Fig. 2. Myocardial ultrastructure is disrupted in the wea mutant atrium and intact in the wea mutant ventricle. (A-D) Longitudinal sections of myocardiocytes at 48 hpf viewed by transmission electron microscopy. (A,B) Atrial cells in wild-type embryos contain myofibrillar arrays (A), but myofibrils are rarely found in wea mutant atrial cells (B). Occasionally, wea mutant atrial cells contain a few disorganized myofilaments (B, arrow). (C,D) Ventricular cells in wild-type (C) and wea mutant embryos (D) contain normal myofibrillar arrays.