ORGANIZATION OF PIONEER RETINAL AXONS WITHIN THE OPTIC TRACT OF THE RHESUS-MONKEY

Retinal ganglion cell axons must make a decision at the embryonic opti c chiasm to grow into the appropriate optic tract. To gain insight int o the cues that play a role in sorting out the crossed from the uncros sed optic axons, we investigated the sequence of their initial ingrowt h in rhesus monkey embryos. Two carbocyanine dyes, -dioctadecyl-3,3,3' ,3'-tetramethylindocarbocyanine perchlorate and 4-(4-dihexadecylaminos tyryl)-N-methylpyridinium iodide, were placed, respectively, into the left and right retinas to identify the course of uncrossed and crossed retinal axons through the optic chiasm and tract. Our results show th at at embryonic day 36 the most advanced retinal projections are uncro ssed. At this age the leading crossed axons are just reaching the chia smatic midline, whereas the uncrossed fibers have already entered the optic tract. This indicates that the pathfinding of these pioneer uncr ossed fibers does not require the presence of retinal axons from the o pposite eye. At subsequent stages of development (embryonic days 40 an d 42) there is a clear partial segregation of the uncrossed and crosse d retinal axons within the optic tract: the uncrossed-component course is in the deeper portion of the optic tract, whereas the crossed comp onent lies in a more superficial region. Thus, the spatial organizatio n of retinal axons within the primordial optic tract reflects the sequ ential addition of the uncrossed and crossed retinal fibers. The order ly and sequential ingrowth of these pioneer retinal axons indicates th at specific chiasmatic cues are expressed early in development and tha t such pioneer fibers may serve as guides for the later-arriving retin al fibers.