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First published online 17 October 2007
doi: 10.1242/dev.005389
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The ARC Special Research Centre for the Molecular Genetics of Development and Molecular Genetics and Evolution Group, Research School of Biological Sciences, The Australian National University, Canberra, ACT, 2601, Australia.
* Author for correspondence (e-mail: robert.saint{at}anu.edu.au)
Accepted 18 August 2007
Mesoderm migration is a pivotal event in the early embryonic development of
animals. One of the best-studied examples occurs during Drosophila
gastrulation. Here, mesodermal cells invaginate, undergo an
epithelial-to-mesenchymal transition (EMT), and spread out dorsally over the
inner surface of the ectoderm. Although several genes required for spreading
have been identified, our inability to visualise mesodermal cells in living
embryos has left us to speculate about the cell rearrangements involved.
Several mechanisms, such as chemotaxis towards a dorsally expressed
attractant, differential affinity between mesodermal cells and the ectoderm,
and convergent extension, have been proposed. Here we resolve the behaviour of
Drosophila mesodermal cells in live embryos using
photoactivatable-GFP fused to
-Tubulin (PAGFP-Tub). By photoactivating
presumptive mesodermal cells before gastrulation, we could observe their
migration over non-fluorescent ectodermal cells. We show that the outermost
(outer) cells, which are in contact with the ectoderm, migrate dorsolaterally
as a group but can be overtaken by more internal (inner) cells. Using
laser-photoactivation of individual cells, we then show that inner cells
adjacent to the centre of the furrow migrate dorsolaterally away from the
midline to reach dorsal positions, while cells at the centre of the furrow
disperse randomly across the mesoderm, before intercalating with outer cells.
These movements are dependent on the FGF receptor Heartless. The results
indicate that chemotactic movement and differential affinity are the primary
drivers of mesodermal cell spreading. These characterisations pave the way for
a more detailed analysis of gene function during early mesoderm
development.
Key words: Mesoderm, Cell migration, Drosophila, Photoactivatable GFP
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