© 1981 by The Society for Integrative and Comparative Biology
Growth of Fish Retinas1
Department of Neurobiology, Harvard Medical School Boston, Massachusetts 02115
SYNOPSIS. This review discusses development and growth of the retina. A geometric model of retinal differentiation is proposed in which four phases are recognized; the first three are common to all vertebrate embryos. The last, post-embryonic growth phase has two alternate routes, one followed by birds and mammals and the other by fish and amphibians. All retinas grow by expansion, and retinal cells are spread apart as the retina enlarges. In fish (and larval amphibians), the retina not only expands but also adds cells. In these retinas a marginal germinal zone persists and continues to produce neurons, which are added appositionally in concentric annuh around the perimeter. The genesis of one class of photoreceptor cell, the retinal rods, is different from all other retinal neurons in that the proportion of rods increases with growth in central retinal regions far from the germinal zone. The source of these centrally-added rods is not yet established; several hypotheses are discussed. Alterations of synaptic connectivity within the retina and between retina and brain are suggested by the pattern of growth and cell addition. The capacity of adult fishes to generate new neurons and to form new synapses is a remarkable property, one which most animals abandon much earlier in life.