Many of the early forms of stem cell therapy involve cell transplants, and seem to produce benefits without those transplanted cells creating replacements for lost native cells. Instead the newcomers are improving the local environment and issuing signals that allow greater survival and repair among the native cell populations. Here is an example of the type:

Promising new research provides evidence that amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, may be treatable using neural stem cells. A consortium of researchers at multiple institutions [have] shown that neural stem cells, when transplanted into the spinal cord of a mouse model with familial ALS, slow disease onset and progression while improving motor function, breathing and survival time compared to untreated mice.

Neural stem cells are the precursors of all brain cells. They can self-renew, making more neural stem cells, and differentiate, becoming nerve cells or other brain cells. These cells can also rescue malfunctioning nerve cells and help preserve and regenerate brain tissue. But they've never before been studied extensively in a good model of adult ALS.

In 11 independent studies [researchers] transplanted neural stem cells into the spinal cord of a mouse model of ALS. The transplanted neural stem cells benefited the mice with ALS by preserving the health and function of the remaining nerve cells. Specifically, the neural stem cells promoted the production of protective molecules that spared remaining nerve cells from destruction. They also reduced inflammation and suppressed the number of toxin-producing and disease-causing cells in the host's spinal cord.

Link: http://www.eurekalert.org/pub_releases/2012-12/uomm-tns121912.php

Source:
http://www.fightaging.org/archives/2012/12/treating-als-with-neural-stem-cell-transplants.php