Stem cells are immature cells with prolonged self-renewal capacity and, depending on their origin, ability to differentiate into multiple cell types or all cells of the body. Transplantation of stem cells or their derivatives, and mobilization of endogenous stem cells within the adult brain, have been proposed as future therapies for neurodegenerative diseases. It may seem unrealistic, though, to induce functional recovery by replacing cells lost through disease, considering the complexity of human brain structure and function. Studies in animal models have nevertheless demonstrated that neuronal replacement and partial reconstruction of damaged neuronal circuitry is possible. There is also evidence from clinical trials that cell replacement in the diseased human brain can lead to symptomatic relief.
Here we review the scientific basis of stem cell therapies and discuss their prospects in Parkinson's disease, stroke, amyotrophic lateral sclerosis (ALS) and Huntington's disease. In each of these neurodegenerative diseases, a different spectrum of cell types is affected; therefore, different types of neurons are required for replacement. We argue that long-term survival of new, functionally integrated neurons is the main goal to achieve maximum symptomatic relief through stem cell therapy. Stem cell transplantation may also lead to clinically valuable improvements through other mechanisms as well (Box 1).