Opisthorchis viverrini: the tegumental cytoskeleton.

The tegumental cytoskeleton of Opisthorchis viverrini was observed using both conventional transmission electron microscopy and Triton X-100 extraction. The cytoskeletal elements of the newly excysted juveniles, first-week and adult stages are composed of 2 components: firstly, the network of knobbed fibres designated as microtrabeculae which form the principal scaffold of the cytoplasm; and secondly, the microtubules. The microtrabeculae are more densely packed in the newly excysted juveniles and become less densely packed later in the first-week and adult stages. Generally, their compactness in the tegument of each stage is higher in the apical and middle zones than in the basal zone. The results from extraction by Triton X-100 suggest that the microtrabeculae may be composed, at the primary level, of thin and straight fibres, partly coiled up to form knobbed fibres, which are highly cross-linked at the secondary level. At the tertiary level, these knobbed fibres may be coiled up further and form closely aggregated globules that appear as dense dots in cross-section. Most microtubules are confined within the tegumental cells' processes and splay out in the basal zone of the tegument. In addition, there are condensed laminae of cytoplasm with intermittent dense plaques underlining the outer membrane, with microtrabecular fibres inserting into them. This organization may help to stabilize the outer membrane and preserve the surface contour. Along the inner membrane of the tegument, there are hemidesmosomes distributed at regular intervals, with fine fibres radiating out from them to intertwine with the microtrabecular network, which may help to anchor the tegument to the basal lamina. Spines, which exist mainly in the newly excysted juveniles, appear as a crystalline lattice structure whose bases are firmly fused to the inner membrane.