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Self‐organization of the limb bud and regeneration blastema: Mechanisms to establish proximal and distal boundaries

Ectopic grafting experiments have established that the anamniote and amniote limb bud and the blastema of the regenerating salamander limb are self‐organizing entities. Marking experiments in limb buds and intercalation experiments in regenerating limbs suggest that this self‐organization relies on the early establishment of cells with proximal and distal positional identities that serve as boundaries within which a normal neighbor map of positional identities is intercalated by local cell interactions. The mechanism by which these boundaries are established, however, is unclear. I hypothesize that either (1) the early limb bud or blastema is composed of equal numbers of randomly interspersed cells that represent proximal and distal boundary positional identities and that these cells then sort out under the influence of the AER or AEC, or (2) that all the cells of the early limb bud or blastema have both proximal and distal boundary properties, but that only those cells in contact with the AER or AEC can maintain distal boundary properties. The subject of this talk will be to advance a series of experiments on regenerating limbs designed to distinguish between these two alternatives. Evidence for one or the other alternative will be a major advance in our understanding of how a limb bud or regeneration blastema self‐organizes its structural pattern in the longitudinal axis of the limb.