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WIREs Dev Biol
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A niche for Drosophila neuroblasts?

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Abstract Stem cells, which can self‐renew and give rise to differentiated daughters, are responsible for the generation of diverse cell types during development and the maintenance of tissue/organ homeostasis in adulthood. Thus, the precise regulation of stem‐cell self‐renewal and proliferative potential is a key aspect of development. The stem‐cell niche confers such control by concentrating localized factors including signaling molecules which favor stem‐cell self‐renew and regulate stem‐cell proliferation in line with developmental programs. In contrast, Drosophila neuroblasts (NBs), often referred to as neural stem cells/progenitors, can undergo asymmetric cell division to self‐renew and produce differentiated daughters even in isolation (or in culture). Furthermore, these isolated NBs can also progress through an intrinsically regulated temporal series (of transcription factor expression) to generate diverse cell types in vitro. These data argue that NBs may depend only to a limited extent, if at all, on local environment (a niche) for their maintenance. On the other hand, there is increasing evidence which indicate that the interaction between NBs and their surrounding glia is critical for the control of NB proliferative potential and these glia, in conjunction with systemic regulation, perform the niche function to regulate NB behavior. Thus, these observations emphasize the importance of coordinated local microenvironment (niche activity) and systemic environment (global activity) on the regulation of NB behavior in vivo, and suggest NBs may conform to an alternative stem‐cell/progenitor maintenance model. WIREs Dev Biol 2012, 1:307–314. doi: 10.1002/wdev.27 This article is categorized under: Nervous System Development > Flies Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Environmental Control of Stem Cells

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Asymmetric division of neuroblasts (NBs). NBs undergo asymmetric cell divisions to produce a self‐renewing neuroblast and a differentiating daughter cell (ganglion mother cell, GMC). The asymmetry of NB divisions is achieved through the establishment of a multi‐protein complex at the apical cortex [including Inscuteable (Insc), Par6–Bazooka (Baz)–Drosophila atypical protein kinase C (DaPKC), and Partner of Insc (Pins)–G protein αi subunit (Gαi) signaling cassettes, in green], and the basal localization of neural cell fate determinants [e.g., Prospero (Pros), brain tumor (Brat), and Numb, in red] and the adaptor proteins Mira and Pon. The GMC then divides terminally to produce two ganglion cells which subsequently differentiate into neurons and/or glial cells.

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Two modes of stem‐cell maintenance. One class of stem cells (a), such as Drosophila germline stem cells, is maintained by the extrinsic signals emanating from the niche, without which these stem cells will be destined for their default mode of differentiation. In other stem‐cell systems such as Drosophila neuroblast (NB) (b), the self‐renewing capacity of the stem cell is intrinsically regulated, while other aspects of stemness such as growth and proliferation are partly controlled by the signaling event associated with their proximal cells.

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The proliferation of the larval neuroblasts (NBs) are controlled by ‘systemic’ signals. The systemic changes of the organism during development signal the fat bodies (orange) to secret fat‐body‐derived signal (FDS) to the surface glial cells (pink) located superficial to the central brain. The signal is then relayed to the quiescent NBs (purple) located in close proximity to the glial cells in the form of insulin‐like peptides (dILPs) such that NBs are reactivated and proliferate in line with systemic requirement.

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Comparison between the local environments in which embryonic neuroblast (NB) and larval NB reside. (a) The embryonic NB (left panel, purple) delaminates from the ventr al neuroectodermal layer such that, during early divisions, its apical pole is always in contact with the epithelial cells (blue) which provide the signaling cue necessary for orientating the division axis of the NB. (b) In contrast, the larval NB (right panel, purple) arises from its quiescent form and divides to form a tight cluster of neuronal progeny (yellow). The proliferative control of the NB as well as the fasciculation and pathfinding of the neurites are regulated by the surface glia (pink) and cortex glia (red), respectively. DE‐Cadherin is in green.

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Transcription factor switching in the embryonic neuroblast (NB). During embryonic neurogenesis, the NB expresses a series of transcription factors (temporal series) sequentially: Hunchback (Hb, red) → Kruppel (Kr, green) → POU homeodomain protein (Pdm, blue) → Castor (Cas, purple) → Grainyhead (Grh, yellow). The temporal transcription factor expressed in the NB is maintained in the ganglion mother cell (GMC) and the subsequent neuronal progeny that arise with the division of the GMC.

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Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches
Nervous System Development > Flies
Adult Stem Cells, Tissue Renewal, and Regeneration > Environmental Control of Stem Cells