3D mitochondrial network formation during postnatal skeletal muscle development of mice

Mitochondrial connectivity plays an important role for muscle energy distribution. However, it is obscure how their structural formation is developed during postnatal muscle growth. Here, using a high‐resolution electron microscopy ( i.e. , FIB‐SEM), we have sought to examine three dimensional (3D) mitochondrial network formations in oxidative (soleus) and glycolytic (gastrocnemius) muscles of C57BL/6N mice at postnatal (P) 1, 7, 14, 21, and 42 day, respectively. After semi‐automated machine learning process, 3D mitochondrial reticulum formations were rendered and quantitatively analyzed by ImageJ software. Both types of skeletal muscles showed similarly fiber parallel mitochondrial network formations during early days (P1–P7). After P14, oxidative muscles developed grid‐like mitochondrial reticulum, while glycolytic muscle mitochondria were found as perpendicular formations along the Z‐lines. Unlikely to glycolytic muscles where individual mitochondrial volumes were similarly maintained across the development, oxidative muscles significantly increased those volumes by P14 and then largely decreased to the level comparable to glycolytic muscles. After P14, oxidative muscles maintained notably higher total mitochondrial volumes as compared to glycolytic muscles. Furthermore, we observed that at P14, glycolytic muscles showed remarkably higher ratio of mitochondrial surface area to volume, as well as sphericity, than oxidative muscles. Another interesting point was that during early muscle growth (P1–P7), mitochondrial elongation, aspect ratio, and length were all shown to be higher in glycolytic muscles than in oxidative muscles. In conclusion, this preliminary study provides how 3D mitochondrial networks are changed over the course of postnatal muscle development, and also suggests that P14 may be a critical time point for the mitochondrial transformation. However, further studies are needed to better understand 3D muscle mitochondrial development along with contractile components and other subcellular elements such as lipid droplets and sarcoplasmic reticulum. Support or Funding Information This work is supported by the Division of Intramural Research of the National Heart, Lung and Blood Institute and the Intramural Research Program of the National Institute of Arthritis and Musculoskeletal and Skin Diseases. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .