One cell is enough
The secret of regeneration of flatworms has been revealed
Biologists have identified the type of cells and a key protein that provide flatworms with an amazing ability to rebuild their body even from a small fragment.
The ability of planarians (Planariidae), one of the families of flatworms, to regenerate has been known to zoologists for a long time. If the planaria is cut into several parts, each of them regenerates into a whole worm, regrowing the missing tissue types. Because of this ability, planarians began to be called "immortals under the blade of a knife." Scientists have found that cells called neoblasts are responsible for regeneration. But it was not possible to determine exactly which of the neoblasts play a leading role until recently.
Developmental biologist Alejandro Sanchez Alvarado from the Stowers Institute for Medical Research in Kansas City used a new method of isolating cells and determining gene activity in them. As a result, he isolated twelve varieties of neoblasts from planarians, the membranes of which contained a certain protein from the tetraspanin group. Tetraspanins are known in many animals, including humans. They are involved in the processes of cell movement and division. Tetraspanins also play an important role in tumors, contributing to the formation of metastases.
The researchers used radioactive labeling of proteins and found that the second of the twelve types of neoblasts (Nb2) is important for the regeneration of planaria. In the experiment, it was these cells that began to multiply rapidly when the planaria was cut. In another experiment, one injected Nb2 cell was able to multiply and diversify in order to save planarians who had received a lethal dose of radiation.
Nb2 cells are a special type of stem cells. Usually, only at the beginning of development, stem cells are able to turn into any kind of cells (such stem cells are called totipotent). Then, as the organism develops, its stem cells are able to give rise to only a few types of cells (pluripotent) or even only one of them. Planarians, however, manage to preserve totipotent stem cells in adulthood.
These cells are distributed throughout the planarian body. If an animal gets injured, they increase the activity of the genes responsible for the synthesis of tetraspanin. Its role turns out to be key in regeneration. If you suppress its production by cells, the damage on the planarian body does not recover. The exact function of the protein is still unknown, but it probably affects the connections between cells and helps them get to where their work is needed.
The results are published in the journal Cell (Zeng et al., Prospectively Isolated Tetraspanin+ Neoblasts Are Adult Pluripotent Stem Cells Underlying Planaria Regeneration).
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