Early formation of a GFAP-positive cell population in the ventricular zone during chicken brain development

It is believed that neurons are generated near the surface of the embryonic cerebral ventricles, whereas glial cell proliferation occurs at sites distant to the ventricles. There is still uncertainty concerning the developmental stages when glial and neuronal cell lines diverge in the proliferative zone. The purpose of this study was to determine whether at early stages of chicken brain development during neurogenesis, cells from the astrocytic lineage are present in relevant amounts, where they are located in the neural tube, and to what extent brain regional differences exist. In this report, we demonstrate that specific markers for astrocytes [glial fibrillary acidic protein (GFAP) and glutamate-aspartate transporter (GLAST)] are expressed and translated during early neurogenesis in the developing chicken embryo at day 4 in the investigated brain regions. Between days 4 and 7, GFAP expression declines, and increases again after day 7. This profile is not entirely paralleled by GLAST expression, suggesting a later acquisition of this functional glial property. Comparison of different brain regions revealed that caudal parts of the developing chicken central nervous system are delayed in development with respect to the switch from neuronal to glial genesis. In conclusion, we show that gliogenesis occurs much earlier in the embryonic chicken brain than hitherto assumed.