Intermitochondrial junctions in the heart of the frog, Rana esculenta. A thin-section and freeze-fracture study.

In muscle fibers of the frog heart, junctions between outer membranes of adjacent mitochondrial profiles are occasionally found. In thin sections of embedded tissue and of mitochondrial pellets, the intermitochondrial junctional space is 5.4 +/- 0.15 nm; the external leaflets of the membranes are joined by periodic structures separated from each other by 16.3 +/- 0.29 nm. There are 65.3 +/- 2 periodic structures per micron of membrane measured on a section perpendicular to the junction. After cryofracture, the outer membrane is cleaved into two parts. Closely packed, parallel rows of large particles and furrows are found either on the P-, or on the E-faces. The rows of particles are 11 +/- 0.3 nm thick and are separated from each other by 16.5 +/- 0.46 nm, their density being 65 +/- 2.28 per micron of the membrane. In junctional areas, rows of particles on one membrane correspond with the furrows on the other membrane. Intermitochondrial junctions appear to be real structures and not artifacts due to preparation procedures. The conditions of their occurrence are discussed.