FIB-SEM used to understand spatial relationships between mitochondria and sites of energy storage, utilization, and signaling.

Skeletal muscle is the most abundant tissue in humans. In an instant, it must coordinate the movement of signals and materials through relatively large muscle cells to generate force through muscle contractions. These contractions are maintained anywhere from seconds to hours. The Muscle Energetics Laboratory, led by Dr. Brian Glancy, at the National Heart, Lung, and Blood Institute at the National Institutes of Health, USA, seeks to understand how mitochondria are optimized within muscle cells to help maintain energy homeostasis during the large change in energy demand caused by muscle contractions. 3D volume electron microscopy (vEM) utilizing ZEISS Crossbeam FIB-SEM is a key technology that Dr. Glancy’s team has utilized to explore mitochondrial and muscle substructure as evidenced in many of their recent publications highlighted below.

There are many, many different physical interactions that are happening within these cells and when we’re doing 3D electron microscopy, we can see all of these things. This allows us to take an integrated viewpoint on muscle cell structure.

In C.K.E. Bleck et al. 2018, Dr. Glancy and team applied a connectomics approach using volume electron microscopy with FIB-SEM and SBF-SEM to quantitatively assess the mitochondrial network in cardiac, oxidative, and glycolytic muscle.

They showed that each muscle type, with its differing contraction demands, had different mitochondrial network configurations. They also assessed mitochondria-lipid droplet interactions and found evidence that individual mitochondria may be tuned to specialize in energy distribution or calcium cycling.

In T.B. Willingham et al. 2020, Dr. Glancy's group uses volume electron microscopy with FIB-SEM to unveil that striated muscle cells form a continuous myofibrillar matrix with frequent, branching sarcomeres. Their work examines changes in branching during postnatal development and in different muscle types. They suggest a new theory for how force is generated based on a mesh-like myofibrillar network rather than many individual, parallel myofibrils.

Mitochondria must supply a constant energy stream to actin and myosin filaments within muscle sarcomeres to sustain muscle contractions over time. In P. Katti et al. 2022, 3D electron microscopy is used to examine variations in sarcomere cross-sectional area and provide evidence that both sarcomere structure and myofilament interactions are influenced by the location and orientation of mitochondria within muscle cells.

Continuing their work from T.B. Willingham et al. 2020 mentioned above, Dr. Glancy and team looked into the extent to which myofibrillar connectivity is evolutionarily conserved as well as mechanisms which regulate the specific architecture of sarcomere branching. In P.T. Ajayi et al. 2022, they present 3D electron microscopy evidence which indicates fruit flies have a myofibrillar connectivity on/off switch that is regulated by both cell-type dependent and independent mechanisms.

3D electron microscopy allows us to see how all the pieces fit together and how this puzzle changes across different cell types and environments. As a result, we have a better understanding of how a muscle cell is built, how it works, and how we may be able to fix it when it doesn’t.