LONGTERM DARK INDUCED FINE-STRUCTURAL CHANGES IN CRAYFISH PHOTORECEPTOR MEMBRANE

1. Fine structural effects of longterm continuous darkness (2-16 weeks) have been quantitatively measured in five rhabdom parameters of adult crayfish (Procambarus) using transmission (Figs. 3-6) and freeze fracture (Figs. 13-15) electron micrographs. The most striking modifications took place during the first four weeks. 2. During the first two weeks in darkness four changes occurred: a) the diameter of rhabdom microvilli increased significantly (Figs. 9, 10), b) the diameter of particles (one or more rhodopsin molecules) visualized on the protoplasmic face of the receptor membrane by freeze fracture (Figs. 13-15) decreased significantly, whereas c) their number increased (Fig. 16) and d) lysosome related bodies near the rhabdom (Figs. 3, 4) in all five retinular regions studied strongly decreased in number (Fig. 12). 3. During weeks 3 and 4 in darkness two further changes occurred: a) the normally regular microvillus pattern of the photoreceptor membrane (Figs. 1, 2) was significantly disrupted and b) the number of membrane particles then fell to about half their initial count (Fig. 16) except in retinular cell eight where they maintained control levels for up to two months in the dark. 4. Most of these effects of prolonged darkness have clear functional implications. Disruption of microvillus pattern and sustained decrease in visual pigment concentration after more than two weeks without light reflect deterioration in vision. Alterations in microvillus diameters and lysosome density imply changes in the membrane turnover steady state resulting from protracted darkness. The data demonstrate that normal photoreceptor membrane could not be maintained in this eye for long in the absence of light. 5. The dark-induced disorganization of microvillus regularity confirms the earlier, disputed demonstration but also shows regional differences in susceptibility which might explain different conclusions drawn from local samples. 6. New distinctive features of retinular cell eight were found in its minimal sensitivity to microvillus disruption compared with the seven regular retinular cells and its maintenance of control densities of membrane particles despite two months continuous darkness. © 1979 Springer-Verlag.