Epimorphosis

Epimorphosis is the regeneration of tissues or organs through the dedifferntiation of existing, differentiated adult tissues. Adult cells dedifferentiate (though not fully into embryonic stem cells) into a mass of cells that then re-differentiates into the new structure. This phenomenon is seen in frog, newt, and salamander limbs. Their limbs may be amputated, but the lost portions of the limb are able to grow back.

Epimorphosis in Salamander Limbs

Immediately after the limb is amputated, a plasma clot forms over the wound. The epidermal cells from around the wound then grow to cover the wound in 6–12 hours; this is called the wound epidermis. No scar tissue forms, as it would in mammals.

An apical ectodermal cap (AEC) forms on the tip of the stump. This is similar to the embryonic apical ectodermal ridge, which forms during normal limb development. The AEC causes the progress zone to re-establish; this means the cells under the AEC dedifferentiate and become mesenchymal. The AEC releases factors that drives the development of the new limb. It is basically resetting the limb back to its embryonic development stage.

But even though some of the limb cells are able to dedifferntiate, they are not able to fully dedifferentiate to the level of multipotent progenitor cells. During regeneration, only cartilage cells can form new cartilage tissue, only muscle cells can form new muscle tissue, and so on. The dedifferentiated cells still retain their original specification.