Regeneration was studied in the Ambystoma annulatum by the amputation of the right forearm of twenty-four adults, over a twelve month period. At the termination of the experiment the limbs were reamputated 1-2 mm proximal to the original amputation site. The regenerated portions were staged, examined at the gross morphological level, and prepared for histological examination. Gross examination revealed a thickened, ridge-like projection along the distal edge of all regenerating forelimbs at the Early Bud through Middle Palette Stages. Histological examination confirmed the existence of this structure as early as Wound Healing and continuing through Middle Palette to a pseudo-epidermal ridge in the Late Palette Stage. This epidermal ridge is continuous with the adjacent epidermis, but the epidermal ridge extends only along the dorsal distal edge of the regenerate. The epidermis proximal to the original amputation site was, at the maximum, 4-5 cells thick, whereas the epidermal ridge proved to be 60-65 cells at its maximum thickness. It is significant that an epidermal ridge appears on the distal tip of a regenerating adult forelimb in the stages Wound Healing through Middle Palette. A comparable structure, the apical ectodermal ridge, occurs during similar stages in embryonic chick limb development and embryonic amphibian limb development and has been considered a unique, embryonic feature. One might postulate that the function of the epidermal ridge during forelimb regeneration in the adult salamander parallels the function of the apical ectodermal ridge of chick limb bud as an inducer for continued mesodermal outgrowth.

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