The Course of an Axon of a Dermal Photoreceptor Cell in Onchidium : Reconstruction Study through Serial Semi-thin Sections of the Osmium-Impregnated Dorsal Mantle Papilla(Issue Commemorating the Directorship of Prof. Yoko HASHIMOTO, at the Department of Physiology)

Dermal photoreceptor (DP) cells, an extraocular photoreceptor, were distributed abundantly in clusters in the dermis of the dorsal mantle papillae of Onchidium. The DP cell was oval in shape and measured 40～50 μm in diameter. The distal portion of the cell consisted of massive microvilli. The proximal one contained a large, irregularly-shaped nucleus and a great number of photic vesicles of 80 nm in diameter. A single axon extended laterally from the confined boundary area between the distal and proximal portions. The axon of the DP17 cell which was located at the periphery of a cluster containing 19 DP cells was examined in the present study. The axon was observed from the site of emergence up to the site joining the small papillar nerve (SPN) through 222 of 700 serial semi-thin sections (0.4 μm in thickness) of an osmificated specimen at both 200 kV with an H-700 and at 1,000 kV with an H-1250M Hitachi electron microscope. The axon arose from the basal area of the microvilli, traveled in the dermal connective tissue, passed nearby muscle fibers and lumens, and finally joined an SPN which was located outside the cluster. The axon was variable in size and in structure at various parts of its course, and was enclosed with a sheath structure. These results suggest that the axon may conduct the photic information from the DP cell to the central nervous system