Regulatory principles of developmental signaling

Signaling between cells is a widely used mechanism by which cell fate and tissue patterning is determined in development. We review the mechanisms by which signaling between cells is regulated so that a cell receives the right amount of signal, at the right time, to achieve its intended developmental fate and position. In nearly all cases, we find that the supply of signal factor (ligand) is the limiting step in initiating a signaling process. Ligand supply is regulated by the transcription and localization of RNA, the spread of ligand from a source, and by inhibitors that operate at several different levels. We emphasize the different regulatory strategies that operate for threshold as opposed to concentration-dependent (morphogen) signaling. Threshold signaling is extensively regulated by feedback mechanisms. Morphogen signaling is regulated quantitatively by receptor loading and transduction flow.