(A) Mechanosensory mechanism 1: directional flow sensing. Sensory cilia (red) on the left (L) and on the right (R) side are deflected by the rotational flow (arrows). They must be able to distinguish between anterior- and posterior-directed flows. (B–C) Cumulative fraction of cilia with the anterior acting force below (right, green) or above (left, red) the value on the abscissa. The dotted lines show instantaneous values (blurred by oscillatory flows of adjacent cilia), dashed lines show the temporal average, and the continuous line the temporal and ensemble average of 3 immotile cilia on each side. The diagrams show the results on randomly generated (B) and experimentally characterized (C) vesicles. The results show that reliable detection (<5% error) would need a sensitivity threshold of 1 × 10−19 Nm. The upper scale shows the effective flow shear rate above a planar surface that induces the equivalent torque on an isolated passive cilium of the same length. (D) Mechanosensory mechanism 2: detection of a cilium's own movement. According to this mechanism, a cell can sense the torque components caused by the motion of its active cilium through the viscous fluid. The lines show the meridional component towards posterior (blue), parallel component towards dorsal (red), and normal component (green). The meridional component shows a temporal average of 10−17 Nm that could potentially allow discrimination between left (left panel) and right (right panel) side. (E) Chemosensory mechanism, based on flow mediated transport of a signaling molecule. Particles are secreted from a region 30° around the anterior (A) pole and then travel diffusively through the rotating fluid. They get absorbed upon encounter with any cilium outside the anterior region. Eventually, particles absorbed in a 45° region around left-right poles are counted. (F) Average particle concentration (arbitrary units) in the equatorial plane for particles where diffusion dominates fluid circulation (Stokes radius = 0.5 nm, top) and those with drift dominating (3 nm, bottom). In the latter case, an asymmetry in the distribution is clearly visible (Video 3). (G–L) Fraction of particles counted on the left among the total count of left and right for different scenarios. The dotted line shows a proposed detection threshold with a left to right ratio of 2:1. The red line shows the average vesicle and the shadowed region the interval between the 5th and the 95th percentile. (G) Continuous model with uniform circulation (Ω = 0.5 s−1). (H) Randomly generated cilia distributions with natural parameters. (I) Simulation on individual vesicles at 3-SS (red), 8-SS (indigo) and 9–14-SS (black). (J) Same as H, but homogeneous cilia distribution. (K) Same as H, but reduced number of cilia (). (L) Further reduced number of cilia (). The following figure supplement is available for Figure 7.