Nodal is a short-range morphogen with activity that spreads through a relay mechanism in human gastruloids

Morphogens are signaling molecules that convey positional information and dictate cell fates during development. Little is known about how morphogen gradients are created and interpreted during mammalian embryogenesis. Here we take advantage of a human gastruloid model to visualize endogenous Nodal protein in living cells. We show that Nodal is extremely short range so that Nodal protein is limited to the immediate neighborhood of source cells. Nodal activity spreads through a relay mechanism in which Nodal production induces neighboring cells to transcribe Nodal. We further show that the Nodal inhibitor Lefty, while biochemically capable of long-range diffusion, also acts locally to control the timing of Nodal spread and therefore of mesoderm differentiation during patterning. Our study establishes a novel paradigm for tissue patterning by an activator-inhibitor pair.