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SC Chamberlain, JB Schallek, BJ Herloski, BM Michaud, RB Sacunas; In Vivo Rhabdom Shedding in Horseshoe Crab Ventral Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1425.
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Purpose: In summer, transient and light-driven rhabdom shedding are robust processes in the photoreceptors of the lateral of the horseshoe crab. In this study we investigated rhabdom shedding in the photoreceptors of the ventral eye, which normally receive far less light because they are in the shadow of the animal. Methods: Animals exposed to natural lighting were fixed in late afternoon. Other animals were exposed to artificial lighting of the same intensity as the natural lighting, but from above and below to increase the light exposure of the ventral eye. The endorgan was dissected free and fixed in methanol/formalin. Fluorescence immunocytochemistry was performed on frozen sections to show opsin and arrestin IR. Confocal micrographs were obtained and analyzed with NIH Image software. Results: In animals exposed to natural lighting, large amounts of debris from light-driven shedding filled the R-lobe and extended into the A-lobe. No transient shedding was observed. In animals exposed to light from below, evidence of both light-driven and transient shedding was observed; the debris from transient shedding formed a band in the A-lobe at the greatest distance from the rhabdomere. Estimates of the percentage of membrane shed by light-driven shedding under natural lighting averaged about 21%. Conclusion: Even during the summer, when transient shedding in the lateral eye is a robust process, transient shedding seems to be suppressed in the photoreceptors of the ventral eye. The sensitivity of light-driven shedding in ventral photoreceptors is much greater than in the lateral eye under natural summer lighting. Increasing the illumination of the ventral eye to match that of the lateral eye, as for example if the animal were upside down for a protracted period of time, appears to activate transient shedding in addition to light-driven shedding. The percentage of rhabdom shed is similar to that shed in the lateral eye, even though the normal ambient illumination is far less.
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