Intermembrane bridges within membrane organelles revealed by quick-freeze deep-etch electron microscopy.

Intracellular membrane-bounded organelles, such as the endoplasmic reticulum, Golgi apparatus, and mitochondrion, possess and maintain their shape and intrinsic relationship due to the nature of their membrane organization. To reveal the membranous attachments that support these shapes and relationships, we examined various kinds of cells by quick-freeze deep-etch electron microscopy. In the cisternae of the endoplasmic reticula, we found intermembrane bridges linking opposite membranes of the cisternae. Membranes of adjoining rough endoplasmic reticulum cisternae were linked by intermembrane bridges crossing a narrow cytoplasmic gap between cisternae. Intermembrane bridges were also found in and between the Golgi cisternae and in nuclear envelopes. Three kinds of intermembrane bridges were found within mitochondria: one linking between outer and inner mitochondrial membranes and the other two spanning the intracristal space and intercristal matrix space. The presence of intermembrane bridges within membrane organelles, except for those between rough endoplasmic reticulum cisternae, was seen in all cell types examined. Intermembrane bridges within membrane organelles provide a structural basis for the membrane organization of the organelles and thus may contribute to the functional integrity of the organelles.