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Fig. 1. Models for mesoderm spreading in Drosophila. (A)
Following invagination, the mesoderm forms an epithelial tube. (B) The
cells then undergo an EMT and divide once. (C) The mesodermal cells
then collapse down onto the ectoderm and begin to spread out. We notionally
divide cells into outer cells adjacent to the ectoderm (grey) and inner cells
(white). Three possible cellular mechanisms for spreading are depicted
[adapted from Wilson and Leptin (Wilson
and Leptin, 2000)]. In the Chemotaxis model, a chemoattractant
emanating from the dorsal part of the ectoderm (red) attracts mesodermal cells
dorsally. In the Differential Affinity model mesodermal cells have more
affinity for the ectoderm (blue) than for each other, and seek to maximise
their contact with the ectoderm. In this model, activation of the FGF receptor
Htl would simply impart a degree of motility to cells allowing inner cells to
move over, and in between, existing outer cells until they were able to find
contact with the ectoderm. In the Convergent Extension model, inner and outer
cells move towards each other (arrows) and intercalate, resulting in a net,
lateral spreading of the tissue. (D) Eventually the mesoderm forms a
single layer of cells.
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