A salamander’s limbs are smaller and a bit slimier than those of most people, but otherwise they are not that different from their human counterparts. The salamander limb is encased in skin, and inside it is composed of a bony skeleton, muscles, ligaments, tendons, nerves and blood vessels. A loose arrangement of cells called fibroblasts holds all these internal tissues together and gives the limb its shape.
Yet a salamander’s limb is unique in the world of vertebrates in that it can regrow from a stump after an amputation. An adult salamander can regenerate a lost arm or leg this way over and over again, regardless of how many times the part is amputated. Frogs can rebuild a limb during tadpole stages when their limbs are first growing out, but they lose this ability in adulthood. Even mammalian embryos have some ability to replace developing limb buds, but that capacity also disappears well before birth. Indeed, this trend toward declining regenerative capacity over the course of an organism’s development is mirrored in the evolution of higher animal forms, leaving the lowly salamander as the only vertebrate still able to regrow complex body parts throughout its lifetime.