Changes in length and disk shedding rate of Xenopus rod outer segments associated with metamorphosis

Abstract

Histological examination of the retinae of Xenopus tadpoles undergoing the extensive transformations of metamorphic climax revealed a progressive and dramatic decrease in the length of rod outer segments (r. o. s.) (by 1.22 µm/day), which was reversed after the completion of metamorphosis, when r. o. s. grew longer (by 1.11 µm/day). The rate of r. o. s. disk addition during these two periods was determined by examining the incorporation of [ 3 H]-leucine by light microscopic autoradiography. The band of labelled protein in r. o. s. was displaced sclerally at a rate of 1.70 µm/day during the first half of metamorphic climax, and of 1.56 µm/day in young juveniles during the second month after metamorphosis. The similarity of the rate of band displacement at these times indicates that the changes in r. o. s. length associated with metamorphosis result from major changes in the rate of disk shedding and/or phagocytosis, which was about 2.92 µm/day pre-metamorphically and 0.45 µm/day post-metamorphically. E. m. observation at these stages and during the final stages of metamorphic climax revealed no significant alterations in the cellular organization or ultrastructure of rods or pigment epithelium, even though some r. o. s. were only 3 µm long. This large change in r. o. s. length undoubtedly influences the animal’s scotopic sensitivity and the relative mesopic activity of its rods and cones, and may have important effects on the animal’s visual physiology.