It sounds like something from science fiction, but if you cut a planaria flatworm into lots of little pieces, each piece will start to grow into an entirely new worm. This extraordinary power of regeneration is the envy of many scientists, and now a team have managed to identify exactly which cells are responsible, and the protein that makes it possible.
The incredible ability hinges on a particular pluripotent stem cell known as a neoblast. In normal development, these cells have the ability to turn into any other cell in the body, but as the organism develops, the cells become more and more specialized until it reaches the point at which when born, the body no longer contains any neoblasts. The adult flatworms, however, seem to be able to retain them.
This has been observed for a long while now, and we've known for some time that if you cut the head off a planaria worm, for example, it will simply regrow a new one. But no one really knew why or how, until now. “Our finding essentially says that this is no longer an abstraction, that there truly is a cellular entity that can restore regenerative capacities to animals that have lost it and that such entity can now be purified alive and studied in detail,” explained Alejandro Sánchez Alvarado, senior author of the study now published in Cell.
There are a few different types of neoblast cells in planaria bodies, however, and the researchers needed a way to identify which ones were responsible for the regeneration. To do this, they looked at the gene activity going on within the cells, and discarded any cells that showed signs of being destined for a particular fate, such as becoming muscle of skin.
From this, they were able to single out a subgroup known as Nb2. Designing a green fluorescent protein that stuck to the surface of these cells, they could then track where exactly in the body of the worms these cells were and watch what happened when they were wounded. And indeed, if the researchers cut the planaria, the Nb2 cells multiplied at the site of injury and healed the wound.
What is more, the team then did another experiment in which they refined the Nb2 cells, before injecting them into a flatworm that had been blasted with lethal levels of radiation. The stem cells then repopulated the planaria, and rescued them from certain death.
Interestingly, the protein that is expressed on the surface of the adult pluripotent stem cells in the worms – known as tspan-1 – is also found in humans. This, the researchers suggest, could hint at some conserved mechanism for regeneration even in our own bodies.
The video below shows an isolated Nb2 cell, with the tspan-1 protein tagged with green fluorescence.
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