THE HUMAN FETAL RETINAL NERVE-FIBER LAYER AND OPTIC-NERVE HEAD - A DII AND DIA TRACING STUDY

The organization of the primate nerve fiber layer and optic nerve head with respect to the positioning of central and peripheral axons remai ns controversial. Data were obtained from 32 human fetal retinae aged between 15 and 21 weeks of gestation. Crystals of the carbocyanine dye s, DiI or DiA, and fluorescence microscopy were used to identify axona l populations from peripheral retinal ganglion cells. Peripheral gangl ion cell axons were scattered throughout the vitreal-scleral depth of the nerve fiber layer. Such a scattered distribution was maintained as the fibers passed through the optic nerve head and along the optic ne rve. There was a rough topographic representation within the optic ner ve head according to retinal quadrant such that both peripheral and ce ntral fibers were mixed within a wedge extending from the periphery to the center of the nerve. There was no indication that the fibers were reorganized in any way as they passed through the optic disc and into the nerve. The present results suggest that any degree of order prese nt within the fiber layer and optic nerve is not an active process but a passive consequence of combining the fascicles of the retinal nerve fiber layer. Optic axons are not instructed to establish a retinotopi c order and the effect of guidance cues in reordering fibers, particul arly evident prechiasmatically and postchiasmatically, does nor appear to be present within the nerve fiber layer or optic nerve head in hum ans.