ORGANIZATION OF RETINAL GANGLION-CELL AXONS IN THE OPTIC FIBER LAYER AND NERVE OF FETAL FERRETS

Previous authors have hypothesized that retinotopic projections may be influenced by 'preordering' of the axons as they grow towards their t argets. In some nonmammalian species, axons are reorganized at or near the optic nerve head to establish a retinotopic order. Data are ambig uous concerning the retinotopy of the mammalian retinal nerve fiber la yer and whether fibers become reorganized at the optic nerve head. We have examined this question in fetal and newborn ferrets (Mustela puto rius furo) by comparing the arrangement of axons in the retinal nerve fiber layer with that in the optic nerve. DiI or DiA crystals were imp lanted into fixed tissue in the innermost layers of the retinal periph ery, or at a location midway between the periphery and the optic nerve head. Fluorescence labelling was examined in 100-200 mu m Vibratome s ections, or the eyecup and nerve were photooxidized and 1-2 mu m longi tudinal or transverse sections were examined. Regardless of fetal age, eccentricity or quadrant of the implant site, a segregation of labell ed peripheral axons from unlabelled central ones was not detected with in the nerve fiber layer. Axons coursed into the nerve head along the margin of their retinal quadrant of origin, often entering the optic n erve as a radial wedge, thus preserving a rough map of retinal circumf erence. However, peripheral axons were in no way restricted to the per ipheral (nor central) portions of the nerve head or nerve, indicating that the optic axons do not establish a map of retinal eccentricity. O ur results demonstrate that (1) the nerve fiber layer is retinotopic o nly with respect to circumferential position and (2) optic axons are n ot actively reorganized to establish a retinotopic ordering at the ner ve head. The present results suggest that any degree of order present within the optic nerve is a passive consequence of combining the fasci cles of the retinal nerve fiber layer; optic axons are not instructed to establish, nor constrained to maintain, a retinotopic order within the optic nerve.