The a-p axis of the worm embryo is defined at the very first division, which results in an anterior AB cell and a posterior P1 cell. How this axis is maintained as the embryo rapidly amasses more cells is unclear. Now, descendents of P1 are shown to provide a continuous posterior-defining signal to the AB-derived cells.
Left alone, AB descendents were polarity blind. They divided with spindles that oriented with an average orientation of ∼60 degrees with respect to the a-p axis. The addition of the P1 daughter cell called P2 reduced the average to 45 degrees.
The resulting cleavage plane is not perfectly perpendicular to the a-p axis, but Schnabel says that, without some built-in sloppiness “after eight divisions, the end cells would be 40 times the original cell diameter apart. They would have to migrate far back to stay within the egg. 45 degrees fills the gaps, and the required transport is minimal.”
The polarity instructions from P2 require Wnt signals. Mutant AB descendents that either did not make or did not perceive Wnt ligands did not polarize. Wnt's effects were far-reaching—cells that did not touch P2 still polarized.
Wnt's effect at a distance is not due to its diffusion, as a single layer of Wnt-blind cells blocked the polarization of cells lying beyond it. Wnt must therefore be made anew by each cell upon sensing the ligand coming from its posterior neighbor. After setting its own polarity, it then passes on the baton by making and secreting Wnt for its anterior neighbor to sense.
This sort of relay mechanism has the advantage of speed over diffusion/gradient models (the establishment of a gradient through an embryo by diffusion is estimated to take longer than development). Relays might thus also direct polarity in cell fields such as the fly wing—a process that “people call planar polarity,” says Schnabel. “It's just that, in my case, it's a round embryo.”