THE REESTABLISHMENT OF THE REPRESENTATION OF THE DORSOVENTRAL RETINALAXIS IN THE CHIASMATIC REGION OF THE FERRET

This study has examined the representation of the dorso-ventral retina l axis in the optic nerve and tract of the ferret, as well as the asso ciated fiber transformations which take place within the chiasmatic re gion. In one series of experiments, dorsal or ventral retinal lesions were made to induce fiber degeneration along the pathway, from which s emi-thin sections were then stained for degenerating myelin. In a seco nd series, implants of the carbocyanine dye, DiI, were made into the c audo-medial or rostro-lateral optic tract in order to label retrograde ly the axons as they course through the chiasmatic region. Additional observations were made from the optic pathways of ferrets that had bee n similarly lesioned or implanted, but employing either a reduced-silv er technique to reveal the degenerating axons or horseradish peroxidas e as the retrograde label. The axons arising from the dorsal and ventr al retina course in the dorsal and ventral parts of the optic nerve po sterior to the eye, but as they continue along the nerve they disperse producing a highly impoverished retinotopy in the prechiasmatic porti on of the nerve. As they course through the chiasmatic region, however , they become segregated again: dorsal fibers cross the midline relati vely caudally while ventral fibers cross further rostrally, although t here 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 o f retinotopic order implicates a fiber-substrate interaction as being responsible for the axonal reordering, and suggests that fiber pre-ord ering in the tract contributes to the formation of the orderly project ion of the dorso-ventral retinal axis upon central visual targets.