Studies on the Structure and Function of the Digestive System in Teleost Larvae-V.

Epithelial Changes in the Posterior-Gut and Protein Ingestion

Abstract

The condition of the posterior-gut (rectum) in 22 teleost postlarvae was compared with one another to observe changes in the epithelium after feeding.The posterior-gut is distinguished from the mid-gut by the constriction with the sphincter.After feeding, many acidophilic granules which are well stained with eosine, orange G and azocarmine G, are found in the supranuclear region of epithelial cells at the posterior-gut both in rearing and natural larvae.
These granules are generally 1-6μ in diameter and sometimes over 10μ in well growing larvae of halfbeak and puffer.The larvae which have fed more actively have more numerous and larger granules, which are not in the lower half of the mucosal folds and the bottom region but are in the upper half.In some species the posterior-gut is provided with villf-iike mucosal projections different from mucosal folds in common species.These facts suggest that the appearance of acidophilic granules relates intimately with ingestion of nutrients from the gut lumen.
Since these acidophilic granules show positive reactions in detecting phenol group, indolyl group, and guanidyl group, they may be proteins or the substances which are chiefly made of proteins.It is well known that in the new-born mammals intact protein ingestion is brought by pinocytosis to receive antibodies from their mothers through colostrum.Then, large globules which are stained with eosine appear in epithelial cells of the small intestine.The fact that these globules are similar to acidophilic granules of teleost larvae both in morphological and histochemical features indicates that proteins are ingested intactly in teleost larvae.
At the transitional stage from larvae to juveniles when the gastric gland becomes functional and pyloric caeca differentiate, acidophilic granules gradually disappear.It is speculated that postlarval digestive system attains to the adult level not only in structure but also in function, and that according to the change in digestive mechanisms proteins are not ingested intactly in the posterior-gut.Food items are quickly transported backward and are interrupted by the posterior constriction, and excreted from the anus in a short time.The fact that epithelial cells in the mid-gut are occupied by fat droplets which are absorbed from food items suggests the difficulty of their displaying many-sided functions at the same time.Based on these facts, it is conceivable that intact protein ingestion in the posterior-gut plays an important role in protein ingestion at the postlarval stage.
The fact that intracellular digestion which is common in the lower groups of invertebrates is recognized in teleost larvae indicates the peculiarity of the larval digestive and absorptive mechanisms, and reflects the history of the developmental process of the animal digestive system.