Intermediate cells of the pancreas: I. ultrastructural characterization

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Abstract
The normal existence of cells in the pancreas with a structure intermediate between those of exocrine and endocrine cell types has long been a matter of dispute. The present study shows that, based upon morphological criteria, such intermediate cells are present in both the endocrine and exocrine tissues of the normal pancreas of the rat, guinea-pig, rhesus monkey, goat, chicken and frog. There is a tendency for intermediate cells to occur most frequently in the frog, where exocrine and endocrine cells are intermingled, and least frequently in higher species such as the rat, guinea-pig, monkey and goat where the endocrine cells are localized in discrete islets. Their occurrence in the chick appears to lie between these 2 extremes. The existence of intermediate cell types has been attributed to a ‘transformation’ of one form of specialized cell in the pancreas into another in response to a metabolic demand. However, the widespread occurrence of inter mediate cells in the normal pancreas suggests that they represent, ab initio, a distinct category of cell, the existence of which poses interesting questions concerning the genetic control of their specialized functions and of developmental processes in the pancreas. Moreover, intermediate cells, such as acinar cells containing endocrine β-granules might serve as a source of insulin additional or alternative to that provided by cells that are wholly endocrine in character.