Ultrastructure of intercellular junctions in the rat exocrine pancreas stimulated by pancreozymin

Intercellular junctions in the exocrine pancreas, through which pancreatic enzymes may leak out from the lumen into the intercellular spaces, were studied in the rat by means of electron microscopy using tannic acid as a tracer and freeze-fracture methods as well as conventional thin sections. In thin sections and freeze-fracture replicas from the pancreas stimulated or not stimulated by exogenous pancreozymin, tight junctions were located between adjacent cells of the acinus or duct at the level just below the lumen. Observation of freeze-fracture replicas revealed that the number of strands or furrows in the tight junction did not differ significantly between the two groups but the width of the tight junction area was narrower and the distance between neighboring strands or furrows in the junction was smaller in the stimulated pancreas than in the control pancreas. Tannic acid, the molecular weight of which is smaller than that of pancreatic enzymes, could not pass beyond the tight junction area into the lumen. This study suggests that the pancreatic enzymes can not leak out from the glandular lumen into the intercellular spaces by dissociation of intercellular junctions even if the pancreas is stimulated by a high concentration of pancreozymin.