Clonal and population analyses demonstrate that an EGF-responsive mammalian embryonic CNS precursor is a stem cell

In cultures of embryonic striatum, we previously reported that EGF induces the proliferation of single precursor cells, which give rise to spheres of undifferentiated cells that can generate neurons and glia. We report here that, in vitro, these embryonic precursor cells exhibit properties and satisfy criteria representative of stem cells. The EGF-responsive cell was able to generate the three major phenotypes of the mammalian CNS--neurons, astrocytes, and oligodendrocytes. Approximately 90% of both primary spheres and secondary expanded clones, derived from the primary spheres, contained all three cell types. The increase in frequency of EGF-generated spheres, from 1% in primary culture to close to 20% in secondary culture, and the large number of clonally derived secondary spheres that could be generated from a single primary sphere indicate that EGF induces both renewal and expansion of the precursor cell itself. In population studies, the EGF-responsive cells were carried through 10 passages, resulting in a 10(7)-fold increase in cell number, without losing their proliferative and multilineage potential. Thus, this study describes the first demonstration, through clonal and population analyses in vitro, of a mammalian CNS stem cell that proliferates in response to an identified growth factor (EGF) and produces the three principal cell types of the CNS.