Fig. 4. amhc is expressed only in atrial myocardium. (A-L) Whole-mount in situ hybridization compares expression of cmlc2 (A,E,I), vmhc (B,F,J) and amhc (C,D,G,H,K,L). (A-D) Dorsal views at the 21-somite stage, anterior to the top. In wild-type embryos (A-C), cmlc2 expression (A) is observed throughout the cardiac cone. Presumed ventricular precursors express vmhc (B) and are found in the central portion of the cardiac cone. At this stage, presumed atrial precursors are beginning to express amhc (C) and are found in the outer portion of the cardiac cone. (E-H) Dorsal views through the head at 24 hpf, anterior to the bottom. In wild-type embryos (E-G), cmlc2 (E) is expressed throughout the heart tube, vmhc (F) is expressed in the ventricular precursors, and amhc (G) is expressed in the atrial precursors. (I-L) Frontal views at 48 hpf, head to the top. In wild-type embryos, cmlc2 (I) is expressed throughout the heart, vmhc (J) is expressed in the ventricle, and amhc (K) is expressed in the atrium. wea mutant embryos express amhc in atrial precursors initially (D), but do not maintain amhc expression (H,L). At all stages examined, ranging from the 19-somite stage through adulthood, there appears to be little, if any, overlap between vmhc and amhc expression. We have not observed amhc expression anywhere outside of the atrium.

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