Fig. 4. Precocious differentiation of Xaf1o ventricular cardiomyocytes. (a) The subepicardial myocytes of E9.5 wild-type (WT) embryos contain bundles of myofilaments, occasionally showing isolated Z lines and connected only by desmosomes. (b) In 70% of the subepicardial myocytes of this E9.5 Xaf1o mutant the myofilaments are already organized into sarcomeres (S) that are connected between cells by a series of adherens junctions forming the intercalated discs (ID). D, desmosome; F, bundles of myofilaments; M, mitochondria; S, sarcomere; Z, Z line. Scale bar: 0.5 µm.