Fig. 9. Morphological changes accompanying segmental groove formation. Schematic representation of the changes in cell morphology and genetic interactions before and during boundary formation. Here, drawings are oriented such that the apical side of the epithelium is upwards, according to convention. (A) Groove formation is initiated by Hedgehog signalling in cells adjoining the most posterior engrailed-expressing cells. Signalling by Hedgehog prevents repression by Ci[75], leading to the expression of gene(s) x. (B) The groove founder cells loose contact on their apical side and an unknown signal (Y) feeds back on the engrailed-expressing cell. (C) The most posterior engrailed-expressing cell constricts its apical surface and moves inwards. (D) It comes to lie at the bottom of the forming groove while continuing to constrict its apical surface. (E) As the groove reaches its deepest point, the most posterior engrailed-expressing cell acquires a bottle shape. At the same time, it turns off engrailed expression.

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