TEM assessment demonstrates alterations of mitochondrial morphologies in LV sections of 4 and 10 week WT and KO littermates. Representative TEM micrographs showing 4 and 10 week WT and KO LV cardiomyocytes in: (A-E) transverse sections and (F-J) longitudinal sections. Mitochondria (m) were found within the myofiber (M) with condensed mitochondrial cristae (white arrows) in the KO at both ages, while iron-aggregates (black arrowheads) were only in 10 week KO mitochondria. Quantitative assessment of (B, G) the average number of mitochondria, (C, H) mitochondria: myocyte area ratio, (D, I) mitochondrial cross-sectional area, and (E, J) mitochondrial aspect ratio, between 4 and 10 week KO and WT mice. Data are mean ± SEM (n = 6 mice/genotype/age). One-way ANOVA with Tukey's post hoc tests were used. *, p < 0.05; **, p < 0.01; ***, p < 0.001, versus WT of respective age. #, p < 0.05; ##, p < 0.01; ###, p < 0.001 versus 4 week mice. n.s., not significant. Scale bars in (A, F) are 2 μm, or 500 nm, as indicated.