A multiple photoreceptive system in a marine gastropod <I>Onchidium:</I> (2) Three-dimensional reconstruction study through serial semithin and ultrathin sections observed with TEM and HVTEM

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  • イソアワモチの多重光受容系：（２）連続切片の電子顕微鏡画像の三次元構造解析
  • イソアワモチ ノ タジュウ ヒカリ ジュヨウケイ 2 レンゾク セッペン ノ デンシ ケンビキョウ ガゾウ ノ 3ジゲン コウゾウ カイセキ

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Abstract

Three-dimensional structures of the multiple photoreceptive system were investigated through serial sections. Manual reconstruction was performed by overlapping and piling up the traced figures from light microscope micrographs of serial sections (1-5μm thickness), from TEM micrographs of serial ultrathin sections (0.1μm), from HVTEM micrographs of serial semithin sections (0.2 and 0.4μm). The 3D reconstruction system OZ95 (Rise Co.) and DeltaViewer software were also used on above specimens. Manual reconstruction clarified that an optical lens in the dorsal eye was consisted of the complex of several lens cells variously arranged. In the ciliary-type photoreceptor cells of the retina, the outer segment was composed of concentrically arranged 30-50 lamellae of modified ciliary membrane. One modified ciliary membrane originated from one cilium that transformed the rod to the ladle shape toward the periphery, and increased remarkably the surface area. Nine pair of microtubules in the cilium were arranged regularly at the base and divided to two groups (4 and 5 pairs) towards the periphery. Two poles of microtubules may support the outer segment mechanically. The 3D reconstruction of two series of semithin sections (adult and juvenile dorsal mantle) clarified: a thin axon arose from the lateral side of the dermal photoreceptor (DP) cell body and joined into the common nerve bundle together with several axons from different DP cells. In juvenile, the giant muscle fibers (MFs) surrounded the DP cells with their long processes, and the unique stellate shape of MF was reconstructed. By electron tomography method on the ultra-thin section (0.1μm), the relation of photic vesicles and Golgi lamella in microvillous-type photoreceptor cell of the stalk eye was shown.

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