Glial cell progenitors in the Drosophila embryo

Abstract

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.

The ontogenetic saga of a social brain

“…The GMC is committed to differentiate into a neuron and glial cell. Type II NBs can also selfrenew or can form intermediate neural progenitors (INPs), which give rise to GMCs (Boone and Doe 2008;Homem et al 2015;Altenhein 2015;Koniszewski et al 2016).…”

Origins of glial cell populations in the insect nervous system

“…Most neuroblasts generate exclusively neurons (which, unlike in vertebrates, outnumber glia by a large margin). Some neuroblasts produce both neurons and glia (“neuro-glioblasts”); very few give rise to glia only (“glioblasts”; Beckervordersandforth et al, 2008; Altenhein et al, 2015). Drosophila exhibits a highly stereotyped nervous system along with a wealth of glial markers and genetic lineage tracing methods allowing for the specific and stable labeling of cells, frequently in a temporally-controlled manner.…”

“…Glioblasts only generate glia, and express the glial determinants gcm and repo right after they delaminate from the neurectoderm. Neuro-glioblasts show different modes of gliogenesis (Altenhein et al, 2015; Fig.2B). In type 1 neuro-glioblasts, no early separation of neural vs glial fate takes place; instead, neurons and glial cells derive as siblings from the division of ganglion mother cells.…”

“…Primary surface glia of the ventral nerve cord is derived from five neuro-glioblasts (Type 1: NB1-1, NB2-2, NB5-6; Type 2: NB1-3, NB7-4), located at the boundary between two adjacent segments (Beckervordersandforth et al, 2008; Altenhein et al, 2015; Fig.3A). The contribution of primary surface glia to the insulating layers around the peripheral nerves have been revealed by Flybow, a stable, multicolor labeling technique (von Hilchen et al, 2008, 2013), and its origins traced back to NB2-5 and NB5-6, or NB1-3, respectively (Fig.3A, C).…”

“…Primary cell body glial cells of the ventral nerve cord, numbering 3 per hemisegment, are generated by the neuro-glioblasts NB6-4 (Type 3) and 7-4 (Type 2; Fig.3A-C; Altenhain et al, 2015). For the brain, the specific neuro-glioblasts which give rise to the primary cell body glia, as well as surface glia, have not yet been identified.…”