Myofibrillar ultrastructure in the developing atrial myocardium of WT (A, C, E) and calreticulin KO hearts (B, D, F). Magnification 10,000×. In atrial myocardium, there was no difference between the degree of organization following myofibrillar disarray between the calreticulin phenotypes. Atrial myocardium of both phenotypes contains large amounts of cytosol. Consequently, the myofibrillar arrangement of the atrial myocardium is less compact compared to the corresponding ventricular myocardial tissue (A, B, C, D, E, and F). By 18.5 dpc, most of the WT and KO myofibrils of atrial myocardium run in straight courses aligned parallel to each other with their Z-lines in register (E and F).