Cytoplasmic streaming in C. elegans: forces that drive oogenesis.

In the gonad of C. elegans worms, germ cells form the outer lining of the central tube, the rachis, which carries a cytoplasmic flow. The porous interface between the germ cells and rachis allows a radial flow of cytoplasm across the interface. In the upstream, the flow at the interface is radially inwards boosting the main cytoplasmic flow in the rachis, while in the downstream the flow is radially outward feeding the germ cells to become oocytes. We analyze the cytoplasmic flow in rachis as a flow in a porous tube of varying width, with spatially heterogeneous flow injection or leakage rates. Taking the geometry of the rachis into account, we predict properties of the rachis flow, such as pressure gradients and spatially heterogeneous flow speeds. Based on these, we speculate how directions of sub-flows between germ cells and rachis could be regulated for the development of healthy oocytes. Knowledge of this connection between micro-scale flow mechanics and physiological parameters may then also help us understand the abnormalities of reproduction in C. elegans worms with mutations or pathologies.