The Ontogeny of Masticatory Muscle Architecture in Microcebus murinus

Abstract

Many studies pertaining to the masticatory apparatus have focused on the mandibular adductors (the temporalis, masseter, and medial pterygoid) ‐ the muscles that work cooperatively to produce bite force. Specifically, the architectural properties of these muscles have been correlated with diet. For instance, felids that eat relatively large prey have relatively longer fascicles to allow for the necessary gape. Primate diet also correlates with muscle architecture and, in some instances (e.g., tree gouging in Callithrix), the mode of foraging. However, there is little known about the ontogenetic development of these muscles. To characterize age‐related changes, we used an intraspecific sample of 28 specimens from a captive colony, spanning the entire lifespan of the small Malagasy primate, Microcebus murinus.We dissected each of the jaw adductors and obtained their masses as well as linear morphometrics. We then chemically dissected them using 35% nitric acid for 15–30 hours until their constituent fascicles could be separated. Then the reaction was neutralized in 50% glycerol and fascicles were photographed and measured using ImageJ (NIH) and all logged variables were statistically analyzed in JMP (SAS).A quadratic curvilinear model is significant when muscle mass is regressed against age with p‐values <0.01 for each adductor. The trends demonstrate that the muscle mass of the adductors peak during early to mid‐adulthood with a point of inflection occurring at late adulthood to early senescence. When physiological cross‐sectional area (PCSA) is scaled against age, a quadratic model represents the data significantly better for the masseter and medial pterygoid with p‐values <0.01. However, a linear model is more appropriate when characterizing changes in the PCSA of temporalis as the quadratic model was found to be insignificant. The general trend for changes in PCSA for masseter and medial pterygoid are consistent with the patterns observed in muscle mass, but temporalis PCSA does not appear to atrophy significantly with age. When fiber length is scaled against age for each mandibular adductor, the curvilinear relationship was observed to be a good fit with p‐values <0.05. However, for the temporalis and medial pterygoid this trend appears to be driven by the inclusion of a low outlier. When this specimen is excluded, the trend line is horizontal suggesting that, once fiber length reaches maximum length early in life, it remains relatively constant. The trend for medial pterygoid also suggests that fiber length peaks early in life; however, it also indicates a point of inflection occurring in early adulthood.From these results, representing the most complete ontogenetic sample of primate masticatory muscle architecture, we have successfully characterized myological changes across the lifespan of M. murinus. Our findings suggest that the changes in PCSA are driven mostly by changes in muscle mass including growth through early adulthood and slight but clear reduction into senescence, while masticatory fascicle lengths remain surprisingly constant throughout life.Support or Funding InformationThis project was supported by the NSF (IOS‐15‐57125 and BCS‐14‐40599).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.