Local application of retinoic acid to the limb bond mimics the action of the polarizing region

Abstract

The polarizing region, a small group of cells at the posterior margin of the limb bud, acts as a signalling region to specify the pattern of structures which develop across the anteroposterior axis of the limb¹. When a polarizing region is grafted to the anterior margin of a second limb, a mirror-image symmetrical limb develops^1,2. There is indirect evidence that the signal from the polarizing region is a diffusible morphogen³. In an attempt to identify the nature of the putative morphogen, we have developed a method whereby extracts of polarizing region cells and chemicals can be locally applied to the wing bud by being bound to implanted beads and other inert carriers. The chemicals tested included hyaluronidase, dibutyryl cyclic AMP and thalidomide. All experiments have been negative until, at the suggestion of Dr J. Pitts, we tested retinoic acid because of its effects on cell to cell communication and cell differentiation, and we have now for the first tune been able to mimic the action of the polarizing region with a defined chemical.