Stem cells of the planarian flatworm shown in yellow, with the nervous system shown in red, and cell nuclei shown in blue. Image credit: Tisha Bohr (CC BY 4.0)
Many animals can repair and regrow body parts through a process called regeneration. Tiny flatworms called planaria have some of the greatest regenerative abilities and can regrow their whole bodies from just a small part. They can do this because around a fifth of their body is made of stem cells, which are cells that continuously produce new cells and turn into other cell types through a process called differentiation.
Measuring the gene activity in stem cells from planaria shows that these cells are not all the same. Different groups of stem cells have specific genes turned on which are needed to regrow certain body parts. It is unclear whether all stem cells respond to injuries in the same way, or whether the stem cells that respond are specific to the type of injury. For example, stem cells needed to repair the gut may respond more specifically to gut injuries than to other damage.
Bohr et al. studied how stem cells in planaria respond to different injuries, by comparing an injury to a specific organ to a more serious injury involving several organs. The specific injury was the loss of the pharynx, the feeding organ of the flatworm, while the more serious injury was the loss of the entire head. Within hours of removing the pharynx, stem cells that were poised to develop into pharyngeal cells became much more active than other stem cell types. When the head was removed, however, a wide range of stem cells became active to make the different cell types required to build a head. This suggests that stem cells monitor all body parts and respond rapidly and specifically to injuries.
These findings add to the understanding of regeneration in animal species, which is of great interest for medicine given humans’ limited ability to heal. Many of the genetic systems that control regeneration in planaria also exist in humans, but are only active before birth. In the long-term, understanding the key genes in these processes and how they are controlled could allow regeneration to be used to treat human injuries.
