They appear to be making proteins that nerve cells make, but this is not definite proof that these cells are acting as nerve cells. Most had short non-branching outgrowths, not the long extensions of mature neurons. The donor cells were injected into the the bloodstream, so the cells had a long road to reach the brain. These factors may include growth factors and tropic factors . For example, how does age of the recipient mouse and the radiation used to kill the host bone marrow cells affect signals that tell cells to migrate.
Human cells grow more slowly, and divide less often than mouse cells.
This work will be complicated by ethical considerations, since research involving cells from human embryos and fetuses is limited. Current guidelines by the National Institutes of Health permit federally funded scientists in the US to work with stem cells from human embryos. The creation of embryos for research purposed is strictly prohibited. One advantage of this approach as a treatment for disease is that the cells would come from a patient's own body. In other words, a patient's bone marrow cells would be used to make his or her brain cells.