ABSTRACT
Organelles in cells are appropriately positioned, despite crowding in the cytoplasm. However, our understanding of the force required to move large organelles, such as the nucleus, inside the cytoplasm is limited, in part owing to a lack of accurate methods for measurement. We devised a novel method to apply forces to the nucleus of living, wild-type Caenorhabditis elegans embryos to measure the force generated inside the cell. We utilized a centrifuge polarizing microscope (CPM) to apply centrifugal force and orientation-independent differential interference contrast (OI-DIC) microscopy to characterize the mass density of the nucleus and cytoplasm. The cellular forces moving the nucleus toward the cell center increased linearly at ~14 pN/µm depending on the distance from the center. The frictional coefficient was ~1,100 pN s/µm. The measured values were smaller than previously reported estimates for sea urchin embryos. The forces were consistent with the centrosome-organelle mutual pulling model for nuclear centration. Frictional coefficient was reduced when microtubules were shorter or detached from nuclei in mutant embryos, demonstrating the contribution of astral microtubules. Finally, the frictional coefficient was higher than a theoretical estimate, indicating the contribution of uncharacterized properties of the cytoplasm.
