Position of growth cones within the retinal nerve fibre layer of fetal ferrets

Optic axons are added to the retinal nerve fibre layer of fish along its vitreal border in a chronotopic manner. Likewise, the optic tract of all vertebrate species acquires axons preferentially along the superficial surface of the pathway. We have examined the developing retina of fetal ferrets (Mustela putorius furo) aged between embryonic day 27 (E27) and E34 to see whether a similar segregation of growth cones is apparent within the mammalian retinal nerve fibre layer. The distributions of growth cone, "wrist" (thick trailing portion of the growth cone), axonal, and glial profiles were determined from electron micrographs, and expressed as a percentage of neural profiles for several retinal locations. The retinal nerve fibre layer of fetal ferrets contains radially elongated bundles of fibres composed of axonal, wrist, and growth cone profiles. Glial processes of varying density divide the adjacent bundles, occasionally subdividing them in the plane of the retina, and give rise to endfeet lining the basal lamina and separating the optic axons from the latter. Growth cones within the developing fibre layer represented about 2.4% of profiles at E28, while at later developmental stages (E34), this value fell to about 0.6%. During this period of axonal outgrowth, growth cones were not preferentially segregated toward the vitreal basal lamina or the glial endfeet within the nerve fibre layer. Rather, they were found scattered throughout the axon bundles of the fibre layer. While there were differences in the proportion of immature profiles found within the vitreal half compared to the scleral half of the fibre layer, such that more growth cones and wrists were found vitreally, there was no clear accumulation of them in association with features of the vitreal margin. The present results show that young and old optic axons course together throughout the depth of the nerve fibre layer. A chronotopic mode of pathway genesis such as seen in the optic fibre layer of fish or in the optic tract of mammals is not present in the nerve fibre layer of ferrets. Differences in growth cone behaviour in the optic fibre layer and tract indicate that the mechanisms governing pathway formation differ along its course.