Electron microscopic study on gut epithelium of the tench (Tinca tinca L.) with respect to its absorptive functions.

Few morphological differences are seen along the intestinal tract of the adult tench (Tinca tinca L.) a stomachless freshwater teleost. However, three segments can be distinguished, when function and structure of enterocytes are studied. The enterocytes of the proximal segment are found to be concerned with dietary lipids absorption. In the cell, absorbed fats are seen in two inclusion bodies: lipid particles and lipid droplets. Only lipid particles are involved in direct transport of absorbed fatty acids in the blood circulation, as in lymphatic vessels. Lipid droplets seem to be involved in temporary storage of fatty acids. Special features are found in enterocytes of the short middle segment; these cells show many invaginations and pinocytosis figures, a well-developed tubulo-vesicular network and large vacuoles in the supranuclear hyaloplasm. Such characters bear a resemblance to descriptions of the gut of some newborn mammals. The great permeability of this epithelium to macromolecules is demonstrated by the administration of horseradish peroxidase (HRP). Enterocytes of the distal segment show, at their basal pole, numerous invaginations of the plasma membrane, and a large mitochondrial population. Morphological similarity suggests a functional analogy with epithelia involved in water and ions transport.