Light and electron microscopic studies on the absorptive cells of the intestine, caeca and rectum of the adult rainbow trout, Salmo gairdneri, Rich

The absorptive cells lining the mucosa of the intestine, caeca and rectum of farm‐reared adult rainbow trout (Salmo gairdneri Rich.) were studied by light and electron microscopy. Most of the intestinal absorptive cells showed morphological characteristics of lipid absorption. The cytoplasm of a few cells was closely packed with mitochondria. The caecal absorptive cells resembled those of the intestine but contained more apical dense bodies and more complex lamellar structures which, in conjunction with the intercellular spaces, appeared to play more active roles in lipid absorption. The rectal lumen was divided into peripheral and central parts by the complex mucosal folds. The surface of the peripheral part was mostly lined by vacuolated cells except for small patches of non–vacuolated (generative) cells between the bases of the folds. The central lumen was lined mainly by a second type of non‐vacuolated cell. The vacuolated cells showed structural indications of a function of absorption of protein macromolecules. There is morphological evidence to suggest that the pattern of absorption of lipids and proteins in the adult rainbow trout is similar to that demonstrated in stomachless teleosts and that both intraluminal and intracellular digestion of proteins co‐exist in this teleost.