Pollen Tube Guidance--Right on Target

How does the pollen tube emanating from a pollen grain on the stigma of a female plant know in which direction to migrate on its long journey down to the depths of the ovary? According to Cheung and Wu in their Perspective, the paired synergid cells that guard the entrance to the embryo sac of the ovule release guidance cues that direct the pollen tip toward the entrance. As the tip enters the embryo sac it ruptures, releasing the two male gametes into the vicinity of the egg cell, and also ruptures one of the synergid cells resulting in the halting of guidance cue release and preventing other pollen tubes from entering the fertilized ovule.