blastema

A blastema is the mass of de-specialized cells that forms at an amputation site — the stump of a salamander's lost leg, for example — and then rebuilds the missing part. It is, in a real sense, nature's regeneration kit: a small bud of flexible cells that knows how to regrow a whole limb. Where humans would seal the wound with a scar, these animals form a blastema and grow the limb back instead.

What is remarkable is how it forms. Mature cells near the wound appear to step backward, shedding their specialized identity to become flexible, fast-dividing cells again. This bud of cells then multiplies and, guided by positional signals, re-grows exactly what was lost — the right bones, muscles, and nerves, in the right order — rather than a random lump.

The blastema is a kind of holy grail for the field, because it is living proof that a complex body part can be regrown from scratch. Studying how salamanders and zebrafish build one — and why mammals do not — is a central thread in the search to unlock fuller regeneration in humans.