Positional signalling by mouse limb polarising region in the chick wing bud.

Abstract

THE development of the chick limb may be viewed in terms of positional information; that is, the spatial pattern of cellular differentiation results from the cells interpreting their position in a three-dimensional coordinate system1. We have presented evidence that positional information along the antero–posterior axis is specified by a signal from the zone of polarising activity (ZPA), which is at the posterior margin of the wing bud2,3. Grafts of ZPA to anterior regions of an early bud result in mirror image duplication between grafted ZPA and host ZPA which is particularly evident in the pattern of the digits. The three digits of the chick wing4, digits 2, 3, and 4 appear to be specified by their distance from the nearest ZPA, digit 4 being closest, then digit 3 and then digit 2. The ZPA from the leg can also specify additional wing digits when grafted into the wing which suggests that the positional information along the antero–posterior axis in leg and wing is similar, but that interpretation differs. We therefore wanted to know whether a ZPA was present in other classes of vertebrates, and if so whether the signal from it was also similar. We investigated this question by grafting tissue from embryonic mouse limbs into developing chick wing buds.