Fig. 4. Ultrastructure of subsarcolemmal region of control and K19^–/– muscle fibers. (A-C) TA muscles of control (A) and K19^–/– (B,C) mice were fixed in situ, processed for electron microscopy of longitudinal thin sections, and viewed near the fiber surfaces. The distance between the sarcolemma and the nearest Z-disks (white lines) was greater in K19^–/– samples (B,C) than in controls (A). Mitochondria accumulated in the subsarcolemmal gap to moderate (B) or large (C) extents in mutant but not in control (A) muscle. Sarcomeres in the mutant appeared unchanged. (D) Distances between neighboring Z-disks in adjacent myofibrils were measured and are shown in µm. Control values were 180 nm (line). K19^–/– samples showed a mean value of 240 nm (line). These differences are highly significant (see text). (E) The percent myofibers with slight (category 1), moderate (category 2) or large (category 3) accumulations of mitochondria under the sarcolemma (see Materials and Methods) were scored in TA and soleus muscle fibers of wild-type (gray bars) and K19^–/– (white bars) mice. TA but not soleus muscle showed significant increases in the number of myofibers with large numbers of subsarcolemmal mitochondria. (F) Distances between the sarcolemma and the nearest Z-disks were measured and are shown in µm. Control values were 150 nm, with little variation (line). K19^–/– samples showed a mean value of 1 µm, with greater variation (line). These differences are highly significant (see text).