Glial cell progenitors in the Drosophila embryo.

Development and general organization of the nervous system is comparable between insects and vertebrates. Our current knowledge on the formation of neurogenic anlagen and the generation of neural stem cells is deeply influenced by work done in invertebrate model organisms such as Drosophila and Caenorhabditis elegans. It is the aim of this review to summarize the most important steps in neurogenesis in the Drosophila embryo with a special emphasis on glial cell progenitors and the specification of glial cells. Induction of neurogenic regions during early embryogenesis and determination of neural stem cells are briefly described. Special attention is given to the formation of neural precursors called neuroblasts (NB) and their lineages. NBs divide in a stem cell mode to generate a cell clone of either neurons and/or glial cells. The latter require the activation of the transcription factor glial cells missing (gcm), thus providing a binary switch between neuronal and glial cell fates. Further aspects of glial cell specification and the resulting heterogeneity of the glial population in Drosophila are discussed.