Figure 10

Models for forming a gradient of Nodal signals over time. In the ratchet model (A), the morphogen spreads through a tissue over time and cells receive different Nodal doses at different times. Cells respond to the highest dose to which they are exposed. In the spatio-temporal gradient model (B), cells freely move into (not depicted) and out of (arrows) a zone of signaling activity (red rectangle). A short residence time in this zone determines a low cumulative dose of Nodal signals (green circle), whereas longer residence times determine higher cumulative Nodal dose (yellow and blue circles).