There has been a bit of action recently in terms of stem cells, with one human trial in MND which commenced this month and another which hopes to commence soon. There are a two ways for stem cells to be of benefit: Trophic support where the grafted cells support existing neurons by secreting nourishing (trophic) proteins such as GDNF and BDNF and regenerative where new replacement neurons are grown. Each approach has advantages and technical (biological) hurdles.
As I said, trophic is secretion of neurotrophic (“nerve-feeding”) factors. It can also occur from the implanted cells just being a close friend to the diseased neurons. A benefit of trophic therapy is that the graft cells do not need to differentiate or grow. Cells modified to become GDNF “mini-pumps” have been shown to fully protect motor neurons in animal models of ALS. Unfortunately, however, in that study the protection did not extend to the axon. While the cell body was healthy, it made no contact with muscle which is functionally the same thing as the whole cell dying. In the trial underway in the United States, Neuralstem is using fetal stem cells. While they claim their product will differentiate into neurons and glials, they only claim to be pursuing a trophic therapy. In at least one animal trial, the implanted cells extended the life of the model (it is worth noting that due to variability in the model, the ten extra days could just be statistical noise). The cells were found to fully integrate into the host central nervous system. Another group, TCA Cellular Therapy is using autologous cells taken from the bone marrow. Both of these trials involve spinal implantation. A planned trial in Israel will also use autologous cells but will do the implantation in muscle to try axonal uptake.
Regenerative therapy has its own set of issues. While the ability for stem cells to become neurons has long been demonstrated, the new neurons still need to make proper axonal connections to muscle (spine -> muscle) and within the brain to other neurons (motor cortex -> spine). In 2006, a study by Johns-Hopkins achieved that in test animals. The process required not only implantation of prepared cells but also significant treatment on the muscular side of the connection (sharp readers may notice similarities to procedures discussed above). A much more recent study from Stanford showed that proper preparation of the stem cells is critical for proper behavior of the cells after implantation. The cells prepared with retinoic acid (as in the Johns-Hopkins study) failed to create proper connections whereas cells prepared by co-culture with stromal cells grew robustly and made proper connections from the motor cortex to the spinal cord, bypassing neurons from other cortexes (visual, etc.). Caveates of the Stanford study is that the work was done on very young animals (about a week old) so fully adult animals may present a different environment, and that requirements for cortex -> cord connections may be different from spine -> muscle.
On most of the studies above, the links are to press releases. This is done to indicate existence and to give limited information. Copyright prevents me from republishing any studies (except for the PLOS link which is intentionally placed in the Public Domain. The reader should also note that my technical training is in hardware and software, not wetware.