How can starfish regenerate arms

Stem cells can be thought of as "permanently immature" cells that can decide later what fate to take on. Stem cells in your skin are what make you able to heal after you've been cut or bruised.

Starfish and some other animals have cells like this that can do more than just make new skin, they can differentiate into whole new arms. In order for them to do this, they need to receive the right signals from the rest of the body. If we lose a limb, we either don't send the right signals to our stem cells, or our stem cells are not able to differentiate into all the parts necessary for a whole new limb.

Starfish seem to send the right signals, and their cells are able to differentiate properly, so they are able to regenerate whole new limbs. We don't know yet exactly what those signals are, or why some stem cells are able to differentiate more than others. Scientists think that this may be easier for them, because their bodies are not as complex as ours. Generally, stem cells are able to differentiate into any type of cell that are important to the body, including blood cells, muscle cells, skin cells, etc.

But when stem cells differentiate themselves into other cells, they cannot change back or de-differentiated. However, we have found that there is a special creature which its cells do not process like that: starfish! A starfish can regenerate its arms whenever they lose one; this shows that a starfish possess unique cells, which can de-differentiate themselves from skin or muscle cells back into stem cells. If the final phase is interrupted, the resulting new limb may be deformed.

Sea stars have the ability to reproduce sexually and asexually. Sea stars will also amputate their own limbs to escape from their predators. Each fragment of central disc or each detached limb can then undergo the regeneration process and result in fully grown sea stars that are genetically identical to the original. Most species of sea stars must split part of their central disc along with a limb for regeneration to occur.

It is very unlikely that a severed limb will be able to regenerate into a full-grown starfish unless it is already attached to at least a portion of the central disc. Regeneration from a single severed limb with no central disc has been observed in some species of sea stars, including Linckia diplax; it's unlikely but not impossible. The severed arm is able to grow a new central disc and four additional arms, resulting in a full sea star.