a mass of morphologically undifferentiated, proliferating progenitor cells that is covered by epithelium and differentiates to replace the missing structures. Upon amputation, epithelial cells migrate to cover the wound, forming a wound epithelium (WE). The WE thickens via distant epithelial proliferation and continued migration. The thickened structure is called an apical ectodermal cap (AEC), which is thought to be similar in function to the apical ectodermal ridge (AER) that forms in the limb bud during embryonic development. While histolysis and fragmentation occurs near the wound, undifferentiated cells accumulate to form a blastema via proliferation and migration from the stump tissue. This is followed by cell specification and patterning