Ultrastructural analysis of cardiac myocytes at 72 hpf. (A) Cardiac myocytes in control embryos have formed intercalated disks (^) and filled the sarcoplasm with mature myofibrils. (B) Higher magnification of an intercalated disk (ID) from a control embryo demonstrates adherens (>) and gap (^) junctions. (C, D) In contrast, cardiac myocytes from mildly affected obscurin A RhoGEF morphant embryos have reduced myofibrillar content [compare the nuclear (N) area to the myofibrillar area within each myocyte] and very poorly defined regions of cell–cell (^) contact with no identifiable IDs. Examples of recent nuclear division (N+) were more frequent in the obscurin RhoGEF morphant embryos. (E, F) In more severely affected embryos, cardiac myocytes appear markedly abnormal with loosely arranged contractile filaments (F: ∗). (G, H) The mature cardiac myofibrils that did form in the obscurin A RhoGEF morphant embryos were not significantly different than control with a normal appearing lattice of thick filaments on cross-section (G) and laterally aligned Z disks and M bands on longitudinal section (H). Scale bars are 4 μm (A–C), 2 μm (D, E), 1 μm (F) and 0.5 μm (G, H).