More than a century ago, biologist Thomas Hunt Morgan carried out a macabre experiment. Using a scalpel, he cut a flatworm into 279 small pieces.
Each developed into a new flatworm. Amazed, Morgan concluded that the lowly creature had an incredible ability to regenerate itself, and that even a piece with as few as 10,000 cells could develop into a viable individual.
Morgan’s flatworm studies were forgotten for most of the 20th century, but a few years ago the seemingly insignificant worms again caught the attention of scientists, who are now using molecular-biological methods to learn more about the animals’ exceptional viability. They are hoping that new knowledge concerning the never-say-die Schmidtea mediterranea flatworm may benefit humans somewhere down the line.
Special stem cells are the key
Among the tens of thousands of species of flatworms, it is the large group called planarians that possess the ability to regenerate. This quality is due to a special type of stem cell called a neoblast.
All animals have pluripotent stem cells — which can develop into almost any type of cell — during the earliest phase of development, but these are replaced after the embryonic stage with more specialized, less adaptable types of stem cells. Planarians, however, keep their highly versatile neoblasts throughout their bodies their whole lives.
These neoblasts are totipotent, meaning that they can develop into any kind of cell in the body and — equally importantly — integrate themselves into existing tissue. So when a flatworm is cut in half, the neoblasts divide and migrate to the wound, repairing or replacing damaged tissue or amputated parts.
In an experiment conducted at the Massachusetts Institute of Technology, scientists showed that just one stem cell from one piece of the Schmidtea mediterránea flatworm can repair an entire damaged creature. A flatworm was bombarded with radiation until all of its cells were so badly damaged that they could no longer divide, rendering the flatworm unable to regenerate its organs and replace the destroyed tissue. Subsequently, the scientists took a single neoblast cell from a healthy flatworm and transplanted it into the dying flatworm. Within six weeks, the healthy stem cell had replaced all of the dying cells, and the flat-worm was good as new.
An internal fountain of youth
The ability to renew its organs means that, theoretically, a flatworm could live eternally. Old age can be characterized as an accumulation of a lifetime of damage to the body, but since flatworms can continuously replace or repair even severely damaged parts, including their brains, they can remain forever young.
Flatworms are thus thought to age extremely slowly. Scientists from the University of Nottingham in England recently showed that Schmidtea mediterranean cells can divide repeatedly without showing the signs of aging that appear in the stem cells of other organisms.
However, flatworms can be killed, so they are not truly immortal. But if kept in fresh water and fed regularly, they can live for decades – several times longer than most earthworms, which are larger. And since bigger animals tend to live longer than smaller ones, the flatworm’s small size contributes to its reputation for being invulnerable to death.
Potential uses for human health
Scientists have made many advanced discoveries in biology since Morgan’s pioneering flatworm experiments more than 100 years ago, and much more is known today about the biological mechanisms harbored in flatworms’ “super-cells.” Nevertheless, there are many challenges to using this knowledge in medical applications, not least because the organs and tissue of mammals and humans are so much more sophisticated and specialized than those of flatworms and consequently much more complicated to regenerate. However, human stem cells are already used in several types of therapy, and there are many more potential treatments if they can acquire the incredible regenerative ability of flatworm neoblasts.
Experts are convinced that flatworm research will one day benefit mankind but as to whether we will be able to regrow an amputated limb or repair a cancerous liver, only time will tell.