The invention is directed to methods for in vivo amplification of neural progenitor cells using the proliferative environment of glial neoplasms in adult brain. The progenitor cells have the capacity to proliferate and differentiate into mature brain cells which can be used for cell replacement therapies. The invention also provides cell replacement therapy methods for treating central nervous system diseases or injuries, including multiple sclerosis, stroke, Alzheimer's disease and Parkinson's disease.