The B-cell granules in the dog pancreatic islets are characterized by unique crystalline bar-like shapes under a transmission electron microscope, but their three-dimensional structure could not be observed by conventional transmission electron microscopy. In this study, the authors analyzed the three-dimensional structure of these crystalline B-cell granules using a scanning electron microscope and a method for the reconstruction from the electron micrographs of serial sections. Under transmission electron microscopy, electron micrographs were taken from tissues cut into serial sections, and the whole shapes of the core of B-cell granules were reconstructed. On the other hand,2.5% glutaraldehyde-fixed tissues were treated with the tannin-osmium method of Murakami, freeze-fractured, coated with Pt-Pd slightly, and then electron micrographed with scanning electron microscope.
From these observations, at least two kinds of three-dimensional shapes were recognized: one is a very peculiar shape obtained as if by slicing the cylinder obliquely. However, the cut surface was oblong, with fairly long, parallel sides. The second shape is that of a rhombic hexahedron, which has been reported already by the authors in the human pancreatic islet B-cells. Moreover, the fine particles were observed on the surface of the cores of B-cell granules, although their characteristics could not be ascertained.