Prechiasmatic Reordering of Fibre Diameter Classes in the Retinofugal Pathway of Ferrets

Abstract

This study examined the distribution of fibre diameter classes at various sites along the retinofugal pathway of adult ferrets. Light microscopic observations were made on semi-thin sections, and regional fibre diameter spectra were constructed from diameter measurements taken from electron micrographs of thin sections of the intraorbital optic nerve (2.5 mm from the optic disc), the intracranial optic nerve (1 mm rostal to the fusion of the nerves), and the optic tract (just caudal to the optic chiasm). Whereas diameter types are relatively evenly distributed behind the eye in the postoptic nerve, they begin to segregate along its prechiasmatic course. Within this prechiasmatic region, coarse and fine calibre fibers are confined increasingly to more ventral locations in the nerve, leaving a dorsal band populated predominantly by intermediate calibre fibers. In conjunction with this redistribution of axon size classes, the fascicular arrangement of axons which is present distally, changes to a non-fascicular organization. The prechiasmatic organization of fibre types approximates that found in the optic tract where the coarse and fine calibre fibres lie further ventrally towards the pial surface. The prechiasmatic region can be viewed as a region of transition where the order of fibres in the nerve (retinotopic) starts to change to that present in the optic tract (chronotopic), resulting in the first-born beta cell axons becoming segregated dorsally, and rostral to the coarse and fine calibre classes which segregate at further caudal locations. Further, since the sorting of fibres according to diameter appears before the fibres reach the optic chiasm, the segregation of diameter classes is not dependent on the chiasmatic sorting of fibres according to their crossed or uncrossed course.