Generation of neurons, astrocytes, and oligodendrocytes in the nervous system involves a sequential process of differentiation. Initially, multipotent stem cells generate more restricted precursor cells, which go through additional stages of differentiation to generate fully differentiated progeny (1 ). Precursor cells at each stage of differentiation can be distinguished from each other on the basis of cytokine dependence, functional properties, and antigen expression. Using markers to antigens expressed on the cell surface, live multipotent stem cells, intermediate precursor cells, and differentiated cells can be isolated at various stages of development.
