A glial palisade delineates the ipsilateral optic projection in Monodelphis

In developing marsupials, the path taken through the optic chiasm by ipsilaterally projecting retinal ganglion cells is complicated. Just prior to entry into the chiasm, ganglion cells destined for the ipsilateral optic tract separate from the remainder of axons by turning abruptly downwards to take a position in the ventral part of the optic nerve. In this report, it is shown that a discrete population of about 10-15 large glial cells transiently form a linear array across the prechiasmatic part of the optic nerve, precisely at this axon turning point. The distinct morphology of these cells and their novel location may reflect a specialized role in axon guidance.