The Effect of Dietary Loading on Structural Determinants of Force Production in the Rat Masseter




Rossiter, Jeffrey A.


Journal Title

Journal ISSN

Volume Title



Rossiter, Jeffrey A., The Effect of Dietary Loading on Structural Determinants of Force Production in the Rat Masseter. Master of Science in Medical Sciences - Anatomy, May 2020. Biomechanical loading associated with feeding is known to direct cranial bone growth, however less is known about its effects on masticatory muscle growth and performance. Peak muscle contractile forces are determined by a combination of factors including total muscle mass, fiber length, and fiber type. Here, we test two hypotheses: that mechanically challenging diets will (1) increase the physiological cross-sectional area (PCSA), an estimate of maximum contractile force at tetanus, and (2) increase the number and proportion of type II (fast-twitch) muscle fibers in the masseter of the rat. Sprague-Dawley rats were raised on either a hard/tough (overuse) diet or a soft (underuse) diet (n=5/cohort). The superficial masseters were dissected and photographed using a trifocal stereo microscope, and muscle fiber length (6/individual) were measured using ImageJ. Muscle volumes were calculated from in-situ diffusible iodine-based contrast-enhanced μCT scans. Muscles were stained using an IHC protocol for the fast isoform of myosin heavy chain, allowing the number and areas of type II (stained) and type I (unstained) fibers to be quantified in ImageJ. Results from this study do not support our hypotheses, most likely due to the small sample sizes (n=5/treatment group) available for this study. Paradoxical results were found, with rats raised on a soft diet tending to have longer superficial masseter muscle fibers and more type II muscle fibers with larger cross-sectional areas in the posterior masseter. Rats raised on a hard diet tend to have larger masseter muscle volumes. However, these trends were not statistically significant (p > 0.05). Mechanically challenging diets tend to be associated with greater masticatory muscle volumes and thus increased PCSA. The fiber type results from the posterior masseter (with more deep masseter fibers) were the opposite of those previous results from the middle masseter (with more superficial masseter fibers) in the same animals. Future studies with increased sample sizes are needed to better understand the structural determinants of force production in the rat masseter.