Organ Size: The Role of Mobile Signals

Growth regulator X1 promotes growth, is produced locally (purple area) and has limited mobility. This causes a spatial growth gradient (faster growth in darker green). Growth will arrest in a wave from right to left as X1 levels decrease. Growth regulator X2 promotes growth, is produced locally (orange area) and is sufficiently mobile to affect growth in the same way across the tissue. If only X2 affects growth, there is no spatial growth gradient. As X2 levels decrease, growth will arrest everywhere at the same time across the tissue. If X1 and X2 are produced in the tissue, growth will proceed for longer than with X1 alone; therefore, the tissue may grow to a larger size. Anne-Gaëlle Rolland-Lagan