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Influence of Hypoxia Acclimation and Evolutionary History on the Muscle Structure of Peromyscus Mice

At high altitude (>2500 meters above sea level), small mammals are confronted with the challenge of sustaining aerobic processes, such as shivering thermogenesis, in the face of reduced oxygen availability and low temperatures. To overcome these demands, oxygen and metabolic fuel must be supplied to skeletal muscles. This supply requires a sufficient density of muscle capillaries and a high proportion of aerobic muscle fibers (those that use O2). I tested the hypothesis that different species of mice of the genus Peromyscus – which have independently colonized high altitude – have evolved an increase in muscle capillary density and a higher proportion of aerobic fibers relative to closely related low-altitude species. I executed this study by measuring capillarity and muscle fiber types of gastrocnemius skeletal muscle (hereafter, gastroc) samples collected from a previous experiment, where each species of Peromyscus was exposed to stimulated elevations spanning normoxia to hypoxia (1000 m, 3500 m, and 4500 meters above sea level) for six weeks. Histological sections of gastroc were taken, stained for capillaries and muscle fibers, and imaged on a light microscope. I have quantified images for oxidative fibers, leaving the analysis of capillary density remaining. Results from this analysis indicate an increase in density of oxidative fibers in the gastroc of species that are native to high elevation, relative to those living at low altitude. This suggests that the supply of oxygen and metabolic energy required to maintain aerobic processes is a ubiquitous adaptation of small mammals at high altitude, contributing to their success in profoundly hypoxic conditions.