Fig. 4. Ultrastructural analysis of atrial and ventricular myocytes from MLC2a-null and wild-type somite matched embryos. Representative images of cardiac atria and left ventricles of wild-type (+/+) and MLC2a-null (–/–) embryos at sp 22, as assessed by transmission electron microscopy. Black arrowhead indicates the lack of myofibrillar organization in atrial sections from MLC2a-null mutants (C), in contrast to the organized myofibrillar structure observed in atrial sections from wild-type littermates (A). White arrows indicate the normal Z line formation seen in left ventricular sections from both wild-type (B) and MLC2a-null (D) embryos.