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N. Katagiri, T. Arii, Y. Shimatani, Y. Katagiri; Extraocular Photoreception of a Marine Gastropod, Onchidium: Three-dimensional Analysis of the Papilla on the Juvenile Dorsal Mantle Examined with High Voltage Electron Microscope . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2854.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Onchidium responds the small shadow that is given suddenly and contracts locally a part of the mantle. The dermal photoreceptor (DP) cell is one of extraocular photoreceptors. It comprises the unique multiple photoreceptive system of Onchidium. It has fine structures typical of rhabdomeric-type photoreceptor cell of gastropod eyes and depolarized response to light. DP cells are numerously distributed in the dermis of the papillae on the dorsal mantle although they cannot be distinguished from the surface. In adult, a single axon extends from the lateral side of DP cell, takes a meandering course through the intercellular connective tissue, and finally joins a small papillar nerve. To examine the role of the DP cell, a small papilla was three-dimensionally reconstructed. Methods: Tiny juveniles (10mm in body length) were osmium-impregnated and embedded in epoxy resin. Serial semi-thin sections (0.3µm in thickness) of the papilla were examined with high voltage TEM at 1,000kV. For reconstruction study, Rise OZ-95-32 reconstruction system was used. Results: DP cells are situated in a cell cluster in the dermis. The DP is trapezoidal in shape. The distal portion consists of the microvilli and the proximal cytoplasm contains photic vesicles. An axon arises from the lateral side, runs together the axon of neighboring DP cell and joins a small nerve bundle as seen in DP cells of adult dorsal mantle. Giant stellate MCs are found around the cell cluster. The MC has an irregular shaped perikaryon and many processes that orient in various directions and reach partially the basal lamina of the epidermis. The MC forms a complex network with processes of fellow MCs. The MC network surrounds the DP cells and the nerve bundle. Conclusions: Light evoked contraction was observed in the isolated papillae from the dorsal mantle. It seems to be mediated by the peripheral reflex system in which the CNS is not involved. The present study suggests the presence of the intrinsic reflex system in the Onchidium dorsal mantle, which includes DP cells, effector stellate MCs and nerve fibers.
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