Next to the various images that suggest a reticular mitochondrial morphology, mitochondria are also described as highly dynamic, reticular structures.
Mitochondria occur in various forms depending on the cell or tissue type and even within single cells, reflecting the variety of cellular functions localized to these organelles (Yaffe 1999, Collins et al. 2002). In S. cerevisiae, mitochondria can accommodate to environmental and developmental changes by adopting morphologically different forms and undergo dynamic membrane fusion and fission in the course of mitochondrial inheritance in budding cells, for example (Yaffe 1999, Dejean et al. 2000, Egner et al. 2002, Shaw & Nunnari 2002). In yeast grown on glycerol, mitochondria typically exhibit a strongly branched tubular reticulum with elongated structures, while the reticulum is less elongated in cells grown on glucose (Egner et al. 2002). From an excellent overview of the literature in this dissertation.
The roles of mitochondria in cell death and in aging have generated much excitement in recent years. At the same time, however, a quiet revolution in our thinking about mitochondrial ultrastructure has begun. This revolution started with the use of vital dyes and of green fluorescent protein fusion proteins, showing that mitochondria are very dynamic structures that constantly move, divide and fuse throughout the life of a cell. More recently, some of the first proteins contributing to these various processes have been discovered. Our view of the internal structures of mitochondria has also changed. Three-dimensional reconstructions obtained with high voltage electron microscopy show that cristae are often connected to the mitochondrial inner membrane by thin tubules. These new insights are brought to bear on the wealth of data collected by conventional electron microscopic analysis. From here.
A revised model of the mitochondrial double-membrane structure has recently been provided by EM tomographic imaging techniques (Frey & Mannella 2000, Perkins & Frey 2000, Frey et al. 2002). In EM images, the mitochondrial membranes can be distinguished from other cellular membranes due to the characteristic double-membrane structure. According to the current model, the inner-membrane cristae structures are typically attached to the mitochondrial outer membrane by very thin connections only (Sjöstrand 1953). Also chloroplasts and their inner membrane thylakoid structures seem to have very similar membrane organization to that of mitochondria (Frey et al. 2002, Nichols 2002). Cristae appear in various forms under different conditions in mitochondria and are typically envisaged as simple tubular or more complex lamellar structures (Palade 1952). Presumably cristae and mitochondrial membrane dynamics, in general, is required to control many metabolic processes. Cristae reassembly, for example, results in release of cytochrome c, a marker protein for apoptosis, outside the mitochondria (Scorrano et al. 2002). Stressing mitochondrial function in respiration, oligomerization of ATP synthase (complex V) has recently been linked to the generation of the characteristic cristae appearance. In these studies, disruption of the ATP synthase subunit in yeast resulted in onion-ring like malformed cristae structures, indicating the keen structure-function relationship in the function of these organelles (Paumard et al. 2002). Ibid. dissertation
An important aspect of mitochondrial dynamics is its morphology. Microscopy studies of various types now suggest that the organelle’s structure can appear in one of two very different morphological states (9) at different points in the cell cycle (10). At “inter-phase” stages, mitochondria often appear as a continuously branched reticulum of connected tube-like filaments, with an approximate tube diameter of 0.5 µm (11). During cell division, mitochondria are partially fragmented into hundreds of ellipsoid shaped vesicles (12), thus allowing the partitioning of mitochondrial fragments (and mtDNA) into the two daughter cells. It is possible to observe intermediate morphologies between these two states. From here.
Reconstruction of the 3-dimensional structure by serial section and model building showed that a number of unicellular organisms possess a single branched mitochondrion. The freeze-fracture studies reported in this paper reveal that the mitochondrion forms an extensive reticulum partially surrounding the nucleus and vacuole. The bulk of this reticulum is located at the periphery of the cell, beneath the plasma membrane. Under the growth conditions used in the present study, the mitochondria of the yeast Candida utilis always form a reticulum, regardless of the growth period. It is clear that the separate mitochondrial profile seen in thin sections of permanganate-fixed yeast cells are not discrete round or ovoid mitochondria. These represent, rather, cross-sections of a portion of the mitochondrial reticulum. During cell division, portions of mitochondrial reticulum migrate into the emerging bud and are partitioned among the daughter cells at cytokinesis by septum formation between the 2 cells. From here.
The idea that the mitochondrial compartment of the cell can exist as a reticulum has recently attracted increased attention (Bereiter-Hahn and Voth, 1994; Rizzuto et al., 1998a). However, the fundamental properties of this reticulum are poorly understood. In terms of functional significance, it is now clear that the intracellular environment is naturally heterogeneous with spatially varying energetic requirements. A reticulate structure, possibly itself composed of separate interconnected compartments, might be ideally suited to function under heterogeneous conditions. For example, the long thread-like tubes of the reticulum could serve as `cables’ along which the mitochondrial membrane potential (or the ATP gradient) is transported to reach cellular regions with the most urgent energy needs (Bereither-Hahn, 1990; Bereither-Hahn and Voth, 1994; Amchenkova et al., 1988). Alternatively, the network of interconnected tubes could be an important structural support for the strategic location of point contacts with the endoplasmic reticulum where Ca2+ uptake occurs as a means of intracellular regulation of metabolic activity (Fall and Bennett, 1999; Rizzuto et al., 1998a; Romashko et al., 1998). As a support structure, the mechanical behavior of this network system should play a critical role in intracellular energy regulation. The recent finding that different respiratory states of mitochondria coexist in a single cell also support the idea that a single mitochondrial reticulum is a heterogeneous structure (Dall’Asta et al., 1997; Nunnari et al., 1997; Reers et al., 1995; Bereiter-Hahn and Voth, 1994). Different values of the mitochondrial membrane potential and different mitochondrial DNA conformations characterize these states. From here.