An electron microscope study of the development of the exocrine and endocrine pancreas with special reference to intercellular junctions.

Intercellular junctions in pancreatic acinar, duct and endocrine cells were studied by thin section and freeze-fracture methods in developing rats and mice. Undifferentiated cells were joined by the zonula occludens and isolated fragments of tight junctional strands. Small gap junctions were either occasionally associated with tight junctional strands or appeared independent of them. During the morphological differentiation of acinar cells, strands of the zonula occludens developed to form a complicated meshwork while gap junctions rapidly increased in size. Duct cells were joined by the less-developed zonula occludens but gap junctions were rarely seen. In the neonate, intercellular junctions were similar to those in adult acini and intercalated ducts. Endocrine cells were joined by maculae occludentes and small gap junctions. During late prenatal days, the macula occludens increased in size and gap junctions in number. Sometimes tight junctional strands disappeared to leave membrane elevations, some of which were associated with small gap junctions. Maculae occludentes on endocrine cells were gradually fragmentized and diminished during postnatal development. They were completely lost in the rat. These results suggest that intercellular junctions play important roles in pancreatic development. In particular, the transient development of maculae occludentes is associated with endocrine cell development, and intercellular communication mediated by gap junctions may be important for the differentiation of acinar and endocrine cells.