Topographic organization in the retinocollicular pathway of the fetal cat demonstrated by retrograde labeling of ganglion cells.
The topographic organization of the developing retinocollicular pathway was assessed by making focal deposits of a retrograde tracer (usually rhodamine latex beads) into the superficial layers of the superior colliculus of fetal cats at known gestational ages. Subsequently, the distributions of labeled cells in the contralateral and ipsilateral retinas were examined. At all stages of development, a high density of labeled cells was found in a delimited area (core region) of both retinas. The locations of the retinal regions containing the high density of labeled cells varied with the locus of the tracer deposit in the superior colliculus in a manner consistent with the topographic organization of the mature cat's retinocollicular pathway. Additionally, some labeled ganglion cells, considered to be ectopic, were found to be scattered 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 projection error. The incidence of ectopic cells did not differ between the contralateral and ipsilateral retina, even though the overall density of crossed labeled cells was always greater than that of uncrossed labeled cells. In the youngest fetal animals, tracer deposits into the caudal portion of the superior colliculus resulted in a core region of labeled cells in the contralateral nasal retina as well as in the nasal ipsilateral retina. Such uncrossed nasal cells, not seen in more mature animals, appear to innervate the appropriate topographic location of the superior colliculus, but on the wrong side of the brain. Most likely, these uncrossed nasal ganglion cells contribute to the widespread distribution of the ipsilateral retinocollicular pathway observed in fetal cats after intraocular injections of anterograde tracers (Williams and Chalupa, 1982). Collectively, our findings demonstrate that the developing retinocollicular pathway of the fetal cat is characterized by a remarkable degree of topographic precision.
Chalupa
L M
LM
Department of Psychology, University of California, Davis 95616, USA.
Snider
C J
CJ
Kirby
M A
MA
D000818:Animals; D002415:Cats; D005333:Fetus / Q000033:anatomy & histology*; D009434:Neural Pathways / Q000033:anatomy & histology*; D012160:Retina / Q000033:anatomy & histology*; D012165:Retinal Ganglion Cells / Q000166:cytology*; D013477:Superior Colliculi / Q000033:anatomy & histology*