Fine Structure of Gut Epithelial Cells of Larval and Juvenile Carp During Absorption of Fat and Protein

The gut epithelium of larval and juvenile carp is composed principally of columnar cells and intermingled goblet cells. In the mid gut of juveniles so-called pear-shaped cells are occasionally found among the columnar epithelial cells.On the basis of absorption patterns, the gut of larvae or juveniles examined after feeding can be divided into two major portions: the mid gut and the posterior gut. In the mid gut, columnar epithelial cells are crowded with fairly large fat droplets. During fat absorption the microvilli and terminal web of the columnar cells appear normal, and no pinocytotic invaginations are seen. On the other hand, in the posterior gut, the columnar epithelial cells are loaded with peculiar vacuoles containing electron lucent materials related to ingested protein. In these cells, the apical cytoplasm is characterized by vigorous pinocytotic invaginations, small vesicles, tubules, and bundles of fine filaments, all associated with the vacuoles.Electron microscopic observations revealed that fat and protein are absorbed by different pathways in teteost larvae. The mid gut appears to be mainly involved in fat absorption without a process of pinocytosis, and the posterior gut is mainly implicated in protein absorption by a process of pinocytosis.