Interestingly, like neurons, these diverse astrocyte and ependymal cell types occupied distinct domains of the brain with little apparent mixing. The sharpness of the border between Mfge8 (telencephalon astrocytes) and Agt (non-telencephalon atroctyes) expression, for example, and the fact that it coincided with a developmentally recognized boundary distinguishing the telencephalon from the rest of the brain, strongly implies that these astrocyte types are developmentally specified. In order to test this hypothesis, we examined the expression of region-specific neural tube patterning genes, the transcription factors Foxg1 (telencephalon), En1, and Pax3 (midbrain) and Hoxc6 (spinal cord). Each of these genes marked the expected subset of astrocyte and ependymal cell types (Figure 3G). Thus, we have uncovered a diversity of astrocyte and ependymal cell types, showing the hallmarks of developmentally specified identities and regional specialization.
