A cellular funicular: A hydrodynamic coupling between the anterior- and posterior-directed cytoplasmic flows

Organelles inside cells move to position themselves at the right place at the right time. A mechanism for generating active force exists for each of such directed organelle movements. In our recent study on cytoplasmic streaming in the Caenorhabditis elegans one-cell embryo, we demonstrated that an anterior-directed force generated by myosin could drive not only anterior-directed cortical flow but also posterior-directed cytoplasmic flow. This coupling of flows in opposing directions is mediated by the hydrodynamic properties of the cytoplasm. This work provided a good example of an active force generation mechanism that drives organelle movements in two opposite directions inside the cell, just as a funicular moves up and down a slope. Interestingly, the funicular-like coupling of intracellular movements is also seen in our recent studies on centrosome positioning in the C. elegans embryo and on interkinetic nuclear movement during mouse neurogenesis. Thus, funicular-like coupling may be a general strategy used repeatedly in cells. The use of the funicular-like coupling seems advantageous because it is efficient, as one active force generation mechanism can drive movements in two directions, and also because the two movements can be coordinated to have similar speeds.