The number of polarizing region cells required to specify additional digits in the developing chick wing

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Abstract
A group of mesenchyme cells at the posterior margin of the developing wing, the polarizing region, can have a dramatic effect on the pattern of structures which develop across the antero-posterior axis of the limb. If a polarizing region is grafted to the anterior margin of a wing bud so that this bud now has 1 polarizing region at the anterior margin and 1 at the posterior margin, the wing that develops has duplicated structures across the antero-posterior axis in mirror-image symmetry: i.e., the normal pattern of digits 2 3 4 (reading from anterior to posterior) becomes 4 3 2 2 3 4. The ability of the polarizing region to specify additional digits from adjacent tissue can be progressively attenuated by .gamma.-ray radiation. If this attenuation is caused by progressively fewer cells remaining viable to signal, there should be a quantitative relationship between the number of polarizing cells used and the digit specified next to the graft. Two tests are reported which confirm this idea. Apparently apical ridge cooperates with a small number of polarizing region cells in a monolayer to specify structures across the antero-posterior axis of the wing.
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