Temporal dynamics of patterning by morphogen gradients.

Morphogens act as graded positional cues to control cell fate specification in many developing tissues. This concept, in which a signaling gradient regulates differential gene expression in a concentration-dependent manner, has received considerable experimental support. Nevertheless, several recent studies have challenged the straightforward model of morphogen activity. In particular, the observation that pattern formation is a dynamic process has raised questions about the influence of time on morphogen activity. Here we propose that the spatiotemporal dynamics of the cellular response to a morphogen gradient depend on a combination of temporal alterations to the morphogen gradient itself, the dynamics of its signal transduction and downstream interactions between target genes.