Fig. 2. Basal surface of tracheal epithelium continues to migrate in rib mutants. (A) Budding primary branch growing toward Bnl FGF signaling center. Cytoplasmic processes extend from the basal surface of the lead cells, and cell bodies and apical surface follow. (B,D) Four tracheal metameres (Tr1-4) of stage 12 rib⁺ (B) and rib^ex12 (D) embryos stained with TL1 antiserum to show lumen and apical surface. Note limited outgrowth of apical surface in rib mutant. (C,E) Same view of similarly staged rib⁺ and rib^ex12 embryos carrying trachealess-lacZ reporter (1-eve-1/+) and immunostained for ß-galactosidase to show tracheal cells and basal surface. Basal cytoplasmic processes extend from the tips of growing branches (brackets) in both rib⁺ and rib^ex12 embryos; the processes are sometimes more numerous (asterisks) and longer (arrowhead) in the mutant. (F-K) Confocal fluorescence micrographs of tracheal metamere Tr5 in stage 12 rib⁺ (F-H) and rib^ex12 (I-K) embryos carrying trachealess-lacZ (1-eve-1/+) and double stained for the apical marker Crumbs and for ß-galactosidase to show cells and basal surface. Red (Crumbs) channel (F,I), green (ß-galactosidase) channel (G,J), and merged image (H,K). Cytoplasmic processes extend from the tips of growing branches (brackets) in both rib⁺ and rib^ex12 embryos. Processes are sometimes more numerous (asterisk) and longer (arrowheads) in the mutant. (L-O) Ventral view (anterior left) of stage 13 rib⁺ (L,M) and rib^ex12 (N,O) embryos carrying trachealess-lacZ (1-eve-1/+) and immunostained for ß-galactosidase. Visceral tracheal branches (boxed in L,N) migrate internally onto the gut. Detail of boxed regions (M,O) show two visceral branches on the gut surface; cytoplasmic processes are more numerous in mutant. Scale bars: in D, 20 µm for B-E; in I, 40 µm for F-K; in N, 20 µm for L,N.

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