Development of visual cells in the retina of Bufo melanostictus tadpoles under conditions of precociously induced and inhibited metamorphosis

Abstract

The visual cells of the larval retina of the toad Bufo andersonii includes red rods, single cones and double cones. Adult retina contains green rods also. Moreover red rods of the larva are different structurally from those of the adult. Development of green rods and transformation of larval red rods into adult type occurs during metamorphosis which depends on thyroid hormone. The present study was made to investigate whether these changes in visual cells of the retina are under the influence of thyroxine. Experiments were started with tadpoles at stage 30. One group was reared in 10^-6DLthyroxine solution and another in 0.2% solution of potassiuum perchlorate for 25 days. Controls for each group were maintained in water. In 3 days, controls of thyroxine treated tadpoles progressed from stage 30 to larval stage 32 while the treated tadpoles underwent radical metamorphic changes of the body. However, retina of the hormone-treated tadpoles remained typically larval, similar to that of controls, without green rods, paraboloid still present and myoid absent. Perchlorate treated tadpoles were extremely retarded in development and in 25 days they had not grown beyond larval stage 34 while the respective controls had metamorphosed by 20th day. Perchlorate had effectively inhibited metamorphosis. The retina of metamorphosed toadlets of control group had become typically adult type. The retina of perchlorate treated tadpoles showed a mixture of larval and adult features. Paraboloid was still present in the inner segment of most red rods which lacked myoid. The adult features were indicated by general reduction in size of paraboloids and disappearance of this structure in at least some red rods which now possessed distinct myoid and the presence of a fair number of green rods. It is suggested that transformation of larval red rods is perhaps a function of age of the retina and seems to be independent of metamorphosis of the body and the thyroid hormone.