Mesenchymal stem cell transplantation for neurodegenerative diseases.

Neurodegenerative diseases are characterized by a progressive degeneration of selective neural populations. The lack of effective treatment and the characteristic of their pathology make these diseases appropriate candidates for cell therapy. Mesenchymal stem cells (MSCs) are multipotent stem-like cells that are capable of differentiating into mesenchymal and nonmesenchymal lineages. Their regenerative capacity after in vivo transplantation into animal models of neurodegenerative diseases has suggested that they could be useful against human diseases. Human bone marrow-derived MSCs (hMSCs) can be easily amplified in vitro and their transdifferentiation has been claimed in vitro and in vivo in neural cells. There are some doubts concerning the exact mechanisms responsible for the beneficial outcome observed after MSC transplantation into neurodegenerating tissues. Possible interpretations include cell replacement, trophic factor delivery, and immunomodulation. This review mainly concerns hMSCs transplantation in neurodegenerative diseases, because it has proven to be feasible, safe, and potentially effective. Although they have been used in hundreds of clinical trials, mixed results and no functional and long-lasting integration have so far been observed. hMSCs transplantations therefore still have their "dark side." However, the challenge in well-planned clinical trials merits discussion.