![]() ![]() ![]() Nuclei that were positioned within a disproportionately large amount of surrounding cytoplasm grew more rapidly than in normal mononucleated cells, up to the point where a N/C ratio of ∼0.08 was achieved. ![]() To investigate the mechanisms of size control in these multinucleated cells, the authors mechanically displaced nuclei in the cells by centrifugation, and then evaluated the growth of individual nuclei in real time. The nuclei located in central regions of these cells were crowded into a relatively small cytoplasmic area and were proportionally smaller than the peripheral nuclei, which occupied a greater cytoplasmic space. Interestingly, the volume of each nucleus was directly proportional to the amount of “surrounding” cytoplasm. In this situation, nuclei with a range of volumes were present in a single cell. They also examined mutant strains blocked in cytokinesis, in which cells acquired multiple, unevenly distributed nuclei. ![]() First, they demonstrated that cell nuclei undergo a continuous increase in volume from G1 until M phase, extending the conclusions of Jorgensen et al. Neumann and Nurse (2007) then performed a real-time imaging of cells labeled with a GFP-tagged nuclear envelope (NE) marker to directly track nuclear and cellular volume increases during the cell cycle. Mutant strains where mitosis was uncoupled from S phase, which gave rise to cells with up to 16-fold higher DNA content, still maintained a similar N/C ratio. The nuclei turn out to be correspondingly smaller at the time of mitosis, thereby maintaining a normal N/C volume ratio of ∼0.08. 593), using mutants that enter mitosis at a smaller than normal cell size. Related findings were made by Neumann and Nurse in fission yeast (see p. Their studies suggest that nuclear volume increases throughout the cell cycle in concert with cell growth, and does not take place discontinuously at points such as the onset of S phase. (2007) observe that the nuclear/cell (N/C) volume nonetheless remains constant in asynchronously growing populations, with the nuclear volume occupying ∼8% of the cellular volume. By examining mutants of budding yeast that enter into S phase at a cell size smaller than normal, Jorgensen et al. The cell size checkpoint in budding yeast requires that cells reach a critical size for S phase progression ( Umen, 2005). The nucleus is known to increase in volume through the cell cycle (for review see Umen, 2005), but how this is coupled to cell cycle progression and cell growth is largely mysterious. Now, a more penetrating look at this question has been taken in two studies involving quantitative morphometric analysis of yeast ( Jorgensen et al., 2007 Neumann and Nurse, 2007). This volume relationship is found in cells with widely different DNA contents, ranging from single-celled eukaryotes to mammalian cells. Although most organelles, such as the ER and mitochondria, can vary greatly in amounts, it has long been observed that cells maintain a roughly constant “karyoplasmic ratio” (the ratio of the nuclear volume to cell volume) ( Wilson, 1925 Cavalier-Smith, 2005). One of the fundamental properties of eukaryotes is their ability to maintain cell sizes and organelle volumes that are appropriate for different growth and differentiation states. ![]()
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