Fig. 5. Costameres at the sarcolemma of K19^–/– muscle are disrupted. Frozen, longitudinal cryosections of tibialis anterior muscles from wild-type (A-C) and K19^–/– (D-I) mice were immunofluorescently labeled with pairs of antibodies to membrane skeletal proteins at the sarcolemma (βI-spectrin: A,D,G; dystrophin: B,E) and nearby structures (desmin, H). Color overlays (C,F,I) show βI-spectrin in red (C,F) or green (I), and the other proteins in the contrasting color. Regions labeled by both antibodies are shown in yellow. Insets show twofold magnifications of the boxed areas in each panel. The results show that the normally rectilinear pattern of costameres (A-C; large arrow indicates a Z-domain; small arrow indicates an M-domain; arrowhead indicates an L-domain) is disrupted in K19^–/– muscle (these domains are missing in the examples shown in D-I) without, however, altering the organization of desmin in nearby myofibrils (H,I). Bars, 5 µm.