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

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Abstract
Histological examination of the retinae of X. laevis tadpoles undergoing the extensive transformations of metamorphic climax revealed a progressive and dramatic decrease in the length of rod outer segments (ROS) (by 1.22 .mu.m/day), which was reversed after the completion of metamorphosis, when ROS grew longer (by 1.11 .mu.m/day). The rate of ROS disk addition during these 2 periods was determined by examining the incorporation of [3H]-leucine by light microscopic autoradiography. The band of labeled protein in ROS was displaced sclerally at a rate of 1.70 .mu.m/day during the first half of metamorphic climax and of 1.56 .mu.m/day in young juveniles during the 2nd mo. after metamorphosis. The similarity of the rate of band displacement at these times indicated that the changes in ROS length associated with metamorphosis resulted from major changes in the rate of disk shedding and/or phagocytosis, which was about 2.92 .mu.m/day pre-metamorphically and 0.45 .mu.m/day post-metamorphically. EM 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 ROS were only 3 .mu.m long. This large change in ROS length undoubtedly influenced 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.