Two populations of rod photoreceptors in the retina of Xenopus laevis identified with 3H-fucose autoradiography.

The retina of Xenopus laevis contains two populations of rod photoreceptors that differ in their utilization of L-fucose. Following intraperitoneal injections with 3H-fucose, all rod and cone photoreceptors incorporate the label. One day after labeling, much of the radioactivity is associated with the photoreceptor outer segments. In rods, a band of radioactivity is initially located at the base of the outer segment. As the time interval between injection and recovery is extended, the band of radioactivity is progressively displaced toward the outer segment tip. When the autoradiography exposure times are reduced so that cone and most rod outer segments no longer appear labeled, a minor population of rod photoreceptors can be distinguished which remains heavily labeled. The outer segment of the principal rod is 29.8 +/- 0.6 micrometers long and 6.4 +/- 0.6 micrometers in diameter, whereas the outer segment of the minor rod is 19.7 +/- 3.4 micrometers long and 4.5 +/- 0.6 micrometers in diameter, the latter accounting for approximately 2-3% of the total rod photoreceptor population. The rate of 3H-band displacement is 2-fold greater in the minor rod outer segment than the renewal rate of the outer segment in the principal rod. The similarity in relative cell density and size of the minor rod suggests that this photoreceptor corresponds to the blue-sensitive rod of Xenopus recently described by Witkovsky et al. (1981a, b), Vision Res. 21, 863-867, 875-883).