Association between mitochondria and rough endoplasmic reticulum in rat liver

Mitochondria were isolated from rat liver homogenate by both zonal and sedimentation equilibrium centrifugation. From electron microscopic examination of thin sections it was observed that 81% of the isolated mitochondria were in contact with rough endoplasmic reticulum (RER). Intact, non-sectioned mitochondria subjected to negative staining procedures appeared to show points of connection between RER and the outer mitochondrial membrane. After treatment of mitochondria with digitonin to remove outer membranes, some of the resulting mitoplasts (intact inner membranes) remained closely associated with RER. Serial section analysis of intact rat liver indicated that RER saccules fit over mitochondria like caps providing broad areas of contact between the two organelles. The RER saccules were also observed communicating with more than one mitochondrion.