The nerve cells in the brain of Drosophila are generated by neural stem cell-like progenitor cells called neuroblasts. In the currently accepted model of neurogenesis, these neuroblast divide asymmetrically both to self renew and to produce a smaller progenitor cell. This smaller cell then divides only once into two daughter cells, which receive cell fate determinants, causing them to exit the cell cycle and differentiate into postmitotic neural cells.
In the mammalian brain, neural stem cells may also divide asymmetrically but they can then amplify the number of cells they produce through intermediate progenitors. These intermediate progenitors can divide repeatedly in a symmetrical manner, such that each intermediate progenitor gives rise to a number of postmitotic neurons in the brain. A research team from the Biozentrum set out to study whether specific Drosophila neuroblasts might also be able to increase the number of cells generated in the postembryonic brain via a similar mechanism.
Bruno Bello, Natalya Izergina, Emanuel Caussinus and Heinrich Reichert initially discovered that surprisingly large neuroblast lineages were present in the dorsomedial Drosophila brain. They then used cell lineage tracing and genetic marker analysis to show that these large cell lineages were a result of amplified neuroblast proliferation mediated through intermediate progenitors.
In the novel mechanism postulated by the researchers, these intermediate progenitors divide symmetrically in terms of morphology, but asymmetrically in molecular terms. This latter feature assures that key cell fate determinant,s such as the Prospero transcription factor, are segregated into only one daughter cell, leaving the other free to divide several more time, thus amplifying the number of cells generated.
The Reichert lab has been studying the development and evolution of the brain for many years by analyzing the cellular and molecular mechanisms of brain development in the fruitfly and comparing these mechanisms, with those involved in the development of the mammalian brain. Remarkably, all of these comparative developmental studies indicate that the brains of all animals, including the human brain, are based on a similar groundplan and are, thus likely to have a common evolutionary origin.
Journal reference: Bruno C Bello, Natalya Izergina, Emmanuel Caussinus, and Heinrich Reichert. Amplification of neural stem cell proliferation by intermediate progenitor cells in Drosophila brain development. Neural Development 2008, 3:5. doi:10.1186/1749-8104-3-5, 19 February 2008. http://www.neuraldevelopment.com/content/3/1/5/abstract
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