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We humans would also like to see an amputated arm grow back, but the regeneration of limbs is reserved for a few special animal species, such as the Cladonema jellyfish. He is even very good at it: in a day or two he can regrow a severed tentacle.
Regenerating tissue is a clever trick that, for example, certain salamanders and insects are also capable of. In them, a blastema forms at the end of their limbs. This is a clump of undifferentiated cells that can repair damage and grow on the missing body part. Jellyfish, as well as corals and sea anemones, are very good at this, but until now it was a mystery how they were able to form this crucial blastema.
Multiply
Japanese researchers have now discovered that a type of stem cells, which multiply rapidly but have not yet differentiated into specific cell types, appear at the site of the injury and help to form a blastema. “These cells are different from the stem cells already in the tentacle,” said researcher Yuichiro Nakajima of the University of Tokyo. “The cells, which are specifically aimed at repair, mainly contribute to the epithelium, the thin outer layer of the newly formed tentacle.”
The stem cells
The stem cells already in the tentacle are responsible for the regeneration of all the different cells, which means they maintain and repair the cells that the jellyfish needs during its life. But the special, very rapidly multiplying stem cells that repair the damage only appear after an injury. “Together, the stem cells and the repair-specific cells ensure rapid regeneration of the tentacle within a few days,” says Nakajima, who also points out the importance of the tentacles for the animals: they hunt and eat with them.
Special discovery
This discovery in the tiny jellyfish – it is the size of a pinky fingernail – also provides knowledge for other animal species that can grow tissue. Lead researcher Sosuke Fujita: “We aimed to discover the mechanism behind blastema formation with the help of a tentacle from the Cladonema jellyfish. This could serve as a model for other non-bilateral animals or animals that do not grow bilaterally during the embryo phase,” says Fujita, explaining that his work may provide insight from an evolutionary perspective. Bilateral animals are bilaterally symmetrical, as it is called: they have a clear left and right side. Almost all animals belong to this group.
Salamanders, for example, are bilaterians that are able to regrow their limbs. Their legs contain stem cells that are limited to the specific needs of certain cell types. This process appears to work in a similar way to the repair cells observed in jellyfish.
Origin unknown
“Since the repair-specific proliferative cells are analogous to the restricted stem cells in the legs of salamanders, we suspect that blastema formation by the repair-specific cells is a general feature independently acquired for complex organ and limb generation during evolution,” says Fujita.
The origin of these special cells, which were found in the blastema, is still unclear. And it will probably remain that way for a while. According to the researchers, the techniques currently available are too limited to clarify the source of those cells or to identify other stem cell-like cells. “This requires genetic techniques that make it possible to detect and manipulate specific cell lines in Cladonema,” says Nakajima.
But ultimately the discovery and subsequent research should lead to an improvement in our own regenerative capabilities.
Regeneration in animals
This jellyfish is of course not the only one in which an organ or leg sometimes grows back. The zebrafish can thus regenerate its fins within one to two weeks. In earthworms, a severed part grows back, provided the front is largely intact. Mice can regrow their front phalanges. And there is even a species of mouse where a hole in the ear grew closed again. But humans also have some form of regeneration: you can remove 70 percent of the liver. Within three to four weeks it will grow back to its original size.
