OBSERVATIONS ON PRISMATIC-TYPE MITOCHONDRIA WITHIN ASTROCYTES OF THE SYRIAN HAMSTER BRAIN

Pathways shaping the mitochondrial inner membrane

Mitochondria are complex organelles with two membranes. Their architecture is determined by characteristic folds of the inner membrane, termed cristae. Recent studies in yeast and other organisms led to the identification of four major pathways that cooperate to shape cristae membranes. These include dimer formation of the mitochondrial ATP synthase, assembly of the mitochondrial contact site and cristae organizing system (MICOS), inner membrane remodelling by a dynamin-related GTPase (Mgm1/OPA1), and modulation of the mitochondrial lipid composition. In this review, we describe the function of the evolutionarily conserved machineries involved in mitochondrial cristae biogenesis with a focus on yeast and present current models to explain how their coordinated activities establish mitochondrial membrane architecture.

Microbiota and mitobiota. Putting an equal sign between mitochondria and bacteria

The recent revival of old theories and setting them on modern scientific rails to a large extent are also relevant to mitochondrial science. Given the widespread belief that mitochondria are symbionts of ancient bacterial origin, the processes inherent to mitochondrial physiology can be revised based on their comparative analysis with possible involvement of bacteria. Such comparison combined with discussion of the role of microbiota in pathogenesis allows discussion of the role of "mitobiota" (we introduce this term) as the combination of different phenotypic manifestations of mitochondria in the organism reflecting pathological changes in the mitochondrial genome. When putting an equal sign between mitochondria and bacteria, we find similarity between the mitochondrial and bacterial theories of cancer. The presence of the term "bacterial infection" suggests "mitochondrial infection", and mitochondrial (oxidative) theory of aging can in some way be transformed into a "bacterial theory of aging". The possible existence of such processes and the data confirming their presence are discussed in this review. If such a comparison has the right to exist, the homeostasis of "mitobiota" is of not lesser physiological importance than homeostasis of microbiota, which has been so intensively discussed recently.

The fine structure of mitochondria and the mitochondrial cloud during oogenesis on the sea anemone Actinia

The appearance and arrangement of the mitochondria during all stages of oocyte growth in the sea anemone Actinia fragacea (Cnidaria: Anthozoa) have been examined by electron microscopy. In small oocytes, the mitochondria are generally squat, with a dense matrix and numerous cristae, although a proportion may show an unusual arrangement of prismatic cristae. During early oogenesis, the mitochondria tend to be arranged in aggregates rather than randomly scattered, and may be associated with nuage material. With the onset of vitellogenesis, a large mitochondrial aggregate forms next to the nucleus. During early vitellogenesis this aggregate enlarges and comes to resemble the mitochondrial clouds found in some amphibian oocytes. Within the cloud, many mitochondria appear to be highly elongate and irregular in shape. The cloud begins to fragment and disperse midway through vitellogenesis at about the time when cortical granules appear. In fully grown oocytes, some mitochondria may have a much less dense matrix and fewer cristae than the remainder, which may be related to their state of activity.

Fine structure of astrocytic mitochondria in the hypothalamus of the hamster

Astrocytic mitochondria in the hypothalamic region of the adult hamster brain have been studied by electron microscopy. Mitochondria showing triangular prismatic cristae, as seen in transverse section, and other unusual forms of mitochondrial cristae, are described and illustrated. Such mitochondria occur primarily in the suprachiasmatic and paraventricular nuclei, especially in perivascular astrocytic processes. The possibility that these atypical mitochondria might develop a specific function is discussed.

Prismatic cristae and paracrystalline inclusions in mitochondria of myocardial cells of the oyster Crassostrea virginica Gmelin

Several types of unusual mitochondrial configurations were found in myocardial cells of the oyster Crassostrea virginica Gmelin. These mitochondria include, in order of frequency, prismatic cristae, filamentous paracrystals in honeycomb and herringbone configurations, and paracrystals composed of rows of electron dense particles. The long, parallel, evenly spaced prismatic cristae are square or rhomboidal in cross section. In the space between the prismatic cristae are rodlike structures (4--6 nm in diameter) that are regularly spaced about 12 nm apart and appear to pass between adjacent cristae. Filamentous paracrystals observed in slender, elongated mitochondria. The filament spacing and form of these paracrystals suggest that they are composed of the intercristal rods. Alternatively, filamentous paracrystals might be tangential sections of prismatic cristae and intercristal rods. Particulate paracrystals which consist of dense lines or rows of particles are the least frequent type of unusual configuration. The particles are triangular, possibly pyramidal, in shape; their bases are 10-12 nm thick and repeat in rows every 17-18 nm. There is a close association between particulate paracrystals and prismatic cristae plus intercristal rods. Although similar mitochondrial configurations have been associated with disease or altered metabolism in a number of species, we have found so such association in the oyster as yet.

Effect of gonadectomy on the follicular cell and inclusions in mitochondria of rabbit thyroid gland

Thyroid glands of male rabbits were studied 2, 4, and 6 weeks after gonadectomy. In the gonadectomized group of animals, at the end of 2 weeks the follicular cell did not present any significant change except for the reduction of the supranuclear portion of the cell. By the end of 4 weeks there was a marked decrease in the microvilli of the apical border, dense granules, colloid droplets, ribosomes, and ergastoplasmic sacs. The lateral and basal cell margins were less convoluted. These changes reached a peak by the end of 6 weeks. The mitochondria with the inclusions also showed an apparent decrease in incidence at the end of 4 weeks and were rarely encountered by the end of 6 weeks. The thyroid gland morphologially resembled a hypothroid state, indicating that gonadectomy depresses the function and, therefore, may affect the metabolic process controlled by the gland.

Morphometry and ultrastructure of prismatic cristae in mitochondria of a crayfish muscle. A hypothesis of the structural principle

Approximately 40% of the mitochondria in the sphincter muscle of the crayfish vas deferens have prismatic-type cristae. In cross section, the angular cristae have either triangular or rhomboid profiles which are surrounded by a hexagonal array of electron-dense dots. In longitudinal section, these mitochondria exhibit both thick and thin parallel lines, which represent cristae and filaments, respectively. It is postulated that the matrix of the prismatic-type mitochondria is packed with rodlets composed of an electron-dense core and a less dense shell. Close packing of these rodlets results in the regular hexagonal dot array. Deletion of fascicles of 3 or 4 rodlets results in spaces with triangular or rectangular cross sections. Lining of these spaces with membranes results in cristae with triangular or rhomboid cross sections.

The ultrastructure of the sheath around chronically implanted electrodes in brain

Insulated, bipolar stainless steel electrodes were chronically implanted in various regions of the cat brain and the long-term structural changes in the tissue surrounding the electrodes were studied by light and electron microscopy. A sheath surrounded and separated the electrode from normal grey or white matter. A layer of foreign body giant cells of variable thickness was formed adjacent to the electrode. This layer was attenuated in some places so that it was unrecognizable by light microscopy. The bulk of the sheath structure consisted of collagen fibrils, leptomeningeal cells and hypertrophied astrocytes. Areas consisting of modified leptomeningeal cells with long thin processes we designated as spongy areas. These have not been previously reported using the electron microscope. Glycogen bodies were seen in leptomeningeal cells. Astrocytes became greatly enlarged and were more numerous in and around the sheath. Oligodendrocytes contained lamellar bodies, and direct continuity was shown between a lamellar body and an adjacent myelin sheath. Myelin was seen in abnormal sites (around oligodendrocytes and neurons) and in unusual configurations. Neuronal changes near the sheath included whorls and stacks of modified endoplasmic reticulum and the presence of cytoplasmic nucleolus-like bodies. Reactive, regenerative and degenerative axons were observed. Blood vessels were more numerous in the sheath and surrounding tissue than normal. Perivascular spaces were prominent even around capillaries and often plasma cells and monocytes were in these spaces. As compared to normal tissue the extracellular space is noticeably increased. Electrodes passing through ventricles were surrounded with a sheath covered with ependymal cells. This sheath was comparable in structure to the sheath present around the electrode in other locations.

On the fine structure of the small, heavily pigmented non-pyramidal cells in lamina II and upper lamina III of the human isocortex

With the aid of a newly developed technique for the successive examination of both the Golgi and pigment picture of individual neurons (Braak, 1974a) Braak (1974b) demonstrated that within lamina II and upper lamina III of the human isocortex, heavily pigmented non-pyramidal cells are distributed irregularly and sparsely. The lipofuscin pigment granules serve as excellent internal markers to identify these non-pyramidal cells in ultrathin sections. This favourable circumstance facilitates the study of these interneurons in the electron microscope. The heavily pigmented non-pyramidal cells are small, spherical to avoid with diameters of about 12-15 mum. One pole of the cell comprising a large cytoplasmic area gives rise to a few dendrites, while the other pole is occupied by the nucleus and in some cases is in close apposition to another nerve cell body. The nucleus is deeply invaginated by the large cytoplasmic area and occasionally displays nuclear inclusions. Among the usual organelles distributed within the large cytoplasmic area the mitochondria with a moderately electron dense matrix are abundant and the coarse lipofuscin pigment granules are the most striking elements. The latter contain densely packed filamentous or tubular material and a single vacuole. The perikaryon rarely receives more than 3 type I and type tii synapses per section per cell, whereas the dendrites receive numerous synapses of both type I and type II. Within the apposition zone to another nerve cell body (which in no case is a heavily pigmented non-pyramidal cell) puncta adhaerentia occur and also contacts in which the cleft of 8 nm is intersected by a dense stratum. Some of the ultrastructural findings are summarized in the schematic drawing of Figure 15.

Organized inclusions in astrocytic and amorphous inclusions in neuronal mitochondria of human frontal brain tissue

Brain tissue specimens were obtained early post mortem from the frontal lobe of seven unselected elderly subjects known to have been free of neurologic disease. Electron microscopically, two types of intramitochondrial inclusions were seen in four of seven cases. In two cases a few astrocyte mitochondria of the gyral white matter showed dense, elongated inclusions with an ordered linear substructure. These inclusions were, as a rule, accompanied by a row of prismatic cristae. In three cases some nerve cell mitochondria contained amorphous material of medium density and compact appearance. The globular masses often occupied the whole width of the mitochondrion. A relationship between the observed finding and a particular disease or morbid condition was not apparent. The inclusions are regarded as the morphologic substrate of a nonspecific metabolic change or degenerative process of the mitochondrion.

On the fine structure of the external glial layer in the isocortex of man

The surface of the external glial layer of the isocortex in the human temporal lobe is generally slightly undulated, with a few protrusions and indentations. The surface is formed by an uninterrupted basement membrane which is continuous over the surface no matter how tortuous it becomes. The overall thickness of the glial layer is generally 15 to 25 mum, but diminishes to about 5 mum immediately beneath blood vessels. It consists mainly of a variable number of stacked glial cell processes. Two groups of cell bodies are encountered particularly in the middle and lower levels of the glial layer. Most of the cells are specialized fibrous astrocytes. They are characterized by eccentrically placed, rounded nuclei with homogeneously dispersed chromatin, and electron-lucent cytoplasm rich in filaments. Lipofuscin pigment granules occupy large areas of the perikaryon. The astroglial cells give rise to four types of processes: foot-processes, tangential and radial processes, and processes irregular in outline. The foot-processes ascend towards the cortical surface and terminate as flat expansions spreading out immediately beneath the basement membrane. Contiguous terminal expansions are connected by gap junctions. The individual profiles are irregular in form and fit together like in a jig-saw puzzle. The plasmalemma beneath the basement membrane is underlined by a fuzzy material, which is penetrated by glial filaments. In the terminal expansions individual or groups of mitochondria are abundant. The tangential processes are straight and slender and form a lattice within the middle and deep level of the external glial layer. They contain numerous filaments, evenly distributed or fasciculated. The remainder of the lattice is filled up by a considerable number of processes irregular in outline and varying greatly in size. They contain fewer filaments than the tangential processes, coursing in all directions, and glycogen particles. In both types of processes only a few mitochondria are present. These processes are also connected by gap junctions and desmosomes, too. Large cytoplasmic areas of astroglial cells localized in the deepest portion of the glial layer protrude into the neuropil of the molecular layer, giving rise to several radiate processes, which extend deeper into the cortex. The second, heterogeneous group of cell bodies is characterized by elongated nuclei, ovoid or irregular in outline, which are smaller than those of astroglial cells, and contain blocks of condensed chromatin; a thin cytoplasmic rim generating a few appendages surrounds the nucleus. The first sub-type is characterized by a nucleus with large chromatin blocks bordering the inner nuclear membrane and a medium-dense cytoplasmic matrix. The second sub-type displays smaller chromatin condensations at the inner nuclear membrane and many microtubules are scattered throughout an electron-lucent cytoplasm.

Intramitochondrial filamentous bodies in the thick limb of henle of the rat kidney

"Intramitochondrial filamentous bodies" (IMFB) were occasionally found within the matrix of some mitochondria of the thick limb of Henle of the rat kidney, but not elsewhere in the tubular system. Three types were recognized: type I, an accumulation of filaments 55 A thick; type II, a bundle of parallel filaments having the same thickness as those of type I and regular spacing, 87 A apart, from center to center; and type III, consisting of type II with regular light bands of 280 A periodicity and a helical border of prismatic tubular cristae. In addition to these, electron-opaque masses showing variable and faint substructures were found in the matrix of mitochondria. It is suggested that all these IMFB may originate from mitochondrial cristae and that type II IMFB may be an intermediate developmental form between type I and type III. After uranyl acetate staining, IMFB and the membranes of prismatic tubular cristae showed highly increased electron opacity. The literature has been reviewed for reports of intramitochondrial filamentous inclusions in various types of cells. These inclusions have been classified according to their structural characteristics and the localization in the mitochondria and compared with IMFB reported herein.