Molecular signals identified that let some organisms regrow lost heads

Ars Technica » Scientific Method 2013-07-24

Although they're nothing exciting to look at, Planaria are remarkable creatures in at least one regard: when cut in half, they're able to regenerate two complete animals. The tail half will regrow a replacement head, while the head will extend a new tail (you can also cut them in half along their main body axis and each side will grow a new mirror image of itself).

Although this would seem to provide a huge evolutionary advantage—the animal can bounce back after being partly eaten—a number of related species lack this ability. Some have it in a more limited form, where they can regrow a tail but not a head. Now, scientists have used these other species to start to understand what happens when an organism regenerates missing body parts.

Initially, all regeneration goes through a similar process, whether it's the formation of a new limb by amphibians or a new head by a planarian. Once the wound heals over, a group of cells called a "blastema" forms at its surface. These are unspecialized stem cells that divide and grow and gradually spin off all the specialized cells (muscles, nerves) that need to be replaced. As the regeneration is completed, the blastema slowly dissipates, leaving behind replacement tissues.

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