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Fig. 4 Temporal regulation of sm22α-b, smtn-b, cpi-17, and nm-mhc-b during the development of visceral smooth muscle. A-D: nm-mhc-b is first seen in the anterior gut at 50 hours postfertilization (hpf; A, arrow). At 3 days postfertilization (dpf), nm-mhc-b is expressed throughout the gut (B, arrow). By 4 dpf, the expression of nm-mhc-b in the gut (arrows) is down-regulated, but continues in the swim bladder (C, arrowhead). In longitudinal section, nm-mhc-b is expressed in a layer of cells surrounding the gut epithelium (D, arrowheads). E-H: SM22α-b expression at 56 hpf (E), 60 hpf (F), and 4 dpf (G) in the developing gut and swim bladder is similar to that of α-smooth muscle actin (αSMA). The inset in F is a dorsal view. Like nm-mhc-b, it is expressed in a layer surrounding the gut epithelium at 3 dpf (H). I,J,L,M: cpi-17 is expressed throughout the gut at 3 dpf (I), while smtn-b is expressed in the anterior gut (L). By 4 dpf, cpi-17 (J) and smtn-b (M) are no longer expressed in the gut, but are expressed in the swim bladder. K,N: In longitudinal section, both genes are expressed in a layer surrounding the gut epithelium.