Using "Mighty Mouse" to understand masticatory plasticity: myostatin-deficient mice and musculoskeletal function

Abstract

Knockout mice lacking myostatin (Mstn), a negative regulator of the growth of skeletal muscle, develop significant increases in the relative mass of masticatory muscles as well as the ability to generate higher maximal muscle forces. Wild-type and Mstn-deficient mice were compared to investigate the postnatal influence of elevated masticatory loads due to increased jaw-adductor and bite forces on the biomineralization of mandibular articular and cortical bone, the internal structure of the jaw joints, and the composition of temporomandibular joint (TMJ) articular cartilage. To provide an interspecific perspective on the long-term responses of mammalian jaw joints to altered loading conditions, the findings on mice were compared to similar data for growing rabbits subjected to long-term dietary manipulation. Statistically significant differences in joint proportions and bone mineral density between normal and Mstn-deficient mice, which are similar to those observed between rabbit loading cohorts, underscore the need for a comprehensive analysis of masticatory tissue plasticity vis-à-vis altered mechanical loads, one in which variation in external and internal structure are considered. Differences in the expression of proteoglycans and type-II collagen in TMJ articular cartilage between the mouse and rabbit comparisons suggest that the duration and magnitude of the loading stimulus will significantly affect patterns of adaptive and degradative responses. These data on mammals subjected to long-term loading conditions offer novel insights regarding variation in ontogeny, life history, and the ecomorphology of the feeding apparatus.

Phenotypic Plasticity and Function of the Hard Palate in Growing Rabbits

“…Similar to many mammals, rabbit jaw-adductor activity patterns vary with dietary properties (Herring and Scapino, 1973;Luschei and Goodwin, 1974;Gorniak and Gans, 1980;Thexton et al, 1980;Weijs et al, 1987Weijs et al, , 1989Gans et al, 1990;Dessem and Druzinsky, 1992;Hylander et al, 1992Hylander et al, , 2000Hylander et al, , 2005, such that increased jaw-adductor recruitment results in elevated peak strains along the mandible and higher TMJ reaction forces (Weijs and de Jongh, 1977;Hylander, 1979aHylander, ,b,c, 1992Hylander et al, 1998;. Lastly, previous work on rabbit mandibular plasticity responses to postweaning alteration of dietary properties and masticatory stresses is consistent with similar experiments in a variety of other mammals (Beecher and Corruccini, 1981;Bouvier and Hylander, 1981, 1982, 1996aBeecher et al, 1983;Kiliardis et al, 1985;Bouvier, 1987Bouvier, , 1988Bouvier and Zimny, 1987;Block et al, 1988;Yamada and Kimmel, 1991;Ravosa et al, 2007bRavosa et al, , 2008a.…”

“…Such long-term studies regarding the impact of loading regimes upon the skull more closely approximate what is experienced by organisms in the wild and are thus fundamental for inferring behavior and performance in the fossil record and in the field (Ravosa et al, 2007b(Ravosa et al, , 2008a.…”

“…Thus, significant differences should exist in palate thickness, cortical bone thickness, and/or overall palatal bone area between dietary treatment groups. A consideration of the relative degree of palatal plasticity vis-à -vis dietary properties will be facilitated via comparisons with similar data for jaw joints from the same rabbit sample, where increased masticatory loading results in larger joint proportions, greater cortical bone thickness, and elevated hard-tissue biomineralization (Ravosa et al, 2007b(Ravosa et al, , 2008a.…”

“…Segmenting tools in Amira were used to calculate the area of palatal bone, which is equal to the number of voxels defined as bone multiplied by 2D voxel dimensions (0.103 mm 2 ). In addition to microCT, a series of standard linear external dimensions were taken on each rabbit cranium (Ravosa et al, 2007b(Ravosa et al, , 2008a. Some external and microCT data were used for comparing levels of Ground pellets require minimal oral preparation, which reduces the amount of cyclical loading during unilateral mastication (Under-use).…”

“…However, such comparisons may be mitigated by differences in dental formulae and eruption, presence of a dental diastema, daily feeding frequencies, and tongue activity. For this reason, it is important to employ caution when extrapolating from within-species analyses and, when doing so, to account for the extent and limitations of known general mammalian trends of functional adaptation (Lanyon and Rubin, 1985;Biewener, 1993;Ravosa et al, 2007bRavosa et al, , 2008a.…”