The early history of the polarizing region: from classical embryology to molecular biology

The polarizing region of the developing limb bud is one of the best known examples of a cell-cell signalling centre that mediates patterning in vertebrate embryos. This article traces some highlights in the history of the polarizing region from its discovery by John Saunders and early work that defined polarizing activity through a period in which modelling was pre-eminent, right up to the discovery of defined molecules with polarizing activity. There is a particular focus on the discovery that retinoic acid could mimic signalling of the polarizing activity and this finding is then set in the context of more recent work which implicates Shh and BMPs in mediating polarizing activity.