Immunocytochemical identification and localization of APUD cells in the gut of seven stomachless teleost fishes

AIM To study the cell types, localization, distribution density and morphology of APUD cells in the intestinal mucosa of stomachless teleost fishes. METHOD By using the peroxidase-antiperoxidase complex (PAP) immunocytochemical staining technique the identification, localization and morphology of immunoreactive (IR) endocrine cells seattered in the intestinal mucosa of grass cam (Cyenopharyngodon idellus), black carp (Mylopharyngodon piceus) and common carp (Cyprinus carpio) were investigated with 20 kinds of antisera prepared against mammalian peptide hormones of APUD cells, and likewise by using avidin-biotin-peroxidase complex (ABC) method those of silver carp (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis), silver crucian carp (Carassius gibelio) and bluntnose black bream (Megalobrama amblyocephala) were also studied with 5 different antisera. The replacement of the first antiserum by phosphate buffered saline (PBS) was employed as a control. in endocrine cells were counted with a square-mesh ocular micrometer from 10 fields selected randomly in every section of each part of the intestine specimen, The average number of in endocrine cells per mm(2) was counted to quantify their distribution density. RESULT Gastrin (GAS)-, Gastric inhibitory peptide (GIP)-, glucagon (GLU)-, glucagon-like immunoreactants (GLI)-, bovine pancreatic polypeptide (BPP)-, leucine-enkephalin (ENK)- and substance P (SP)-IR endocrine cells were found in the gut of grass carp, black carp and common carp, and somatostatin (SOM)-IR endocrine cells were only seen in common carp. GAS-, GIP and GLU-IR endocrine cells were found in the intestinal mucosa of silver carp, bighead, silver crucian carp and bluntnose black bream. Most of IR endocrine cells had the higher distribution density in the foregut and midgut, and were longer in shape. They had a long apical cytoplasmic process extended to the gut lumen and a basal process extended to adjacent cells or basement membrane and touched with it. Sometimes, the basal cytoplasmic process formed an enlarged synapse-like structure in the contiguous part with basement membrane. This phenomenon provided new morphological evidence for neuroendocrine and paracrine secretory function of these enteroendocrine cells. CONCLUTION At least 8 kinds of IR endocrine cells were found in the gut of stomachless teleost species for the first time in China. These in endocrine cells scattering in the gut mucosa belong to the APUD system. Among them, the hormones secreted by SP-, ENK-, SOM- and GLU-IR endocrine Cells belong to the peptides of dual distribution in the brain and gut, This provided new evidence for the concept of brain-gut peptide, According to the cell types, distribution density, morphological characteristics and variety in shape of APUD cells in the gut of stomachless teleost fishes, it is deemed that the digestive tract of fishes is also an endocrine organ of great importance and complexity.