Morphogen signaling is critical for the growth and patterning of tissues in 
embryos and adults, but how morphogen signaling gradients are generated in 
tissues remains controversial. The morphogen Nodal was proposed to form a 
long-range signaling gradient via a reaction-diffusion system, on the basis of 
differential diffusion rates of Nodal and its antagonist Lefty. Here we use a 
specific zebrafish Nodal biosensor combined with immunofluorescence for 
phosphorylated Smad2 to demonstrate that endogenous Nodal is unlikely to diffuse 
over a long range. Instead, short-range Nodal signaling activation in a temporal 
window is sufficient to determine the dimensions of the Nodal signaling domain. 
The size of this temporal window is set by the differentially timed production 
of Nodal and Lefty, which arises mainly from repression of Lefty translation by 
the microRNA miR-430. Thus, temporal information is transformed into spatial 
information to define the dimensions of the Nodal signaling domain and, 
consequently, to specify mesendoderm.