Birefringent periodicities in amphibian rod outer segments

Birefringence variations, seen as regularly spaced altering light and dark rings (bands), have been observed along the length of unfixed, freshly isolated rod outer segments (ROS) of both Rana pipiens and Xenopus laevis. In our hands, the spatial frequency of the banding pattern is from 1.3-1.6u/band in Rana pipiens and 1.8-2.2u/ band in Xenopus laevis ROS, both corresponding closely to determinations we made in the same animals of the quantity of new ROS disks added each day. To further probe this correlation, Xenopus laevis were maintained at 16°C to lower the disk renewal rate. A similar correspondence was found, with the banding pattern and renewal rate both at 0.8-1. Ou/day. Further experiments involving Xenopus laevis placed on altered lighting cycles have suggested the existence of two normally superimposed periodicities. The more intense component is driven by the environmental light cycle, and thus may be regarded as diurnal. The less intense component is seen infrequently, suggesting lability, and apparently follows a 24-hour period in both constant light and darkness.