Of all the animal models used in biology, the freshwater planarian flatworm is one of the most fascinating: first because roughly 10% of all planarian cells are stem cells, second because these worms can regenerate from almost any injury. This ability to regenerate entire organs (including their own heads!) makes them very popular for stem cell biologists and a key model organism for regenerative medicine. In their dreams, scientists would like to understand the mechanisms by which planarians can regenerate entire organs and use that knowledge to, one day, make organs on demand in the lab. Though making organs on demand for medical applications is still a matter of science fiction, major scientific effort is put towards understanding how planarian stem cell biology works.
A recent example is a study published by Lin and Pearson in Development. They show that, in planarian flatworms, the protein yorkie was important for proliferation of stem cells and organ maintenance.
In this picture, you can observe the expression of the stem cell marker H2B in grey. On the left, you can see a control (normal) planarian flatworm whereas on the right you can see a planarian flatworm in which the expression of yorkie was turned down by genetic engineering. When compared to the normal planarian flatworm, you can see that there is more H2B, thus more stem cells, in the modified planarian flatworm. From this, authors conclude that yorkie is important for maintaining the right amount of stem cells in planarians.
This study is one of many that aim at understanding how planarians regulate their pools of stem cells and how they can regenerate entire organs and limbs. Though the road will be a long one, such scientific effort will hopefully one day teach us how to make organs in the lab, and make regenerative medicine a reality…
Lin, A. Y. T. and Pearson, B. J. (2014). Planarian yorkie/YAP functions to integrate adult stem cell proliferation, organ homeostasis and maintenance of axial patterning. Development 141, 1197-1208.