TOPOGRAPHIC ORGANIZATION IN THE RETINOCOLLICULAR PATHWAY OF THE FETALCAT DEMONSTRATED BY RETROGRADE LABELING OF GANGLION-CELLS

The topographic organization of the developing retinocollicular pathwa y was assessed by making focal deposits of a retrograde tracer (usuall y rhodamine latex beads) into the superficial layers of the superior c olliculus of fetal cats at known gestational ages. Subsequently, the d istributions of labeled cells in the contralateral and ipsilateral ret inas were examined. At all stages of development, a high density of la beled cells was found in a delimited area (core region) of both retina s. The locations of the retinal regions containing the high density of labeled cells varied with the locus of the tracer deposit in the supe rior colliculus in a manner consistent with the topographic organizati on of the mature cat's retinocollicular pathway. Additionally, some la beled ganglion cells, considered to be ectopic, were found to be scatt ered throughout the contralateral and ipsilateral fetal retinas. Such ectopic cells were few in number throughout prenatal development. For every 100 cells projecting to the appropriate region of the colliculus , we estimate that less than one ganglion cell makes a gross projectio n error. The incidence of ectopic cells did not differ between the con tralateral and ipsilateral retina, even though the overall density of crossed labeled cells was always greater than that of uncrossed labele d cells. In the youngest fetal animals, tracer deposits into the cauda l portion of the superior colliculus resulted in a core region of labe led cells in the contralateral nasal retina as well as in the nasal ip silateral retina. Such uncrossed nasal cells, not seen in more mature animals, appear to innervate the appropriate topographic location of t he superior colliculus, but on the wrong side of the brain. Most likel y, these uncrossed nasal ganglion cells contribute to the widespread d istribution of the ipsilateral retinocollicular pathway observed in fe tal cats after intraocular injections of anterograde tracers (Williams and Chalupa, 1982). Collectively, our findings demonstrate that the d eveloping retinocollicular pathway of the fetal cat is characterized b y a remarkable degree of topographic precision. (C) 1996 Wiley-Liss, I nc.