The Course of Fibre Diameter Classes Through the Chiasmatic Region in the Ferret.

Abstract

The present study has examined the transition of fibre order from the optic nerve through the optic chiasm and into the optic tract in the ferret's retinofugal pathway. Semi-thin sections through the chiasmatic region were examined in normal and in monocularly enucleated ferrets in order to display the segregation of the different axon diameter classes as the fibres pass from the optic nerve into the optic tract, and to determine, for each diameter class, where the crossed and uncrossed components become separated in the chiasmatic region. As demonstrated in the preceding manuscript, fine and coarse optic axons begin to segregate from the medium optic axons rostral to the fusion of the two optic nerves. This segregation continues in the chiasmatic region where the different axon diameter classes decussate at different rostro-caudal levels: the fine and coarse diameter axons decussate rostrally, accumulating along the ventral, superficial surface of the contralateral half-chiasm, while the medium diameter axons continue caudally before crossing the midline. These two populations, a ventral, crossed (fine and coarse) population and a dorsal, yet-to-cross (medium) population are discriminable not only by their size in the chiasmatic region, but also by a thin invagination of hypothalamic neuropil separating them at their lateral extreme. The population of ipsilaterally projecting fibres is composed of both fine and medium optic axons. No coarse optic axons project ipsilaterally in the ferret: these fibres all decussate rostrally in the optic chiasm, intermingled with many of the decussating fine fibres. The fibre ordering in the adult ferret's optic chiasm is interpreted as reflecting a gradient of axonal arrival during development, with successively later arriving optic axons entering one of the two optic tracts at a progressively more superficial, rostral and ventral, location in the chiasmatic region. A fibre's position and time of arrival may influence its subsequent crossed or uncrossed course during the period of axonal ingrowth in development.