Structure of Mitochondria

Abstract

Transmission electron microscopy of thin sections has provided most of the basic information responsible for our present general concept of mitochondrial organization. Observations on material from a variety of sources have confirmed that the ground plan of mitochondria is generally the same from one type of cell to another (e.g., see Vogell 1963, Steinert 1969). The view first proposed by Palade (1953) and Sjöstrand (1953) seems correct, at least in general outline. In thin sections, mitochondria appear to be enclosed by two membranes. An outer membrane delineates the mitochondrial-cytoplasmic interface. The inner membrane enclosing the mitochondrial matrix forms infoldings (the cristae) projecting into the mitochondrial lumen. The cristae can be lamellar or tubular. The size of the space between the two membranes differs depending on the technique used. With freeze-substitution of fresh specimens, for example, the membranes appear closely apposed (Malhotra 1966). A close apposition of inner and outer membranes is also shown in freeze-cleavage under at least some conditions (e.g., Hackenbrock 1968, 1972) although the two membranes appear to separate in the condensed state. Generally, however (with negative staining or conventional fixation), a significant space is apparent between the two membranes. Several kinds of junctions between mitochondrial membranes have been reported (e.g., Newcomb et al. 1968, Hall and Crane 1970, Tani et al. 1971, Bartók et al. 1973, Morton et al. 1973, Saito et al. 1974). These structures can take the form of a dense midline in the intracristal space or, alternatively, of ladder-like connections spanning the space between membranes. Apparently, the appearance of these connections depends on physiological conditions (e.g., hydration after a period of dehydration, Bartók et al. 1973). Although they may be virtually absent normally, in most mitochondria they appear after brief delays in fixation (Saito et al. 1974). These results suggest that, in at least some cases, the junctions are not normally present but they do reflect a molecular organization of mitochondrial components. At least in special functional states this molecular organization becomes apparent at these special junctions.