Why can't people grow back body parts?

Researchers grow tissues : Arms that grow back

Fingers, hands, arms, legs - for the axolotl these are almost as interchangeable body tags as the hair or fingernails are for humans. If the salamander loses a leg, it grows back completely. Other tissues, even the spinal cord, are not only scarcely scarred after an injury, but also regenerate. Humans lost this ability at some point in the course of evolution. Wounds only heal, but amputated legs can never grow back. Elly Tanaka has been trying to figure out why for half of her life. The Boston-born scientist has been studying the regeneration phenomenon in the axolotl for years, first at the Max Planck Institute in Dresden, and now at the Research Institute for Molecular Pathology in Vienna. On Monday evening, the 52-year-old received the Ernst Schering Prize endowed with 50,000 euros.

As early as the 18th century, scientists recognized that a number of animal species were able to regenerate tissues, entire organs and even entire limbs - including flatworms, but also more highly developed vertebrates such as newts and salamanders. Basically, every cell in an animal contains the same genetic material, i.e. the complete plan for regenerating an entire body from an (egg) cell. The salamander can reactivate this program. However, only under certain conditions. So the leg must be completely amputated. A deep incision alone is not enough for a new leg to form. But how do the cells know when to close a wound and when to regenerate a limb? How does a cell know where and for what purpose it is needed? And how does the organism stop the arm from growing when it is long enough?

Elly Tanaka has found the first answers to these questions. She identified stem cells that are responsible for limb regrowth. To do this, she is researching white axolotls. Their skin is so light that the researchers can easily observe muscle fibers, nerve tracts and other tissues from the outside. Tanaka also marked the cell types with fluorescent substances. This enabled her to closely follow the fate of the cells in the organism under the microscope.

After severing a limb, the microscope reveals quite a tangle of cells. What was once skin or muscle, nerve or blood vessel cell has to reorient and change so that a functional limb can arise. Tanaka succeeded in deciphering the communication between the cells in this phase of reorientation. She was also able to prove that the cells regress after an amputation. So muscle cells first become stem cell-like precursor cells before they build new muscles again. The same is true for cartilage and other cell types. In this way, a completely functional new limb grows in a few weeks.

Humans and axolotl share the same evolutionary history. Therefore, the basic ability to regenerate should also be applied to humans. Tanaka and her team are now trying to use mice to find out why it has been lost in the course of evolution and why humans cannot grow a new leg. The mammals are more like humans than the axolotl. "Initial results suggest that the immune systems of mice and humans prevent regeneration," says Tanaka. There are also big differences in the regulation of the genetic make-up. "Apparently, mammals have a harder time turning on the genes that allow a new limb to grow."

Elly Tanaka hopes to transfer her research to humans one day: “We want to get human cells to initiate the same processes as in the axolotl,” says the researcher. “Perhaps one day we can produce a tissue from stem cells that can regenerate itself can. " This pile of cells from the Petri dish could then be transplanted so that a missing limb can then grow back on site. But there is still a long way to go until then. The fact that Elly Tanaka will continue to research the phenomenon can be seen from her enthusiasm for this research: "The process of regeneration is simply wonderful."

Florian Schumann

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