TOPOGRAPHIC SPECIFICITY IN THE RETINOCOLLICULAR PROJECTION OF THE DEVELOPING FERRET - AN ANTEROGRADE TRACING STUDY

To assess the degree of order exhibited during development by crossed and uncrossed retinocollicular pathways, focal deposits of -dioctodecy l-3,3,3'3'-tetramethylinodocarbocyanine perchlorate (DiI) were made in to the temporal or nasal retina of prenatal and postnatal ferrets. Thi s procedure revealed that the first retinal fibers (from the ipsilater al temporal retina) grow into the superior colliculus at embryonic (E) day 30. Both crossed and uncrossed fibers innervate the colliculus by E34. At this age, terminal arbors were lacking, and there was no evid ence of extensive axonal branching. Retinocollicular arbors first appe ared at E38, with both the crossed and uncrossed projections forming w ell-defined terminal zones that appeared to be localized to topographi cally appropriate regions. At E38, the ipsilateral terminal zone was s ignificantly larger but notably less dense than the contralateral zone . At this and later ages (postnatal day [P] 0 and P7), a few crossed a nd uncrossed fibers extended beyond the terminal zone. Four days later , at P0, the terminal zone of the uncrossed projection was reduced in size in comparison with that of earlier ages, whereas the crossed proj ection became substantially larger. By P7, the few misprojecting fiber s seen in younger ferrets had been virtually eliminated. When focal re tinal deposits of tracer were made into the nasal retina of E36 and E4 0 ferrets, crossed fibers were found to innervate the caudal segment o f the superior colliculus. These crossed nasal cells appear to project to the topographically appropriate region of the superior colliculus (caudal segment) but on the wrong side of the brain. Collectively, the present findings indicate that throughout development the ferret reti nocollicular pathway is characterized by a remarkable degree of topogr aphic precision as evident by the paucity of axonal branches and the l ow number of grossly misprojecting axons. (C) 1998 Wiley-Liss, Inc.