The distribution of actin immunoreactivity in rhabdomeres of tipulid flies in relation to extracellular membrane shedding

Summary Rhabdomeres of tipulid flies lose membrane during turnover from a shedding zone composed of microvillar tips. These distal domains lack intramicrovillar cytoskeletons and appear to be empty sacs of membrane. Recent concerns about the role of ninaC mechano-enzymes in the architecture of dipteran rhabodomeral microvilli and the dynamic role that they may play in the creation of shedding zones demand an examination of the distribution of actin in tipulid rhabdomeres. We compared rhabdomeres from tipulid retinae incubated before fixation for immunocytochemistry in a buffer without additives and a stabilising buffer that contained a cocktail of cysteine protease inhibitors; both were challenged by an anti-actin antibody for immunogold labelling after embedding in LR White Resin. Shedding zones thus processed collapse to structureless detritus. Stabilised and unstabilised shedding zones were immunonegative to anti-actin. To ensure that the negative results were not consequent upon conformational changes generated by the processing protocol, we examined microvilli of degenerating rhabdomeres of the Drosophila light-dependent retinal degeneration mutant rdgB ^KS222 (which separate and collapse without creating a shedding zone) and found the detritus they generate to be immunopositive to anti-actin. Stabilised and unstabilised regions of basal regions of tipulid rhabdomeres were equally immunopositive. We infer that (a) actin is absent from shedding zones; (b) actin is not degraded by microvillar cysteine proteases. The implications of these conclusions are discussed in relation to some functional models of arthropod photoreceptor microvilli.     

Efferent fibers and daily rhabdomal changes in the anteromedial eye of the liphistiid spider,Heptathela kimurai

The presence of efferent fibers in the retina of liphistiid spiders, kept in natural daily cycles of illuminance, was examined by electron microscopy. The efferent fibers were observed to extend their processes through the ocellar nerve to the retina. They contained characteristic large electron-dense granules and branched repeatedly within the retina with varicosities, to provide synaptoid contacts with the receptor cells. They ran mostly among receptor cells and glial cells but sometimes protruded into receptor cells to establish invaginated synaptoid contacts. The synaptoid structures were characterized by spherical clear vesicles located at the presynaptic region, with electron-dense material adhering to the plasma membranes of the receptor cell and the efferent fiber, and a cleft about 10 nm wide formed by the two opposed parallel membranes. The clear vesicles and the electron-dense granules were secreted by exocytosis. The efferent fiber was characteristically presynaptic in relation to the receptor cell. In addition, the rhabdoms differed in size from day to night.