Individual mitochondria which form the chondriom of eucaryotic cells are highly dynamic systems capable of fusion and fragmentation. These two processes do not exclude one another and can occur concurrently. However, fragmentation and fusion of mitochondria regularly alternate in the cell cycle of some unicellular and multicellular organisms. Mitochondrial shapes are also described which are interpreted as intermediates of their "equational" division, or fission. Unlike the fragmentation, the division of mitochondria, especially synchronous division, is also accompanied by segregation of mitochondrial genomes and production of specific "dumbbell-shaped" intermediates. This review considers molecular components and possible mechanisms of fusion, fragmentation, and fission of mitochondria, and the biological significance of these processes is discussed.
The dynamics of chondriome changes in oogenesis of the sea urchin Paracentrotus lividus were studied by electron microscopy. An oocyte-enriched fraction obtained by gonad mechanical dissociation without protease treatment was used. The shape, size and arrangement of mitochondria (Mt) in cells were quantitatively analysed on the basis of data from reconstruction experiments, with serial sections performed using a specific computer program. At all stages of oogenesis, the chondriome was shown to consist of rod-shaped Mt of various lengths and also of small amounts of globular Mt about 0.3 m in diameter. Chondriome transformation during oogenesis is shown to involve the following processes: (1) a 64-fold increase in number of Mt, with the ratio of cytoplasm to Mt volume quite constant in the course of oogenesis; (2) an increase in length of Mt to a maximum of 1.54 m in medium oocytes and successive considerable mitochondrial division; (3) changes in Mt ultrastructure; and (4) a clustering of Mt. In a mature egg, the modal value of Mt length was reduced and, unlike the oocytes, was more homogeneous, and the Mt were completely clustered.
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