A skeletal muscle model of extreme hypertrophic growth reveals the influence of diffusion on cellular design

Muscle fibers that power swimming in the blue crab Callinectes sapidus are <80 microm in diameter in juveniles but grow hypertrophically, exceeding 600 microm in adults. Therefore, intracellular diffusion distances become progressively greater as the animals grow and, in adults, vastly exceed those in most cells. This developmental trajectory makes C. sapidus an excellent model for characterization of the influence of diffusion on fiber structure. The anaerobic light fibers, which power burst swimming, undergo a prominent shift in organelle distribution with growth. Mitochondria, which require O2 and rely on the transport of small, rapidly diffusing metabolites, are evenly distributed throughout the small fibers of juveniles, but in the large fibers of adults they are located almost exclusively at the fiber periphery where O2 concentrations are high. Nuclei, which do not require O2, but rely on the transport of large, slow-moving macromolecules, have the inverse pattern: they are distributed peripherally in small fibers but are evenly distributed across the large fibers, thereby reducing diffusion path lengths for large macromolecules. The aerobic dark fibers, which power endurance swimming, have evolved an intricate network of cytoplasmically isolated, highly perfused subdivisions that create the short diffusion distances needed to meet the high aerobic ATP turnover demands of sustained contraction. However, fiber innervation patterns are the same in the dark and light fibers. Thus the dark fibers appear to have disparate functional units for metabolism (fiber subdivision) and contraction (entire fiber). Reaction-diffusion mathematical models demonstrate that diffusion would greatly constrain the rate of metabolic processes without these developmental changes in fiber structure.