Development of visual cells in the toad Bufo melanostictus (Schneider)

Abstract

Differentiation of visual cells in Bufo melanostictus begins after hatching at gill bud stage. It is indicated by emergence of cytoplasmic buds from the outer nuclear layer of the retina. Each bud contains an apically located vacuole which soon disappears from all cells. Cones and rods start differentiating simultaneously but the majority of early cells are rods. Single cones, red rods and double cones can be identified by the stage when the operculum is formed. Up to the beginning of hindlimb morphogenesis rods remain the most numerous but later single cones attain approximate numerical equality with rods and the tadpole retina becomes a truly duplex retina. Red rods of tadpoles are structurally different from those of adult. As in the accessory members of double cones a paraboloid develops in the myoid region of the inner segment of red rods also and persists throughout larval life. During metamorphosis green rods appear, cone outer segments become sharply pointed, myoid develops in the red rods replacing the paraboloid which disappears along with its glycogen but there is no change in the double cones. After metamorphosis number and size of red rods increase greatly transforming the duplex retina of the tadpole into a predominantly rod retina adapted for scoptopic vision of the nocturnal adult.