Genesis of rods in teleost fish retina

Abstract

Fish grow throughout life¹ and new neurones are added to the retina as the eyes increase in size^2–6. As the retina expands, the density of cells decreases: ganglion cells, cones and cells in the inner nuclear layer are spaced further apart in retinas from larger (older) fish^2–5,7–9. In contrast, the density of rods increases during larval development and is then maintained approximately constant as the adult eye grows^2–5. Previous developmental studies, in which ³H-thymidine was used to identify proliferating cells, revealed a germinal zone at the margin of the retina. The germinal cells divide to produce new retinal neurones which are added annularly at the perimeter of the growing retina^5,6,10. A similar circumferential pattern of growth has been demonstrated in larval amphibians^11–15. These studies concluded that the retinal margin is the only site of neurogenesis in post-embryonic retinas. In contrast, our observations suggest that new rods originate from mitotic divisions of precursor cells which are interspersed among the nuclei of mature rods within the retina. The selective addition of rods throughout the retina could explain how the proportion of rods relative to other neurones increases as the retinas of fish grow^2–4,7. Preliminary reports of these experiments have appeared elsewhere^16,17.