ULTRASTRUCTURAL-CHANGES OF THE MICROVILLAR CYTOSKELETON IN THE PHOTORECEPTOR OF THE CRAYFISH ORCONECTES-LIMOSUS RELATED TO DIFFERENT ADAPTATION CONDITIONS

In the retina of crayfish microvilli of seven of the eight photorecept or cells build highly organized structures, the rhabdoms. Cytoskeletal elements inside the microvilli were investigated in conventional and slightly extracted electron microscopical preparations. In conventiona l preparations the ultrastructure of these cytoskeletal elements depen ded on the adaptational state of the animal. They appeared as central filament-like structures inside each microvillus when dark-adapted ret inae were prepared and fixed at night in the absence of calcium. Chang es of these conditions (light, daytime, or calcium concentration) impa ired the detectability of these central filaments; in light-adapted ey es prepared at midday they were rarely seen. Nevertheless, single micr ovillar filaments were present in light-adapted retinae after mild cel l permeabilization with the saponin beta-escin. They appeared as a reg ular structure in each microvillus, often attached to the membrane. Th eir fine structure was consistent with the ultrastructure of single ac tin filaments as indicated by fast-fourier-analysis and further suppor ted by the presence of anti-actin immunoreactivity in electron microsc opical and immunocytochemical preparations. These results indicate tha t microvillar filaments are not necessarily destroyed by light as prev iously described; we suggest that their appearance inside the microvil lus might be altered by the properties of associated, maybe sidearm-li ke proteins.