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Fig. 3 α-Smooth muscle actin (αSMA) expression is temporally regulated in the intestine and swim bladder. A,D,G,J,M: αSMA is expressed in the developing gut (arrows) and swim bladder (arrowheads) at 60 (A), 65 (D), and 77 hours postfertilization (hpf) (G), or 4 (J), and 5 days postfertilization (dpf) (M). αSMA expression in the gut is not continuous at 65 hpf (white arrows in inset A), but becomes continuous by 65 hpf (D). G′: αSMA expression is seen in the developing swim bladder at 77 hpf in a dorsal view. B,E,H,K,N: Cross-sections of 60 (B), 65 (E), and 77 hpf (H), or 4 (K) and 5 dpf (N) embryos show αSMA expression in the developing muscularis propria of the gut and swim bladder (arrowhead in E). C,F: Cells expressing αSMA at 60 (C) and 65 hpf (F) are mesenchymal in morphology by hematoxylin and eosin staining. I,L,O: By 77 hpf a layer of circularly arranged cells is seen outside the gut epithelium (arrows in I′ point to nuclei of circularly arranged cells). g, gut; sb, swim bladder.