Morphological changes induced by prolonged darkness in the retinal pigment epithelium of the turtle (Pseudemys scripta elegans)

The retinal pigment epithelium of Vertebrates was shown to be sensitive to cyclic oscillations of light and darkness. The morphological changes induced by prolonged darkness on the retinal epithelial cells of the freshwater turtle were studied, with particular regard to their localization and to their reversibility if animals are recovered under cyclic light. The eyes were processed for light and electron microscopy and a morphological and morphometric analysis was performed on the specimens. After 7 days of prolonged darkness, the vitreal extremity of some epithelial cells was partially detached; on the basal zone the infoldings were missing and vesicles and tubules, often arranged in rows, were observed. After 30 days of prolonged darkness, partial or complete double layers of epithelial cells were present: the superficial layer was connected, by means of the apical fringes, to the photoreceptors, whilst the deepest layer showed vesicles and tubules on its basal zone. After 7 days of recovery to L:D = 12:12, no cyclic activity was demonstrated and only occasional double layers of cells were present; on the basal surface isolated basal infoldings were present where two adjacent cells were joined together. It could be concluded that the detachment of the apical part of some cells, rapidly covered by the lateral sliding of the adjacent cells, and the substitution of the basal infoldings with vesicles and tubules could represent the morphological response of the retinal epithelium to the functional changes induced by prolonged darkness.