Fig. 8. Effect of K19^–/– mutation on costameres and nearby structures. (A) Organization of the costameres in the myofibrils of striated muscle. Image is adapted from Ursitti et al. (Ursitti et al., 2004). For clarity, only the Z- and M-domains of costameres and a few of the integral and peripheral proteins present at costameres are shown. Emphasis is on the alignment of integral and peripheral membrane proteins with filaments composed of -actin, desmin, and K8 and K19. (B) Changes documented by us in K19^–/– TA muscle, including the disruption of costameres, and the increase in the gap between the sarcolemma and the nearest myofibrils in which mitochondria accumulate. The retention of costameres in some K19^–/– myofibers might be owing to filaments composed of -actin and desmin that persist in the absence of K19. Not drawn to scale.