Protein and lipid absorption in the intestinal mucosa of adult lampreys (Petromyzon marinus L.) following induced feeding

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Abstract
Young adult lampreys (Petromyzon marinus) were force-fed either on polyunsaturated lipid or on a protein tracer following a period of starvation to follow the absorption and intracellular transport of these substances in the intestine. Type A and B absorptive cells of the diverticulum, anterior intestine, and transition zone are the principal cells of lipid absorption and transport, whereas protein absorption is confined to caveolated cells of the posterior intestine and hindgut. Lipoprotein, corresponding to very low density lipoprotein (VLDL), becomes localized in smooth and rough endoplasmic reticulum, smooth tubules, and in the Golgi saccules and vesicles, while lipid droplets are localized in the cytoplasmic matrix. The localization of VLDL in the lateral intercellular space and extracellular space, including the submucosal connective tissue and the lumina of the capillaries, are believed to be an expression of the manner in which lipid is absorbed and transported in the anterior segments of the lamprey intestine. Protein absorption in caveolated cells follows the pattern noted in cells of similar function in other fishes and involves pinocytotic vesicles, a tubulovesicular network, heterophagic vacuoles, and likely a lysosomal system for degradation and condensation of the endocytosed material. The presence of regions specialized for lipid and protein absorption in the intestine of adult lampreys is comparable to the situation in many other fishes.