Fig. 3. Matching in embryos with asymmetries between the opposing epithelial sheets. DC zippering in Drosophila embryos with asymmetries as revealed by en-RFP-Moesin (red) and ptc-GFP-Moesin (green) expression, shown merged (top row) and in isolation (middle and bottom rows, respectively). (Ai-iv'') Images from Movie 4 (see Movie 4 in the supplementary material) showing zippering in an embryo with a spontaneous asymmetry. (i) The upper epithelial sheet has two rather than one misplaced ptc cell in the en domain and one of these has associated with the A compartment of the lower sheet blocking matching of en cells. (ii) As a result, the blocked en cells make filopodial contacts with en cells of the neighbouring segment. (iii) These develop into permanent contacts. (iv) Cells that are not associated with matching partners continue to produce filopodia. (Bi-iii'') Zippering in an embryo with a laser-induced asymmetry. (i) An en stripe has been removed from the leading edge of the lower epithelial sheet by laser ablation (asterisk), whereas the opposing en stripe (arrow) is normal. (ii,iii) On contact with the opposing leading edge, the unpartnered en stripe constricts and then withdraws completely from the leading edge. The described filopodial interactions are indicated by arrowheads. Scale bars: 10 µm.