Positional signal transmission in the developing chick limb.

It has been suggested that the pattern along the antero-posterior axis of the embryonic developing chick limb arises from a gradient of diffusible morphogen produced by a special region of cells on the posterior margin of the limb bud, called the polarizing region. Grafts of posterior polarizing-region tissue to an anterior site in an embryonic chick wing bud result in mirror-image duplication of the limb bud; typically three extra wing digits are produced (Fig. 1f, g). Direct evidence for a long-range signal has, however, been lacking, and recently it has been suggested that chick wing development might better be understood as resulting only from local, neighbour-neighbour-like interactions of the sort postulated to occur during regeneration of insect and amphibian limbs and insect imaginal disks. As we report here, operations were performed in which chick wing buds were made host to grafts of leg bud responding tissue adjacent (posteriorly) to a grafted polarizing region (Fig. 2). The results showed that the pattern signal from the polarizing region can be transmitted through the leg tissue over distances of tens of cell diameters.