Scientists have discovered a new kind of stem cell. In the Dec. 10 issue of Nature, a group of international researchers led by Andras Nagy from Mount Sinai Hospital in Toronto reprogrammed adult mouse cells to create F-cells, a new kind of stem cell that grows in fuzzy-looking colonies. The results came as part of Project Grandiose, an effort by Nagy and his team to map out what happens when the previously-discovered induced pluripotent stem cells (iPS cells) are formed.

iPS cells were made by injecting adult cells with genes that would let them form, like embryonic stem cells, any type of cell in the human body. When they were discovered, it was hoped that iPS cells would be an ethical alternative to embryonic stem cells. The iPS cells, however, had a tendency to cause the growth of tumours and only a small fraction of them actually ended up fully reprogrammed. Nagy wanted to analyze the cells that were not deemed fully reprogrammed.

F-cells were produced when the researchers kept supplying reprogramming factors to the iPS cells instead of stopping them after a few days. The F cells were generated so that they could only be reprogrammed if exposed to the antibiotic doxycycline. They either turned into specialized adult cells or died if the medication was taken away. F-cells are different from iPS cells because they do not add to the growing tissues when injected into mouse embryos. They can, however, form “teratomas,” which contain a variety of differentiated cells, and grow more quickly in the test tube than other stem cells. It is unclear whether F-cells exist naturally in mice.

“I think it’s inevitable that human F-class cells can be achieved in the near future through reprogramming,” said Juan Carlos Izpisua Belmonte, a developmental biologist from the Salk Institute for Biological Studies in La Jolla, to the Los Angeles Times. It is also possible, however, that they could lead to the formation of tumours.

“With the arrival of F-class cells on the scene…(who’s) to say that there aren’t additional types of pluripotent cells with unique properties that might be harnessed clinically?” said Paul Knoepfler, a University of California Davis stem cell researcher, to the Los Angeles Times.

“We do believe that if we are opening up this black box and put some light into it and even videotape what’s happening inside we will be able to understand this process better,” said Nagy to CBC News about his project investigating how stem cells are programmed. “That knowledge that we are generating would help us to make safer cells at the end, maybe more efficient cells to treat diseases.”