Progenitors in the ventricular zone of the central nervous system are responsible for producing the vast complexity of cell types in the spinal cord. The developmental trajectories of neurons have been well studied, but mechanisms regulating the switch from neurogenesis to gliogenesis, and the origin of astrocyte diversity is less well known. Now, Guillermo Lanuza and colleagues dissect the developmental pathways of astrocytes derived from Dbx1-expressing progenitors in the p0 domain of the ventricular zone. Using lineage tracing and molecular markers, they find that Dbx1+ progenitors produce cells that populate a specific region in the intermediate portion of mouse spinal cords. They identify three morphologies amongst the astrocytes: protoplasmic astrocytes that populate the grey matter; fibrous astrocytes, which are found in the white matter; and radial astrocytes that are in the white matter, close to the spinal cord surface. They find that an excess of astrocytes is specified in Dbx1 knockout mice and attribute this to a reduction in neuronal production, resulting in a larger progenitor pool with astroglial potential. They discover that this is due to increased Notch signalling during the neurogenic phase in knockout mice. Together, these data show that Dbx1 modulates Notch signalling to control differentiation of progenitors and contribute to creating astrocyte diversity.