Membrane unbound crystals in pancreatic acinar cells of the mouse

The exocrine pancreas of normal mice was investigated electron microscopically. The crystals were encountered in the basal cytoplasm of the acinar cells. They exhibited usually rectangular or rhomboidal profiles, measuring less than 5 mum in length and more than 0.1 mum in width. The matrices of these crystals displayed substructures of parallel dense linear pattern. The center-to-center distances between the dense lines were 100 similar to 150 A. The crystals were not limited at their surface by any membrane, but rather were densely circumscribed or sporadically surrounded by ribosomes. Filaments were interposed between ribosomes and crystal matrices and also between matrices of abutting crystals. The significance of the close relation of ribosomes to crystals was discussed.