Morphogen gradients can convey position and time in growing tissues

During embryonic development, cells coordinate fate decisions based on both position and time. While morphogen gradients have long been recognized as a source of positional information, how timing is synchronized across developing tissues remains unclear. We propose a simple mechanism by which morphogen dynamics can also encode time. If the morphogen source expands with a uniformly growing tissue and the gradient maintains a constant decay length, cells experience transient, hump-shaped signals that convey timing cues. Moreover, when two opposing exponential gradients with equal lengths—such as those in the vertebrate neural tube—interact, their product forms a uniform signal that can synchronize fate decisions across the entire tissue. With increasing gradient amplitudes, cells encounter a transient signal; with constant amplitudes, the signal decays—a feature of a depletion timer. This mechanism provides a generalizable principle by which morphogen gradients may coordinate both spatial and temporal aspects of patterning during development.