The extrarhabdomeral cytoskeleton in photoreceptors of Diptera. I. Labile components in the cytoplasm

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Abstract
Labile cytoskeletal structures in the cytoplasm of photoreceptors of Lucilia and Drosophila were stabilized before primary fixation for EM by retinal infiltration with 2 inhibitors of thiol proteases, Ep-459 or Ep-475. Alternatively, pretreatments employed EGTA in combination with the Ca2+ ionophore A23187. The following cytoskeletal structures were revealed. Radial, robust filaments run roughly parallel to the axes of the rhabdomeral microvilli and may be continuous with the axial microvillar filaments. They have diameters of 8 nm upwards, and are occasionally seen in association with radial microtubules and with pigment granules. Slender radial filaments with diameters in the 6-8 nm range extend for shorter distances from the bases of microvilli, and are also associated with endocytotic structures. The receptor cytoplasm is densely occupied by an ill-defined, filamentous network. Bundles of slender filaments run longitudinally on each side of rhabdoms of R1-6 in Lucilia, close to the plasma membrane. Dimensions cited for all categories of filament must be treated with caution because of problems of resolution. Photoreceptors do not bind the fluorescent F-actin probe NBD-phallacidin either without or after treatment with thiol protease inhibitors, and slender filaments are of greater diameter than the 4-5 nm obtained for identified actin filaments in the basement membrane of the compound eye of Lucilia. Infiltration of retinae with Ep-459 or Ep-475 neither prejudices phototransduction, nor impairs the radial migrations of granules of screening pigment in response to light or dark adaption. The status of these cytoskeletal elements is discussed in terms of the dynamic processes of the photoreceptors, and of various labile filaments described from recent studies of vertebrate material using the deep-etch freeze-fracture technique.