Tip dominance can be established in different manners: (1) higher frequency oscillators entrain cells that oscillate at lower frequency, (50) and (2) tips produce a diffusible inhibitor that reduces the excitability of surrounding cells. (51) The cAMP hydrolysis product adenosine was proposed to fulfill this role. (52, 53) Dominance will break down if there are physical or biochemical barriers that prevent propagation of cAMP waves or diffusion of the inhibitor.