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

Jak2 and Jaw Muscles Are Required for Buccopharyngeal Membrane Perforation during Mouth Development

The mouth is a central feature of our face, without which we could not eat, breathe, or communicate. A critical and early event in mouth formation is the creation of a "hole" which connects the digestive system and the external environment. This hole, which has also been called the primary or embryonic mouth in vertebrates, is initially covered by a 1-2 cell layer thick structure called the buccopharyngeal membrane. When the buccopharyngeal membrane does not rupture, it impairs early mouth functions and may also lead to further craniofacial malformations. Using a chemical screen in an animal model (Xenopus laevis) and genetic data from humans, we determined that Janus kinase 2 (Jak2) has a role in buccopharyngeal membrane rupture. We have determined that decreased Jak2 function, using antisense morpholinos or a pharmacological antagonist, caused a persistent buccopharyngeal membrane as well as the loss of jaw muscles. Surprisingly, we observed that the jaw muscle compartments were connected to the oral epithelium that is continuous with the buccopharyngeal membrane. Severing such connections resulted in buccopharyngeal membrane buckling and persistence. We also noted puncta accumulation of F-actin, an indicator of tension, in the buccopharyngeal membrane during perforation. Taken together, the data has led us to a hypothesis that muscles are required to exert tension across the buccopharyngeal membrane, and such tension is necessary for its perforation.

FACEts of mechanical regulation in the morphogenesis of craniofacial structures

During embryonic development, organs undergo distinct and programmed morphological changes as they develop into their functional forms. While genetics and biochemical signals are well recognized regulators of morphogenesis, mechanical forces and the physical properties of tissues are now emerging as integral parts of this process as well. These physical factors drive coordinated cell movements and reorganizations, shape and size changes, proliferation and differentiation, as well as gene expression changes, and ultimately sculpt any developing structure by guiding correct cellular architectures and compositions. In this review we focus on several craniofacial structures, including the tooth, the mandible, the palate, and the cranium. We discuss the spatiotemporal regulation of different mechanical cues at both the cellular and tissue scales during craniofacial development and examine how tissue mechanics control various aspects of cell biology and signaling to shape a developing craniofacial organ.

Anatomical similarity between the Sost-knockout mouse and sclerosteosis in humans

Sclerosteosis, a rare autosomal recessive genetic disorder caused by a mutation of the Sost gene, manifests in the facial skeleton by gigantism, facial distortion, mandibular prognathism, cranial nerve palsy, and, in extreme cases, compression of the medulla oblongata. Mice lacking sclerostin reflect some symptoms of sclerosteosis, but this is the first report of the effect on the facial skeleton. We used geometric morphometrics (GMM) to analyze the deformations of the murine facial skeleton from the wild‐type to the Sost gene knockout. Landmark coordinates were obtained by surface reconstructions from micro‐computed tomography. Centroid size, principal component scores in shape space and form space, and asymmetry were computed by the standard GMM formulas, and dental and skeletal jaw lengths were examined as ratios. We show here that, compared to wild type controls, mice lacking Sost have larger centroid size (effect size, p‐value: 4.59, <.001), higher mean asymmetry (1.14, .065), dental and skeletal mandibular prognathism (1.36, .010 and 5.92, <.001), a smaller foramen magnum (−1.71, .015), and calvaria that are more highly curved (form space p = 4.09, .002; shape space p = 12.82, .002). These features of mice lacking sclerostin largely correspond to the changes of the facial skeleton observed in sclerosteosis. This alignment further supports claims that the Sost gene plays a fundamental role in bony facial development in rodents and humans alike.

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.

Disruption of Src Is Associated with Phenotypes Related to Williams-Beuren Syndrome and Altered Cellular Localization of TFII-I

Src is a nonreceptor protein tyrosine kinase that is expressed widely throughout the central nervous system and is involved in diverse biological functions. Mice homozygous for a spontaneous mutation in Src (Src (thl/thl) ) exhibited hypersociability and hyperactivity along with impairments in visuospatial, amygdala-dependent, and motor learning as well as an increased startle response to loud tones. The phenotype of Src (thl/thl) mice showed significant overlap with Williams-Beuren syndrome (WBS), a disorder caused by the deletion of several genes, including General Transcription Factor 2-I (GTF2I). Src phosphorylation regulates the movement of GTF2I protein (TFII-I) between the nucleus, where it is a transcriptional activator, and the cytoplasm, where it regulates trafficking of transient receptor potential cation channel, subfamily C, member 3 (TRPC3) subunits to the plasma membrane. Here, we demonstrate altered cellular localization of both TFII-I and TRPC3 in the Src mutants, suggesting that disruption of Src can phenocopy behavioral phenotypes observed in WBS through its regulation of TFII-I.

The complex ontogenetic trajectory of mandibular shape in a laboratory mouse

The mouse mandible is a popular model system that continues to be the focus of studies in evo-devo and other fields. Yet, little attention has been given to the role of postnatal growth in producing the adult form. Using cleared and stained specimens, we describe the timing of tooth and jaw development and changes in jaw size and shape from postnatal day 1 (p1) through weaning to adulthood. We found that tooth development is relatively advanced at birth, and that the functional adult dentition is in place by p15 (just before the start of weaning). Shape analysis showed that the trajectory of mandible shape changes direction at least twice between birth and adulthood, at p7 and p15. At each stage there are changes in shape to all tooth- and muscle-bearing regions and, at each change of direction, all of these regions change their pattern of growth. The timing of the changes in direction in Mus suggests there are signals that redirect growth patterns independently of changes in function and loading associated with weaning and jaw muscle growth. A better understanding of these signals and how they produce a functionally integrated mandible may help explain the mechanisms guiding evolutionary trends and patterns of plasticity and may also provide valuable clues to therapeutic manipulation of growth to alleviate the consequences of trauma or disease.

Epigenetic effects on the mouse mandible: common features and discrepancies in remodeling due to muscular dystrophy and response to food consistency

Background

In wild populations phenotypic differentiation of skeletal structures is influenced by many factors including epigenetic interactions and plastic response to environmental influences, possibly blurring the expression of genetic differences. In contrast, laboratory animals provide the opportunity to separate environmental from genetic effects. The mouse mandible is particularly prone to such plastic variations because bone remodeling occurs late in postnatal ontogeny, in interaction with muscular loading. In order to understand the impact of this process on mandible morphology, we investigated how change in the masticatory function affects the mandible shape, and its pattern of variation. Breeding laboratory mice on food of different consistencies mimicked a natural variation in feeding ecology, whereas mice affected by the murine analogue of the Duchenne muscular dystrophy provided a case of pathological modification of the mastication process.

Results

Food consistency as well as dystrophy caused significant shape changes in the mouse mandible. Further differences were observed between laboratory strains and between sexes within strains, muscular dystrophy causing the largest morphological change. The directions of the morphological changes due to food consistency and muscular dystrophy were discrepant, despite the fact that both are related to bone remodeling. In contrast, directions of greatest variance were comparable among most groups, and the direction of the change due to sexual dimorphism was parallel to the direction of main variance.

Conclusions

Bone remodeling is confirmed as an important factor driving mandible shape differences, evidenced by differences due to both the consistency of the food ingested and muscular dystrophy. However, the resulting shape change will depend on how the masticatory function is affected. Muscular dystrophy caused shape changes distributed all over the mandible, all muscles being affected although possibly to a different degree. In contrast, the chewing function was mostly affected when the mice were fed on hard vs. soft food, whereas grinding likely occurred normally; accordingly, shape change was more localized. The direction of greatest variance, however, was remarkably comparable among groups, although we found a residual variance discarding age, sex, and food differences. This suggests that whatever the context in which bone remodeling occurs, some parts of the mandible such as the angular process are more prone to remodeling during late postnatal growth.

The effects of muscular dystrophy on the craniofacial shape of Mus musculus

Skeletal anomalies are common in patients with muscular dystrophy, despite an absence of mutations to genes that specifically direct skeletogenesis. In order to understand these anomalies further, we examined two strains of muscular dystrophy (laminin- and merosin-deficient) relative to controls, to determine how the weakened muscle forces affected skull shape in a mouse model. Shape was characterized with geometric morphometric techniques, improving upon the limited analytical power of the standard linear measurements. Through these techniques, we document the specific types of cranial skeletal deformation produced by the two strains, each with individual shape abnormalities. The mice with merosin deficiency (with an earlier age of onset) developed skulls with more deformation, probably related to the earlier ontogenetic timing of disease onset. Future examinations of these mouse models may provide insight regarding the impact of muscular forces and the production and maintenance of craniofacial integration and modularity.

Life-long protein malnutrition in the rat (Rattus norvegicus) results in altered patterns of craniofacial growth and smaller individuals

Dietary protein is a limiting factor in mammalian growth, significantly affecting the non-linear trajectories of skeletal growth. Young females may be particularly vulnerable to protein malnutrition if the restriction is not lifted before they become reproductive. With such early malnutrition, limited amino acids would be partitioned between two physiological objectives, successful reproduction vs. continued growth. Thus, the consequences of protein malnutrition could affect more than one generation. However, few studies have quantified these cross-generational effects. Our objective was to test for differences in skeletal growth in a second generation of malnourished rats compared with rats malnourished only post-weaning, the first generation and with controls. In this longitudinal study we modelled the growth of 22 craniofacial measurements with the logistic Gompertz equation, and tested for differences in the equation's parameters among the diet groups. The female offspring of post-weaning malnourished dams did not catch up in size to the first generation or to controls, although certain aspects of their craniofacial skeleton were less affected than others. The second generation's growth trajectories resembled the longer and slower growth of the first malnourished generation. There was a complex interaction between developmental processes and early nutritional environment, which affected variation of adult size.

Myf5-/- :MyoD-/- amyogenic fetuses reveal the importance of early contraction and static loading by striated muscle in mouse skeletogenesis

The mechanical loading of striated muscle is thought to play an important role in shaping bones and joints. Here, we examine skeletogenesis in late embryogenesis (embryonic day 18.5) in Myf5-/-:MyoD-/- fetuses completely lacking striated muscle. The phenotype includes enlarged and fused cervical vertebrae and postural anomalies, some viscerocranial anomalies, long bone truncation and fusion, absent deltoid tuberosity of the humerus, scapular and clavicular hypoplasia, cleft palate, and cleft sternum. In contrast, neurocranial bone development was essentially normal. While the magnitude of individual effects varied throughout the skeletal system, the results are consistent with skeletal development depending on functional muscles. Novel abnormalities in the amyogenic fetuses relative to less severely paralyzed phenotypes extend our understanding of skeletogenic dependence on embryonic muscle contraction and static loading.

Sexual dimorphism in the craniofacial growth of the guinea pig (Cavia porcellus)

Variation between the sexes during ontogeny is frequently overlooked in discussions of the phylogenetic patterns of adult sexual dimorphism. Different growth trajectories can produce identical degrees and direction of adult dimorphism and the possibility exists that similarities in adults may be the result of differing growth patterns, suggesting independent evolutionary pathways among species to the seemingly identical adult morphology. We quantified the sexual dimorphism in craniofacial skeletal growth of Cavia porcellus, the guinea pig, using longitudinally collected radiographs. Guinea pigs have male-biased sexual dimorphism in size and in growth parameters, despite literature reports to the contrary. These results, analyzed with equivalent data for five species of rodents, and two outgroups representing similarly sized mammals, a rabbit and a marsupial, indicate that some aspects of sexual differences in growth follow phylogenetic lines, while others are a function of whether the species has male- or female-biased dimorphism.

Prenatal development of the human mandible

In an effort to better understand the interrelationship of the growth and development pattern of the mandible and condyle, a sequential growth pattern of human mandibles in 38 embryos and 111 fetuses were examined by serial histological sections and soft X-ray views. The basic growth pattern of the mandibular body and condyle appeared in week 7 of fertilization. Histologically, the embryonal mandible originated from primary intramembranous ossification in the fibrous mesenchymal tissue around the Meckel cartilage. From this initial ossification, the ramifying trabecular bones developed forward, backward and upward, to form the symphysis, mandibular body, and coronoid process, respectively. We named this initial ossification site of embryonal mandible as the mandibular primary growth center (MdPGC). During week 8 of fertilization, the trabecular bone of the mandibular body grew rapidly to form muscular attachments to the masseter, temporalis, and pterygoid muscles. The mandible was then rapidly separated from the Meckel cartilage and formed a condyle blastema at the posterior end of linear mandibular trabeculae. The condyle blastema, attached to the upper part of pterygoid muscle, grew backward and upward and concurrent endochondral ossification resulted in the formation of the condyle. From week 14 of fertilization, the growth of conical structure of condyle became apparent on histological and radiological examinations. The mandibular body showed a conspicuous radiating trabecular growth pattern centered at the MdPGC, located around the apical area of deciduous first molar. The condyle growth showed characteristic conical structure and abundant hematopoietic tissue in the marrow. The growth of the proximal end of condyle was also approximated to the MdPGC on radiograms. Taken together, we hypothesized that the MdPGC has an important morphogenetic affect for the development of the human mandible, providing a growth center for the trabecular bone of mandibular body and also indicating the initial growth of endochondral ossification of the condyle.

The impact of muscular dystrophy on limb bone growth and scaling in mice

Muscular loading affects bone growth and the factors determining size and shape. However, it is not known what epigenetic impact muscular dystrophy (dystrophia muscularis) has on limb bone growth or ontogenetic scaling. To assess the effects of two types of muscular dystrophy (genotypes dy/dy and dy2J/dy2J) on limb bone growth, we measured lengths and widths of the right humerus, femur and tibia, and lengths of the ulna and radius from dorsal/ventral radiographs of mice taken over a period of 270 days. Radiographs were taken approximately 3 times a week, and the sampling frequency was gradually reduced to once a month. We plotted measurements from each individual against time and fit a Gompertz equation to the growth of each bone. Parameters of the equation were compared using ANOVA across genotypes and between sexes. Slopes of length versus width were calculated for the limb bones of each individual using linear regression. Slope differences among genotypes and between sexes were tested using ANOVA. Control and dy2J values were significantly longer than those of dy mice in all bones, but there was considerable variation across genotypes for the various width measurements. Sexual dimorphism was found in several measurements, where males were always larger than females. There were few significant differences in limb scaling (lengths vs. widths) among genotypes and almost no scaling differences between sexes despite the size differences. Differences among widths suggest that muscular dystrophy affects different parts of limb bones in different ways. This may be the effect of the type and number of muscular attachments, as well as the usage of the limb. The sexually dimorphic measurements suggest that there are size differences in the skeleton between sexes, regardless of the genotype. Our ontogenetic allometry results indicate that size is affected by the muscular dystrophic condition and by sexual dimorphism, while shape remains largely unchanged.

Growth pattern of the maxillary sinus in the miniature pig (Sus scrofa)

The biological role of the paranasal sinuses is obscure, can be elucidated through a cross-sectional growth study of the maxillary sinus in miniature pigs. The maxillary sinus area was obtained from lateral cephalograms of left skull halves of 103 female miniature pigs of known ages, from newborn to 24 months. Out of several nonlinear models, the growth of the maxillary sinus was best described with the Gompertz model. The first derivative of the Gompertz curve revealed an increase in the growth rates of the maxillary sinus until 4 months, after which sinus growth slowed down. The eruption of the permanent molars did not seem to have a significant influence on this growth pattern. Furthermore, growth in maxillary sinus size in the miniature pig does not follow growth in skull size closely, which showed the highest growth rates in newborn animals. In addition, a correlation analysis revealed that the relationship between maxillary sinus area and different characteristics of the masticatory apparatus (including linear cranial dimensions, and the dry weight of the masseter and zygomatico-mandibularis muscles) were influenced greatly by skull size. These results suggest that the existence of pneumatic cavities within the mammalian skull is not satisfactorily explained solely by an architectural theory. Epigenetic factors are likely to influence the final shape of the maxillary sinus.

Protein malnutrition affects the growth trajectories of the craniofacial skeleton in rats

To investigate the effects of protein malnutrition on a normal growth trajectory, we radiographed Rattus norvegicus from 22 d (weaning) and continuing past adult size. We took measurements from longitudinal radiographs of rats fed a control diet and littermates fed an isocaloric low protein experimental diet. A Gompertz model was fit to each individual rat for body weight and 22 measurements of the craniofacial skeleton, producing parameters that described the rate and timing of growth. We tested for differences in these parameters due to diet, sex and litter with a mixed-model three-way ANOVA. Allometric analysis examined the scaling relationships between and within various regions of the skull. For most measurements, final sizes predicted by the model were not significantly different between rats fed the two diets, although the differences in final measurements showed small, but significant differences in growth between rats in the two diet groups. The instantaneous initial rate of growth, maximum rate of growth and deceleration of growth were significantly higher in the control rats for every measurement. Rats fed the low protein diet grew for a significantly longer period of time. The shape of the neurocranium was relatively conserved between diet groups; however, rats fed the low protein diet had shorter and relatively wider skulls than the controls. These results suggest that functional demands of the viscerocranium were greater after birth, and that growth in this area was faster. The viscerocranium reached functional adult proportions earlier and was therefore more susceptible to epigenetic perturbations such as dietary protein level. Protein malnutrition did not affect many aspects of adult size, but strongly altered the growth trajectory to achieve that size.

Sexual dimorphism and ontogenetic allometry of soft tissues in Rattus norvegicus

Most studies of sexual dimorphism in mammals focus on overall body size. However, relatively little is known about the differences in growth trajectories that produce dimorphism in organ and muscle size. We weighed six organs and four muscles in Rattus norvegicus to determine what heterochronic and allometric scaling differences exist between the sexes. This cross-sectional growth study included 113 males and 109 females with ages ranging from birth to 200 days of age. All muscle and organ weights were ultimately greater in males than in females, because males grew for a longer period of time, had a greater maximum rate of growth, and spent more time near the maximum rate. No ontogenetic scaling differences existed between the sexes in organ weight except for lungs and gonads. During growth, organ weights were negatively allometric to body weight. No scaling differences relative to body weight existed between the sexes for muscles; however, there was variation in the allometric relations among muscles relative to body weight. Sexual dimorphism in muscles and organs appears to be a size difference resulting from differences in the duration and rates of growth.