Freeze-fracture and thin section study of the rough ER-Golgi interface in the pancreatic acinar cell. Resemblance between the intramembranal architecture of the outermost Golgi cisterna and the post-rough ER vesicular and tubular elements.

Post-ER membranous structures are clearly observed in pancreases fixed with aldehydes and subsequently with reduced osmium. Close to the transitional rough ER, clusters of vesicles of approximately 56 nm diameter are consistently present. In some cells, tortuous tubules appear enmeshed by the approximately 56 nm vesicles and by irregular, vesicular formations. In freeze-fracture replicas, the membranes of the bulges and tubules that protrude from the transitional rough ER differ from those of the donor compartment. These protrusions are herein designated as the budding chamber of the transitional rough ER. Quantitative and qualitative observations performed previously and in the present study show that the P and E freeze-fracture faces of the outermost Golgi cisternal membrane possess patterns of texture that are unique among membranes. The P-face exhibits a very high density of intramembranous particles of dimensions among the smallest yet described; E-faces show rugosities and an unusually high density of intramembranous particles of normal size. The membranes of the budding chamber, the putative transport vesicles of approximately 56 nm diameter, the sinuous tubules and the vesicles of irregular size and shape exhibit P and E fracture faces with textures indistinguishable from those of the corresponding P and E faces of the outermost Golgi cisterna.