This article was contributed by Johanna Kowalko
Limb regeneration, when an animal that has lost a limb completely regrows it, is a phenomenon that has fascinated scientists for decades. Few animals are able to regenerate limbs. However, the axolotl, a type of salamander, can regenerate its limbs throughout adulthood. This capability has resulted in the axolotl becoming an important system in which to study limb regeneration.
When an axolotl loses a limb, a group of cells called the blastema forms at the injured end of the limb. From this mass of cells grows a fully functional new limb, comprised of multiple cell types – cells that make bone, muscles, connective tissue, skin, nerves and blood vessels. For a long time, scientists thought that the blastema consisted of cells that had lost their identity and were reprogrammed so that each cell could become any type of tissue. A new study [you may need special permissions to view] by Kragl et al. shows that this is not the case. Individual cells in the blastema seem to remember the tissue they came from, and this restricts what tissues they can produce.
Researchers discovered that blastema cells had memory by marking cells from specific tissues with a green fluorescent protein (GFP). They then analyzed limbs after amputation and regeneration to determine which tissues had green labeled cells. They found that cells from a tissue made a subset of tissues, rather than all of the tissue types in the limb. For example, skin cells can make skin and cartilage, but cannot make muscle. This means that the cells have a restricted potential for what they can become.
This study shows that during regeneration, cells do not need to become reprogrammed to completely forget their tissue of origin, as was previously thought. However, researchers have much more to learn about regeneration, including the important question of why salamanders can regenerate while humans cannot.