[Ultrastructure of the air-blood-intestinal respiration barrier of the American sheatfish Hoplosternum thoracatum].

The fish, Hoplosternum thoracatum, lives in water with a permanently reduced content of oxygen. The respiratory gut as an accessory respiratory organ makes it possible to breath atmospheric oxygen and facilitates gaseous interchange. It forms a large, dilated tube containing a voluminous bubble of air. The wall of the respiratory gut is extremely thin. Below the peritoneum there is situated a branching plexus of blood vessels. The tributaries extent perpendicularly and reach the epithelium. They form a rich anastomosing capillary network. The capillaries are covered with attenuated processes of squamous epithelium. The thickness of epithelial processes ranges roughly from 0.5-1.0 micron. The basal part of the respiratory epithelium is lined with basal lamina which, in many places fuses with basal lamina of the capillary endothelium. The air-blood barrier of the respiratory gut is, in principle, built up according to the known general bauplan and is formed by thin epithelial and endothelial components and complex of basal laminae. The nonrespiratory area of the gut is lined with columnar epithelium of enterocyte type.