a small subpopulation of retinal ganglion cell axons forms a persistent projection to the ipsilateral half of the brain. These fibres originate almost exclusively from the ventrotemporal margin of the retina. In contrast to all other retinal axons they seem to be deflected from the midline of the optic chiasm and thereby led into the ipsilateral optic tract. In order to analyse the interactions between growing fibres and chiasm midline, we have developed the following in vitro model. Axons of the embryonic rat retina are grown on a carpet of tectal cell membranes used as a general growth-permissive substratum. At a certain distance from the explant (200-450 microns), the advancing fibres are confronted with two stripes of cell membranes prepared from the chiasm midline. Such chiasm membranes are shown to act as a barrier for the presumptive non-crossing axons, while they do not influence growth of fibres originating from any other regions of the retina, including the dorsotemporal part. The repulsion of non-crossing fibres by chiasm membranes is observed in vitro only when retinal explants from embryonic day (E) 17/18 and chiasm preparations from E14/15 are used. Fibres and tissue from different regions of the brain as well as from different developmental ages, and even from different species, can be combined in this assay system. In a first attempt to characterize the molecular basis of the repulsive effect of chiasm membranes on ventrotemporal fibres, similar assays were performed with membranes derived from other regions of the central nervous system midline, some of which are known to have repulsive properties against certain axon populations. Since these cell membranes did not act as a barrier for the ventrotemporal retinal axons, we suggest that the guidance cues at the chiasm are very specific. Our results are consistent with the hypothesis that certain cells at the chiasm midline (very likely radial glial cells) express 'repulsive or inhibitory' molecules, which act in a specific way on ipsilaterally projecting axons.