Bone-muscle interactions in the muscular dystrophic mouse

Congenital muscle dystrophy and diet consistency affect mouse skull shape differently

The bones of the mammalian skull respond plastically to changes in masticatory function. However, the extent to which muscle function affects the growth and development of the skull, whose regions have different maturity patterns, remains unclear. Using muscle dissection and 3D landmark-based geometric morphometrics we investigated the effect of changes in muscle function established either before or after weaning, on skull shape and muscle mass in adult mice. We compared temporalis and masseter mass and skull shape in mice with a congenital muscle dystrophy (mdx) and wild type (wt) mice fed on either a hard or a soft diet. We found that dystrophy and diet have distinct effects on the morphology of the skull and the masticatory muscles. Mdx mice show a flattened neurocranium with a more dorsally displaced foramen magnum and an anteriorly placed mandibular condyle compared with wt mice. Compared with hard diet mice, soft diet mice had lower masseter mass and a face with more gracile features as well as labially inclined incisors, suggesting reduced bite strength. Thus, while the early-maturing neurocranium and the posterior portion of the mandible are affected by the congenital dystrophy, the late-maturing face including the anterior part of the mandible responds to dietary differences irrespective of the mdx mutation. Our study confirms a hierarchical, tripartite organisation of the skull (comprising neurocranium, face and mandible) with a modular division based on development and function. Moreover, we provide further experimental evidence that masticatory loading is one of the main environmental stimuli that generate craniofacial variation.

Temporalis muscle hypertrophy and reduced skull eccentricity in Duchenne muscular dystrophy

Muscle hypertrophy and muscle weakness are well known in Duchenne muscular dystrophy. Decreased muscle force can have secondary effects on skeletal growth and development such as facial and dental morphology changes. In this study, we quantified temporal muscle thickness, circumference, and eccentricity of the skull and the head on T1-weighted magnetic resonance imaging (MRI) scans of the head of 15 Duchenne muscular dystrophy patients and 15 controls. Average temporal muscle thickness was significantly increased in patients (12.9 ± 5.2 mm) compared to controls (6.8 ± 1.4 mm) (P < .0001), whereas the shape of the skull was significantly rounder compared to controls. Temporal muscle thickness and skull eccentricity were significantly negatively correlated in patients, and positively in controls. Hypertrophy of the temporal muscles and changes in skull eccentricity appear to occur early in the course of Duchenne muscular dystrophy. Further studies in younger patients are needed to confirm a causal relationship.

Developmental changes in craniofacial morphology in subjects with Duchenne muscular dystrophy

Lateral cephalometric radiographs of 35 Japanese male patients suffering from Duchenne muscular dystrophy (DMD) were taken longitudinally from 10 to 20 years of age. Eighteen landmarks were placed and 15 angles and four linear distances calculated. Profile diagrams (profilograms) were produced to analyse changes in craniofacial morphological growth in the DMD subjects. The measurements were then compared with Japanese standards. In young patients with DMD, compared with the controls, the following were observed: a large gonial angle; clockwise rotation of the mandible; short sagittal length of the cranial base and protrusion of the upper incisors. In adult patients, the maxillary alveolus and the upper incisors were protruded, compared with the controls. Overbite in DMD subjects also showed a tendency to decrease. In the controls, mandibular growth direction tended to be straight down and forward, while in patients with DMD, the growth direction was down until approximately 16 years of age and, after that, a forward vector of growth was apparent. As a result, the tendency towards a clockwise rotation of the mandible in the adults was less than in the young patients. These findings showed that DMD significantly affects craniofacial morphology.

Craniofacial growth in the ferret (Mustela putorius furo)—a cephalometric study

OBJECTIVE

When suggesting the ferret as a valid laboratory model in craniofacial research, it is essential to know about its normal craniofacial growth.

DESIGN

Sixteen ferret kits (eight male and eight female) were selected for the present investigation. Serial lateral and dorsoventral cephalograms were taken on each animal at a mean age of 25, 35, 55, 80 and 300 days. The cephalograms were then digitised and the coordinates of 33 landmarks were derived on each set of cephalograms. Thirty-four variables were then calculated on each set of cephalograms by computer image programs with the coordinate data. Results were analysed statistically, and the craniofacial growth pattern and related sexual dimorphism were described in three perspectives: lateral and dorsoventral viscero- and neurocranium, and lateral mandible.

FINDINGS

In both sexes, the viscero- and neurocranium follow an orderly pattern of expansive growth in three dimensions. The growth of the mandible is mainly characterised by an anteroposterior elongation of the mandibular body, an enlargement of the coronoid process, and an increase in height of the alveolar process. The growth rate varies with site. Craniofacial growth in ferrets starts to slow down and finally ceases earlier in female than in male animals.

Morphometric analysis of the mandible in growing rats with different masticatory functional demands: adaptation to an upper posterior bite block

Functional appliances displace the mandible forward and/or downward, causing a stretching of the orofacial soft tissues, muscles included. The resulting forces are directly or indirectly transmitted to the underlying dento-skeletal tissues. The hypothesis underlying the present investigation was that the insertion of a bite-opening appliance influences the lateral morphology of the rat mandible during growth, and that, moreover, this influence depends on the masticatory functional demands. One-hundred and four 4-wk-old male albino rats were divided into two groups, fed a hard and soft diet, respectively. After 2 wk, half of the animals in each experimental group were fitted with upper posterior blocks, and 4 wk later they were killed. Their left hemi-mandibles were transilluminated, photographed under magnification, and digitized on screen. A total of 170 points were used to draw the lateral outline of the mandible. In addition to the inhibitory effect on the height of the dento-alveolar process, the upper bite block resulted in significant changes in the condyle inclination, the length of the coronoid process, and the occlusal plane inclination. Masticatory functional demands influenced this adaptation in an additive way. The results raise the question of whether orthodontic treatment with posterior bite blocks might have different effects on the mandible, depending on the characteristics of the orofacial soft tissues.

Changes in cortical bone mineralization in the developing mandible: a three-dimensional quantitative computed tomography study

Quantitative computed tomography (QCT) was completed in 34 subjects between the ages of 9 and 33 years with symmetrical mandibles in order to investigate the three-dimensional cortical bone mineral density (BMD) distribution in the mandible. The number and distribution of the pixels were determined at three levels: (1) representing the entire mandibular bone; (2) the cortical bone at 60% above the baseline defined as the segmentation level (around 1050 mg/cm3) and representative of only cortical bone; and (3) the highest mineralized cortical bone (>1250 mg/cm3). The geometrical distribution of the highest mineralized areas was evaluated by three-dimensional reconstruction of the images. The total number of pixels for the entire mandible increased significantly at each time point represented at four increasing ages groups (9-11 years of age, 12-14 years of age, 15-17 years of age, and >18 years of age). The male and female subjects had a similar total number of pixels for the entire mandible before the age of 11, but the male subjects showed a significantly larger total number of mandibular pixels after that age. Comparison of the number of pixels for pure cortical bone (60% segmentation level) and the highest mineralized cortical bone indicated a significant increase with maturation with the greatest change occurring between the 13-year and 16-year age groups. However, the ratio of cortical bone/total bone increased at a more rapid rate in the male subjects and reached a plateau by the 16-year age group, showing distinct differences in mineralization of the mandible between the sexes.

The effects of muscular dystrophy on craniofacial growth in mice: a study of heterochrony and ontogenetic allometry

Mechanical loading of muscles on bones at their sites of attachment can regulate skeletal morphology. The present study examined the effects of muscle degeneration on craniofacial growth, using two strains of muscular dystrophic mice, Mus musculus, differing in pathological severity. We collected radiographic and weight data longitudinally and digitized radiographs to obtain distances between anatomical landmarks in different functional regions of the skull. We then quantified heterochronic and allometric differences among genotypes and between sexes. Because growth is nonlinear with respect to time, we first used the Gompertz model to obtain heterochronic growth parameters, which were then tested with ANOVA. Ontogenetic allometric analyses examined the scaling relationships between various measurements with linear regressions. For most measurements the severely dystrophic mice are significantly smaller in final size than both the control and the mildly dystrophic mice, which are statistically indistinguishable. Measures of total growth and the neurocranium exhibit more differences among groups in heterochronic parameters of early ontogeny because growth in these regions is controlled primarily by brain expansion that ceases early in development. In contrast, the face and mandible exhibit more differences in later growth parameters possibly because of the increased influence of muscles on these regions as growth progresses. The severely dystrophic mice have flatter, more elongate skulls and mandibles than those of the other two genotypes, concurrent with an absence of muscular forces to stimulate growth in a superior-inferior direction.

The epigenetic impact of weaning on craniofacial morphology during growth

Miniature pigs (Sus scrofa) were used as a model to investigate whether the time of weaning (a nongenetic factor) affects skeletal growth rates for both pre- and postweaning time periods. Control litters were weaned at the normal time of 32 days. Two litters were weaned early (at 20 days) and two late (at 46 days). We digitized cranial landmarks from radiographs taken three times a week for a total of 70 days. We used analysis of covariance to test for differences in growth rates between pre- and post-weaning periods, as well as differences in growth rates among treatments. In both the late weaned pigs and the controls, facial length, facial width, basicranial length, and basicranial width growth rates slowed significantly at the time of weaning. However, in the early weaned pigs, there were no significant changes in growth rates for any of the facial or basicranial measurements at weaning. Furthermore, the postweaning rates of growth were different among treatments. One possible implication is that early growth rates could be under tight genetic control while later growth rates can be epigenetically regulated through nutritional changes.

Implications of temporomandibular disorders for facial growth and orthodontic treatment

The purpose of this article is to review the literature concerning the possible associations between temporomandibular disorders (TMD), orthodontic therapy, mandibular growth, and facial form. Consideration of the association between TMD and orthodontic treatment leads to one conclusion: there is no evidence that orthodontic treatment generally increases or decreases the chances of developing TMD later in life. Still, our understanding of TMD is not final, and the expanding diagnostic knowledge continues to call for new longitudinal studies focusing on the developmental basis of temporomandibular disorders. Little is known about the potential effect of TMD on facial development. However, information relating to the normal development, growth, and adaptation of the temporomandibular joint all tend to emphasize the significant ontogenetic plasticity of the growth-related secondary cartilage associated with the TMJ, within the bounds of normal function and histophysiology. With the exception of such diseases as juvenile rheumatoid arthritis and osteoarthritis, little is known about the influence of TM pathology or myofascial disorders on facial growth. Also, little is known about the possible influence of disc interferences on facial growth. However, the condyle is known to play a prominent role in normal mandibular growth and, consequently, facial development. Thus, categories of TMD that involve dysplasia of the condylar cartilage could be associated with aberrant facial growth and form.

Development of craniofacial musculature in Monodelphis domestica (Marsupialia, Didelphidae)

Development of craniofacial muscles of Monodelphis domestica (Marsupialia, Didelphidae) is described. In a period of 4-6 days all craniofacial muscles in M. domestica progress from myoblast condensation, to striated myofibers that are aligned in the direction of adult muscles and possess multiple, lateral nuclei. This process begins 1 to 2 days before birth and continues during the first few days after birth. Compared to other aspects of cranial development, muscle development in M. domestica is rapid. This rapid and more or less simultaneous emergence of craniofacial muscles differs from the previously described pattern of development of the cranial skeleton in marsupials, which displays a mosaic of acceleration and deceleration of regions and individual elements. Unlike the skeletal system, craniofacial muscles show no evidence of regional specialization during development. M. domestica resembles eutherian mammals in the relatively rapid and more or less simultaneous differentiation of all craniofacial muscles. It differs from eutherian taxa in that most stages of myogenesis occur postnatally, following the onset of function. The timing of the development of muscular and skeletal structures is compared and it is concluded that the relatively early development of muscle is not reflected by any particular acceleration of the differentiation or growth of skeletal structures. Finally, the difficulties in accounting for complex internal arrangements of muscles such as the tongue, given current models of myogenesis are summarized.

Histomorphometrical analysis of the influence of soft diet on masticatory muscle development in the muscular dystrophic mouse

The known difference in the severity of dystrophy between the masseter and the digastric muscle of the mouse (dy/dy C57BL/J6) may be attributed to the differences in muscle work load. This possibility was tested by subjecting 3-week-old mice (normal and dystrophic) to a soft diet for 4 weeks. Microscopic examination of haematoxylin-eosin stained sections of these muscles showed that the fibre size dispersion (a measure of disease severity) decreased slightly but significantly in the masseters of mice on a soft diet. It was thus possible to improve the condition of dystrophic masticatory muscles by changing their function. Body weight curves measured during the experimental period suggest that the dystrophic mice may have been under weight because of malnutrition due to lack of sufficient masticatory power.

Condylar cartilage in the muscular dystrophic mouse

The adaptability of condylar cartilage has been demonstrated previously after experimentally created functional alteration. This study was undertaken to examine the morphology of condylar cartilage in animals affected with a progressive muscular disease. Muscular dystrophic male mice (genotype: dy/dy, dy/+ x dy/+, Jackson Laboratory, Maine) and corresponding unaffected control mice were decapitated at ages 3, 6, 9, and 12 weeks, and their heads processed for histology. The cellular morphology of the condylar cartilage in the youngest age group was similar in dystrophic and control mice: the cartilage was a hypertrophic type. At ages 6 and 9 weeks, the maturational progression toward the nonhypertrophic form was observed in the dystrophic and control groups, the latter having flatter condylar heads. Differences were still evident at age 12 weeks. These observations were supported by measurements of the ratios of the mean cartilage thickness to condylar width (c/w), mean condylar height to width (h/w), and mean cartilage thickness to condylar height (c/h). This study supports the hypothesis that the adaptive nature of condylar cartilage may be regulated by the force levels to which the condyle is subjected.