Development of ganglion cell topography in ferret retina.

The adult ferret has approximately 90,000 retinal ganglion cells, arranged in a prominent area centralis and visual streak. The role of differential cell generation, cell death, and retinal growth in the control of adult retinal ganglion cell number and distribution was evaluated by examining basic aspects of retinogenesis, including growth in retinal area, developmental changes in the number, size, and distribution of retinal ganglion cells (identification aided by retrograde transport of HRP), and the incidence of degenerating cells in the ganglion cell layer. Retinal development in the ferret was also compared to retinal development in the cat (which has an even more differentiated area centralis) to determine what alterations of developmental parameters are most closely associated with this species difference in adult morphology. The area of the retina increases linearly from birth (12 mm2) to postnatal day 24 (54 mm2), reaching an eventual adult value of 64 mm2. Ganglion cell numbers peak at 155,000 (approximately twice the adult number) on postnatal day 3, and fall to adult numbers by postnatal day 6. The remaining cells of the ganglion cell layer, principally displaced amacrine cells, reach their peak number on postnatal day 10 (approximately 280,000), falling to 200,000 by adulthood. Degenerating cells are abundant in the ganglion cell layer in the immediate postnatal period. A difference in the incidence of degenerating cells in the presumptive area centralis versus that in the retinal periphery was not observed postnatally, though there were other striking spatial nonuniformities, suggesting that differential cell loss might contribute to other features of retinal topographic organization. Ganglion cell density is virtually uniform across the retina at birth. Cell density is first reduced in the dorsal retina, resulting in a dorsal-to-ventral gradient in cell density that persists until day 10, when ganglion cell number has stabilized. By postnatal day 24, an area centralis and visual streak has emerged, but not of adult magnitude. Because ganglion cell number has stabilized long before the area centralis and visual streak emerge, we conclude that differential retinal growth is the principal mechanism producing this feature of retinal topography. Comparison with the cat suggests that the proportionately greater nonuniform growth of the cat's eye accounts for the greater differentiation of its area centralis.