THE INITIAL-STAGES OF DEVELOPMENT OF THE RETINOCOLLICULAR PROJECTION IN THE WALLABY (MACROPUS-EUGENII) - DISTRIBUTION OF GANGLION-CELLS IN THE RETINA AND THEIR AXONS IN THE SUPERIOR COLLICULUS

The time course of ingrowth of retinal projections to the superior col liculus in the marsupial mammal, the wallaby (Macropus eugenii), was d etermined by anterograde labelling of axons from the eye with horserad ish peroxidase, from birth to 46 days, when axons cover the colliculus contralaterally and ipsilaterally. The position of retinal ganglion c ells giving rise to these projections over this period was determined in fixed tissue by retrograde labelling from the colliculus with a car bocyanine dye. Axons first reach the rostrolateral contralateral colli culus 4 days after birth and extend caudally and medially, reaching th e caudal pole at 18 days and the far caudomedial pole at 46 days. The first contralaterally projecting cells are in the central dorsal and t emporal retina, followed by cells in the nasal and finally the ventral retina. They are distributed closer to the periphery with increasing age. The first sign of a visual streak appears by 18 days. Axons reach the ipsilateral projection reaches the caudal and medial collicular m argins by 46 days but by 16-18 days, ganglion cells giving rise to thi s transient projection are already concentrated in the temporoventral retina. The orderly recruitment of ganglion cells from retinotopically appropriate regions of the retinal as axons advance across the contra lateral colliculus suggests that the projection is topographically ord ered from the beginning. The ipsilateral projection is less ordered as cells are located in the temporoventral crescent at a time when their axons are still transiently covering the colliculus prior to becoming restricted to the rostral colliculus. Features of mature retinal topo graphy such as the visual streak and the location of ipsilaterally pro jecting cells begin to be established very early in development, befor e the period of ganglion cell loss and long before eye opening at 140 days.