Postnatal development of the mitochondrial reticulum in the cardiac muscles of mice

Adult cardiac muscles are characterized by well‐developed mitochondrial networks, which supports high energy demand in the heart. However, it is obscure how these structures are formed during postnatal development. Using focused ion beam scanning electron microscopy (FIB‐SEM), we collected high‐resolution image stacks of cardiac muscles of mice at postnatal (P) day 1, 7, 14, and 42, respectively (N=2‐3 per group). Then, we conducted a semi‐automated machine running to extract 3D mitochondrial formations and analyzed its morphological characteristics across postnatal days. After forming more stretched structures by P7, it was shown that cardiac muscles further develop spherical shaped‐mitochondria and that they are overall arranged in the fiber‐parallel direction. Although total mitochondrial volumes tend to increase across P1‐P42, individual mitochondrial volumes were relatively smaller during early postnatal days (P1‐P14) than in the matured muscles. Notably, we observed that individual mitochondrial surface area to volume ratios as well as aspect ratios are significantly downregulated during the entire developmental period by ~34% and ~31%, respectively. Furthermore, it was also shown that individual mitochondrial lengths are ~31% decreased across the postnatal days. In conclusion, these preliminary results provide dynamic transitions of individual mitochondrial structures during postnatal cardiac muscle development. Nevertheless, further analyses will be needed to study intermitochondrial interactions, as well as mitochondrial interactions with other subcellular components (e.g., lipid droplets), by which we can better understand functional aspects of mitochondria in developing cardiac muscles.