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.