SPECIFICITY OF RETINAL GANGLION-CELL PROJECTIONS IN THE EMBRYONIC RHESUS-MONKEY

Recent studies dealing with the organization of retinal projections in the developing rhesus monkey brain have revealed a high degree of dev elopmental specificity. This is demonstrated by the ingrowth patterns of the initial contingents of crossed and uncrossed fibers that form t he primordial optic tract as well as by the adult-like nasotemporal re tinal decussation pattern evident even before the period of ganglion c ell death. On the basis of these observations, it is suggested that ea rly generated retinal fibers are guided through the optic chiasm by a transiently expressed decussation signal, and that later generated fib ers utilize retinal position-dependent cues to innervate the appropria te hemisphere. Furthermore, the first retinal fibers to arrive at the dorsal lateral geniculate nucleus invade only the presumed parvocellul ar layers. Thus, the initial innervation of the lateral geniculate nuc leus appears to reflect the birth order of retinal ganglion cell class es. It is suggested that the high degree of precision evident in the m acaque monkey nasotemporal retinal decussation pattern relates to the adultlike distribution of callosal projection neurons in the developin g striate cortex of the primate.