Intermitochondrial junctions in the extraocular muscle of the rat.

Intermitochondrial junctions with a spacing of 17-21 nm were observed in the superior rectus muscle of a rat. Periodic rounded densities are aligned midway between the apposed outer mitochondrial membranes at some of these junctions. Such densities have a diameter of about 8-10 nm and a center-to-center spacing of about 26-30 nm. These junctions occur in cases where one mitochondrial profile is enclosed within another or where two profiles are interlocked so that their combined overall form has a smoothly contoured profile. Intermitochondrial junctions seem not to have been previously described in muscle, but have been reported in other kinds of tissues. In agreement with those previous reports, the presently observed intermitochondrial junctions usually involve mitochondria that display atypical features indicative of tissue abnormality or stress. Such junctions were never observed in normal extraocular muscle.