Modification of the zone of polarizing activity signal by trypsin.

Patterning of the developing vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) via the expression of Sonic hedgehog (Shh) and along the proximal-distal axis by the apical ectodermal ridge (AER) through the production of fibroblast growth factors (FGFs). ZPA grafting, as well as ectopic application of SHH to the anterior chick limb bud, demonstrate that digit patterning is largely influenced by these secreted factors. Although signal transduction pathways have been well characterized for SHH and for FGFs, little is known of how these signals are regulated extracellularly in the limb. The present study shows that alteration of the extracellular environment through trypsin treatment can have profound effects on digit patterning. These effects appear to be mediated by the induction of Shh in host tissues and by ectopic AER formation, implicating the extracellular matrix in regulating the signaling activities of key patterning genes in the limb.

Limb regeneration in higher vertebrates: developing a roadmap.

We review what is known about amphibian limb regeneration from the prospective of developing strategies for the induction of regeneration in adult mammals. Prominent in urodele amphibian limb regeneration is the formation of a blastema of undifferentiated cells that goes on to reform the limb. The blastema shares many properties with the developing limb bud; thus, the outgrowth phase of regeneration can be thought of as cells going through development again, i.e., redevelopment. Getting to a redevelopment phase in mammals would be a major breakthrough given our extensive understanding of limb development. The formation of the blastema itself represents a transition phase in which limb cells respond to injury by dedifferentiating to become embryonic limb progenitor cells that can undergo redevelopment. During this phase, rapid wound closure is followed by the dedifferentiation of limb cells to form the blastema. Thus, the regeneration process can be divided into a wound-healing/dedifferentiation phase and a redevelopment phase, and we propose that the interface between the wound-healing response and gaining access to developmentally regulated programs (dedifferentiation) lies at the heart of the regeneration problem in mammals. In urodele amphibians, dedifferentiation can occur in all of the tissues of the limb; however, numerous studies lead us to focus on the epidermis, the dermis, and muscle as key regulators of regeneration. Among higher vertebrates, the digit tip in mammals, including humans, is regeneration-competent and offers a unique mammalian model for regeneration. Recent genetic studies in mice identify the Msx1 gene as playing a critical role in the injury response leading to digit tip regeneration. The results from regeneration studies ranging from amphibians to mammals can be integrated to develop a roadmap for mammalian regeneration that has as its focus understanding the phenomenon of dedifferentiation.