The re-establishment of the representation of the dorso-ventral retinal axis in the chiasmatic region of the ferret

Abstract

This study has examined the representation of the dorso-ventral retinal axis in the optic nerve and tract of the ferret, as well as the associated fiber transformations which take place within the chiasmatic region. In one series of experiments, dorsal or ventral retinal lesions were made to induce fiber degeneration along the pathway, from which semi-thin sections were then stained for degenerating myelin. In a second series, implants of the carbocyanine dye, Dil, were made into the caudo-medial or rostro-lateral optic tract in order to label retrogradely the axons as they course through the chiasmatic region. Additional observations were made from the optic pathways of ferrets that had been similarly lesioned or implanted, but employing either a reduced-silver technique to reveal the degenerating axons or horseradish peroxidase as the retrograde label.The axons arising from the dorsal and ventral retina course in the dorsal and ventral parts of the optic nerve posterior to the eye, but as they continue along the nerve they disperse producing a highly impoverished retinotopy in the prechiasmatic portion of the nerve. As they course through the chiasmatic region, however, they become segregated again: dorsal fibers cross the midline relatively caudally while ventral fibers cross further rostrally, although there is overlap between them. Nearer the threshold of the optic tract, the fibers from dorsal and ventral retina undergo a further and more striking segregation, placing the dorsal fibers caudo-medially and the ventral fibers rostro-laterally within the tract. This re-emergence of retinotopic order implicates a fiber-substrate interaction as being responsible for the axonal reordering, and suggests that fiber pre-ordering in the tract contributes to the formation of the orderly projection of the dorso-ventral retinal axis upon central visual targets.