Positional signal transmission in the developing chick limb

Abstract

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^1,2 produced by a special region of cells on the posterior margin of the limb bud, called the polarizing region³. 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 suggested⁴ 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 disks⁵. 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.