1. A method of producing mammalian PDGF-responsive neural precursor (PRP) cells that express PDGF receptor alpha, comprising: (a) culturing brain tissue from a mammalian embryo in a culture medium containing PDGF and allowing proliferation of the PRP cells to produce primary neurospheres of the PRP cells, wherein the culture medium does not contain EGF; and (b) passaging the primary neurospheres of the PRP cells with PDGF and FGF-2 for self-renewal of the PRP cells.