Isoparorchis hypselobagri: ultrastructure of the tegument and associated tissues in a digenean inhabiting a gaseous aerobic environment

Adult I. hypselobagri live in the swim bladder of the Indian catfish Wallago attu, a gaseous environment with a relatively high oxygen content. The ventral tegument, which in life is applied close to the swim bladder wall, is relatively unspecialized, showing typical ultrastructural features of the digenean surface. The dorsal tegument, which is exposed to the oxygen-rich surroundings, has numerous pyriform extensions of superficial parenchymal cells closely applied to the base of the surface syncytium. These extensions bear numerous mitochondria and send finger-like processes deep into the basal cytoplasm of the syncytium where they interdigitate with corresponding infolds of the basal tegument membrane. The pyriform parenchymal extensions are connected with underlying nucleated cell bodies via irregular glycogen-filled tubular processes, many of which end blindly in the interstitial tissue or expand into glycogen-filled bulbs beneath the cell bodies. These superficial parenchymal processes associate at gap junctions with ramifications of a distinct deeper parenchymal tissue which contains lipid, residual bodies and glycogen. The dorsal tegument and associated structures may constitute a respiratory organ, taking advantage of molecular oxygen diffusing across the surface syncytium to carry out aerobic energy transduction in the superficial parenchymal extensions. ATP so generated may diffuse inwards for distribution throughout the body in the deep parenchymal tissue. The extensive network of ramifying cytoplasmic tubules is supported by a fibrous matrix of interstitial tissue.