The mitochondrion was identified at the end of the 19th century by Altmann. Benda coined the term mitochondrion, from mito, thread, and khóndrion, granule, because these organelles appeared as thread-forming granules under the light microscope. Only a few years later, most of the relevant functions of mitochondria, such as energy production, cell respiration and inheritance, were suggested by Michaelis, Regaud and Kingsbury. Even the biological origin of mitochondria as a symbiotic fusion between a prokaryote and a primitive eukaryote cell was hypothesized by Portier in these early years, decades before it could be more firmly established (Fig. 1).
Figure 1. Endocytotic vesicles engulfing E. coli in a macrophage from a case of malakoplakia simulate the process that could have given rise to the origin of the mitochondrion: the outer mitochodrial membrane has many similarities to the cell membrane while the inner mitochondrial membrane shares many features with the bacterial membrane. With the advent of the electron microscope, the structure of mitochondria was characterized, and the many variations in their basic organization were recognized. A combination of ultrastructural and biochemical methods allowed a detailed knowledge of mitochondrial physiology and it is still providing us with new information. Comparatively, abnormalities in mitochondrial function are not so thoroughly understood and, in many situations, they are only recognized by subtle, probably non-specific ultrastructural changes. In this review, the main features of mitochondrial structure and function, as well as related physiological and pathological changes, will be summarized.
Origin, structure and function of the mitochondrion.
Current evolutionary knowledge, supported by analogies between mitochondria and bacteria, favor the hypothesis that the mitochondrion originated from primitive aerobic non-photosynthetic bacteria, that entered a primitive eukaryote cell, thus establishing a symbiotic process in which mitochondria provided the cell with a very effective way of obtaining energy and, in its turn, the cell synthesized many of the mitochondrial constituents. Thus, the outer mitochondrial membrane would be the remnant of the cell-derived endocytotic vesicle and the inner mitochondrial membrane would be the correlate of the bacterial membrane. Both mitochondrial membranes have important differences in their biochemical composition: the outer membrane contains a transport protein called porin, that makes it permeable to all molecules of 5000 daltons or less; the inner membrane contains a high proportion of cardiolipin, that makes this membrane especially impermeable to ions, along with specific transport proteins for the molecules required by the mitochondrial enzymes in the inner mitochondrial compartment. Thus, the inner mitochondrial compartment remains relatively isolated and it has different biochemical composition than the outer mitochondrial compartment.It is in the inner mitochondrial membrane were the three enzyme complexes (NADH dehydrogenase, b-c1 cytochrome, and cytochrome oxidase) and the three electron carriers (iron-sulfur centers, ubiquinone, cytochrome c) of the respiratory chain are located. The electron transfer through this chain creates a high proton concentration in the outer mitochondrial compartment, resulting in an electro-chemical gradient. The passage of these protons to the inner mitochondrial compartment through the ATP synthase complex drives the synthesis of ATP. This is a very efficient energy obtaining machinery that results in fifteen times more ATP molecules than anaerobic glycolysis.
Other important functions take place in the mitochondria: intracellular calcium homeostasis in matrix granules, heat production by uncoupling of oxidative respiration and ATP synthesis in brown fat, synthesis of heme group in liver and red blood cell precursors, intracellular location for peripheral benzodiazepine receptors, and apoptosis. Most of the components of the Bcl-2 family (including apoptosis antagonists Bcl-2, Bcl-XL, Bcl-w, Mcl-1 and A-1, and agonists Bax, Bk, and Bcl-XS) are attached to both the nuclear and the outer mitochondrial membranes. In the early steps of apoptosis, there is a severe change in transmembrane potential at the inner mitochondrial membrane that alters its permeability and results in disruption of the outer mitochondrial membrane (Fig. 2). Thus, a protease that has been shown to induce nuclear chromatin condensation and fragmentation is released from the outer mitochondrial compartment. Apoptosis antagonists such as Bcl-2 act through the stabilization of the inner mitochondrial transmembrane potential. It is remarkable that the organelle that is most sensitive to low oxygen concentrations is involved in triggering programmed cell death.
Mitochondrial plasticity
Figure 2. The early events of apoptosis take place in the mitochodrion. Bcl-2 prevents apoptosis through stabilization of inner mitochodrial membrane potential. Bax and other apoptosis antagonists reverse this potential and the apoptogenic protease contained in the outer mitochondrial compartment is released after disruption of inner and outer membranes.
Mitochondria are able to modify their structure to meet the changing requirements of the cell. Some of these changes are typical of specialized cells, i.e. tubulo-vesicular cristae in steroid-producing cells. In other instances, there is an increase in the number of cristae or a change in their shape that results in a larger active surface for energy conversion, such as in zigzag, fenestrated, longitudinal or prismatic cristae, the latter resulting in a 75% increase in active membrane surface. Mitochondria may fuse or increase in size to form giant mitochondria or megamitochondria, and they are also able to divide in a sequence that morphologically resembles bacterial division. Thus, increased numbers of mitochondria are generated in situations with high metabolic activity. Conversely, the number of mitochondria can be reduced by pyknosis, ballooning, or autophagolysosome formation. Distribution of mitochondria in the cell is regulated by the cytoskeleton. Mitochondria tend to locate near the structures were the energy is required, as illustrated by the close association with rough endoplasmic reticulum in cells with high synthetic activity.Primary mitochondrial disorders
Since the first report of a mitochondrial myopathy by Luft in 1962, several mitochondrial diseases involving respiratory chain enzymes, ATP synthase or calcium transport have been identified. Most of these disorders result in clinical manifestations involving the central nervous system and the skeletal or cardiac muscle. In many of them, there are abnormalities in mitochondrial DNA, either deletion mutations, point mutations or quantitative changes. The clinical variability of these disorders is related to several peculiar features of mitochondrial DNA alterations: maternal inheritance, heteroplasmy (coexistence of both normal and abnormal DNA), high mutation index, mitochondrial segregation during cell division, and phenotypic threshold. The different mitochondrial encephalomyopathies have similar ultrastructural features, mostly in the way of non-specific changes such as mitochondrial hyperplasia and variable proportions of mitochondria with crystalline inclusions. In some cases, the muscle may even have normal ultrastructural appearance.Secondary abnormalities of mitochondria
Changes in matrical dense granules. These are osmiophylic granules measuring from 20 to 50 nm in diameter. They are able to retain ions, mostly calcium and magnesium. As their size is markedly increased in situations associated with abnormally high intracellular calcium concentrations, they have been suggested to be involved in the homeostasis of calcium. In extreme situations, however, matrix dense granules fuse with large crystalline calcium aggregates that fill the inner mitochondrial compartment.Wooly densities. Disruption of cristae results in irregular accumulations of lipid or lipoprotein, the so called flocculent or wooly densities, that are found in lethally injured cells. They are related to cell hypoxia. A relationship has been shown between the amount and size of these densities and the duration of ischemia.
Lipidic inclusions. They can be single or multiple and are not membrane-bound. They must be distinguished from cytoplasmic lipid droplets adjacent to an invagination of the mitochondrial envelope. True lipidic inclusions are uncommon and can be encountered in mitochondria from brown fat, adrenal cortex, kidney, and liver, as well as in tumors from these sites (Fig. 3). This can be a helpful feature in a metastatic neoplasm of unknown origin.
Intramitochondrial glycogen. Pools of intramitochondrial glycogen can be found in cardiac muscle under certain conditions, as well as in oncocytic neoplasms. It is usually located in the outer mitochondrial compartment. There is no agreement regarding the way in which they are formed nor their biological significance.
Figure 3. Mitochondrial hyperplasia can be encountered in tumors from different organs, so-called oncocytomas, and it has been suggested to be related to defective mitochondrial function. The finding of true lipidic inclusions in mitochodria, however, is a strong suggestion that a given lesion has originated in an upper abdominal organ. This electron micrograph shows intramitochodrial lipidic inclusions in a renal oncocytoma. (figure kindly supplied by Bruce Mackay, MD Anderson Cancer Center, Houston, Texas). Iron inclusions. Several mitochondrial enzymes are involved in the synthesis of heme group. Deficient porphyrin or heme synthesis occurs in sideroblastic anemia and this results in large iron accumulations in the inner mitochondrial compartment of erythroid precursors, giving the appearance of ring sideroblasts under the light microscope.
Crystalline inclusions. There is a variety of crystalline and paracrystalline inclusions in mitochondria. Rectangular crystalline inclusions are located in the outer mitochondrial compartment (usually in the intracristal space) and are made up of parallel arrays of plates or filaments with periodical constrictions between them. Their composition and mechanism of formation remain unsettled. They were initially thought to be typical of mitochondrial myopathy and primarily involved in it. They have subsequently been found in different situations, including hyperthyroid myopathy, diaphragm of patients with severe chronic obstructive pulmonary disease (COPD), and adrenocortical oncocytoma, suggesting that they could be a result rather than the cause of mitochondrial disfunction.
Mitochondrial hyperplasia in tumors. Malignant tumor cells with high proliferation index contain fewer mitochondria and, in spite of their higher metabolic activity, they obtain most of their ATP from anaerobic glycolysis, probably reflecting an adaptive phenomenon to low oxygen concentrations. Conversely, increased numbers of mitochondria resulting in an oncocytic phenotype are usually encountered in benign or low grade malignant tumors. Oncocytic tumors have been reported in many sites, mainly in kidney, salivary gland, hypophysis, thyroid and parathyroid glands, lung, adrenal gland, and liver. In most of these tumors, mitochondrial hyperplasia is the result of a compensatory mechanism related to abnormalities in mitochondrial function rather than an increase in energy production by tumor cells. Non-neoplastic oncocytic cells are also well known to occur mostly in salivary gland, liver, pancreas, parathyroid gland, and thyroid gland of patients with Graves disease or Hashimoto's thyroiditis, and none of these cells has been shown to have increased energy production as a result of mitochondrial hyperplasia.
Mitochondrial hyperplasia in non-neoplastic tissues. Mitochondrial plasticity plays an important role in the adaptation of cells to changing energy demands. This is particularly true in skeletal muscle, that reacts to aerobic endurance training by increasing the number of capillaries and mitochondria. However, there are situations in which this process is impaired and the desired effect is not obtained. In patients with COPD, a change in shape of diaphragm occurs as a result of overinflation. In response to this change plus the need to overcome obstruction, diaphragm sarcomeres shorten, capillary network increases, and a striking mitochondrial hyperplasia occurs (Fig. 4). However, these changes are not associated with functional improvement, and non-specific structural abnormalities of mitochondria develop. This situation could reflect exhaustion of the energetic machinery of diaphragm. It has also been suggested that primary abnormalities in diaphragm, probably involving mitochondria, could precede and actually predispose patients to COPD, but this hypothesis remains to be proven.
References
Figure 4. Striking degrees of mitochodrial hyperplasia can be observed in the diaphragm from patients with chronic obstructive pulmonary disease. It is still unclear whether this finding reflects a physiologic compensatory mechanism, wasting of the energetic machinery as a result of muscle overload or even a primary deficiency of mitochonria that would predispose these patients to obstruction through inadequate diaphragm function.
