Prolonged cyclical loading induces Haversian remodeling in mandibles of growing rabbits.

Bone adaptation to mechanical loading happens predominantly via modeling and remodeling, but the latter is poorly understood. Haversian remodeling (cortical bone replacement resulting in secondary osteons) is thought to occur in regions of low strain as part of bone maintenance or high strain in response to microdamage. However, analyses of remodeling in primates have revealed an unappreciated association with the number of daily load cycles. We tested this relationship by raising 30 male domestic rabbits (Oryctolagus cuniculus) on disparate diets from weaning to adulthood (48 weeks), facilitating a naturalistic perspective on mandibular bone adaptation. A control group consumed only rabbit pellets and an 'overuse' group ate hay in addition to pellets. To process hay, which is tougher and stiffer, rabbits increase chewing investment and duration without increasing bite force (i.e. corpus mean peak strain is similar for the two foods). Corpus remodeling in overuse rabbits was ∼1.5 times that of controls, measured as osteon population density and percentage Haversian bone. In the same subjects, there was a significant increase in overuse corpus bone formation (ratio of cortical area to cranial length), consistent with previous reports on the same dietary manipulation and bone formation in rabbits. This is the first evidence that both modeling and remodeling are simultaneously driven by the number of load cycles, independent of strain magnitude. This novel finding provides unique data on the feeding apparatus, challenges traditional thought on Haversian remodeling, and highlights the need for experimental studies of skeletal adaptation that examine mechanical factors beyond strain magnitude.

Absence of secondary osteons in femora of aged rats: Implications of lifespan on Haversian remodeling in mammals.

Bone is a dynamic tissue capable of adapting to its loading environment, allowing the skeleton to remain structurally sound throughout life. One way adaptation occurs in mammals is via Haversian remodeling: the site-specific, coupled resorption and formation of cortical bone that results in secondary osteons. Remodeling occurs at a baseline rate in most mammals, but it also occurs in relation to strain by repairing deleterious microdamage. Yet, not all animals with bony skeletons remodel. Among mammals, there is inconsistent or absent evidence for Haversian remodeling among monotremes, insectivores, chiropterans, cingulates, and rodents. Three possible explanations for this disparity are discussed: the capacity for Haversian remodeling, body size as a constraint, and age and lifespan as constraints. It is generally accepted, although not thoroughly documented, that rats (a common model used in bone research) do not typically exhibit Haversian remodeling. The present aim is to more specifically test the hypothesis that rats of advanced age do remodel intracortically because of the longer lifespan over which baseline remodeling could occur. Most published histological descriptions of rat bone only include young (3-6 months) rats. Excluding aged rats possibly overlooks a transition from modeling (i.e., bone growth) to Haversian remodeling as the primary mode of bone adaptation. Here, midshaft and distal femora (typical sites for remodeling in other mammals) of 24-month-old rats were examined for presence of secondary osteons. None were found, suggesting that Haversian remodeling does not occur in rats under normal physiological conditions at any age. A likely explanation is that modeling of cortical bone continues throughout most of the short rat lifespan, negating the stimulus for Haversian remodeling. Thorough sampling of key rodent taxa of varying body sizes and lifespans is key to elucidating the reasons why (i.e., body size, age/lifespan, phylogenetic factors) Haversian remodeling might not occur in all mammals.

Bone remodeling and cyclical loading in maxillae of New Zealand white rabbits (Oryctolagus cuniculus).

Mammalian feeding behaviors are altered when mechanically challenging (e.g., tough, stiff) foods require large bite forces or prolonged mastication. Bony responses to high bite forces are well-documented for the mammalian skull, but osteogenesis due to cyclical loading, caused by repetitive chewing, is more poorly understood. Previous studies demonstrate that cyclical loading results in greater bone formation in the rabbit masticatory apparatus and in substantial Haversian remodeling in primate postcrania. Here we assess the relationship between cyclical loading and remodeling in the rabbit maxilla. Twenty male New Zealand white rabbits (Oryctolagus cuniculus) were raised on either an overuse or control diet (10 per group) for 48 weeks, beginning at weaning onset. The control group was raised on a diet of rabbit pellets (E = 29 MPa, R = 1031 J/m2 ), whereas the overuse group ate rabbit pellets and hay, which has high stiffness (E = 3336 MPa) and toughness (R = 2760 J/m2 ) properties. Hay requires greater chewing investment (475 chews/g) and longer chewing durations (568 s/g) than pellets (161 chews/g and 173 s/g), therefore causing cyclical loading of the jaws. Remodeling was measured as osteon population density (OPD), percent Haversian bone (%HAV), and osteon cross-sectional area (On.Ar). The only significant difference found was greater On.Ar in the alveolar region of the maxilla (p < 0.001) in the overuse group. The hypothesis that cyclical loading engenders Haversian remodeling in the developing maxilla is not supported. The continuation of modeling throughout the experimental duration may negate the need for remodeling as newly laid bone tends to be more compliant and resistant to crack propagation.

Precocious chondrocyte differentiation disrupts skeletal growth in Kabuki syndrome mice.

Kabuki syndrome 1 (KS1) is a Mendelian disorder of the epigenetic machinery caused by mutations in the gene encoding KMT2D, which methylates lysine 4 on histone H3 (H3K4). KS1 is characterized by intellectual disability, postnatal growth retardation, and distinct craniofacial dysmorphisms. A mouse model (Kmt2d+/ßGeo) exhibits features of the human disorder and has provided insight into other phenotypes; however, the mechanistic basis of skeletal abnormalities and growth retardation remains elusive. Using high-resolution micro-CT, we show that Kmt2d+/ßGeo mice have shortened long bones and ventral bowing of skulls. In vivo expansion of growth plates within skulls and long bones suggests disrupted endochondral ossification as a common disease mechanism. Stable chondrocyte cell lines harboring inactivating mutations in Kmt2d exhibit precocious differentiation, further supporting this mechanism. A known inducer of chondrogenesis, SOX9, and its targets show markedly increased expression in Kmt2d-/- chondrocytes. By transcriptome profiling, we identify Shox2 as a putative KMT2D target. We propose that decreased KMT2D-mediated H3K4me3 at Shox2 releases Sox9 inhibition and thereby leads to enhanced chondrogenesis, providing a potentially novel and plausible explanation for precocious chondrocyte differentiation. Our findings provide insight into the pathogenesis of growth retardation in KS1 and suggest therapeutic approaches for this and related disorders.

Haversian remodeling corresponds to load frequency but not strain magnitude in the macaque (Macaca fascicularis) skeleton.

One way bone adapts to its mechanical environment is by Haversian remodeling, a repair process in which existing bone is resorbed and replaced by new bone. Haversian remodeling forms interconnected, cylindrical structures called secondary osteons. The amount of remodeling that occurs is related to the nature of mechanical loading and accrual of microdamage, but it is uncertain whether habitual loads of high magnitude versus high frequency result in more remodeling. The answer to this question is important if remodeling is to be a tool for inferring loading environments, and thus behavior, in past populations. Here, secondary osteon population density (OPD), osteon cross-sectional area (On.Ar), and percent Haversian bone (%HAV) were compared among mid-diaphysis femora, tibia, fibulae, and mid-level ribs of five adult crab-eating macaques (Macaca fascicularis). Ribs experience relatively low strains but have a high daily loading frequency (~33 times per minute). Limb bones are loaded for fewer cycles per day, but the femur and tibia have high load magnitudes due to gravitational forces. Strain magnitudes in the fibula are a fraction of those in the femur and tibia. Analyses of variance demonstrated significant differences in OPD (P = 0.010) and On.Ar (P < 0.001) among the bones. Pairwise t-tests revealed greater OPD but lower On.Ar in the rib than all other bones. The high rib OPD suggests that Haversian remodeling is more responsive to load frequency than strain magnitude. The fact that osteons are smaller in ribs than any other bone may be an effect of remodeling in comparatively narrow cortices.

The Influence of Leaping Frequency on Secondary Bone in Cercopithecid Primates.

Bone remodeling is at least partially mediated by the mechanical environment created by an animal's behavior. Here, we test the hypothesis that bone remodeling is primarily induced by high magnitude loads, likely encountered during leaping/bounding behaviors. Osteon population density (OPD), osteon cross-sectional area (On.Ar), and relative osteonal area (%HAV) were measured from femoral and humeral midshaft thin sections of four cercopithecids (N = 5 per species) from Taï Forest, Côte d'Ivoire: Colobus polykomos, Piliocolobus badius, Cercopithecus diana, and Cercocebus atys. All species are generalized quadrupeds but vary in leaping frequency and overall activity budget. Differences between taxa with high (C. polykomos and P. badius) and low leaping frequency (C. diana and C. atys) were assessed via a phylogenetically informed generalized linear mixed model using Markov Chain Monte Carlo methods. Femoral OPD and %HAV are greater in the high frequency leapers than in low frequency leapers, suggesting that frequent high magnitude loads engender remodeling, however, there is no similar pattern in the humerus, which presumably also experiences high magnitude loads during leaping. Additionally, OPD and %HAV are greater in the humerus than the femur, despite load magnitude being presumably higher in the femur. These results provide conflicting support for hypotheses about load magnitude and load frequency as they relate to bone remodeling activity. Future work is proposed to parse out the respective effects of load magnitude and frequency on bone remodeling. Anat Rec, 302:1116-1126, 2019. © 2018 Wiley Periodicals, Inc.

Functional morphology of the douc langur (Pygathrix spp.) scapula.

Most colobine monkeys primarily move through their arboreal environment quadrupedally. Douc langurs (Pygathrix spp.), however, are regularly observed to use suspensory behaviors at the Endangered Primate Rescue Center (EPRC) in Northern Vietnam. Previous work has linked variation in scapular morphology to different modes of primate arboreal locomotion. Here we investigate whether the shape of the Pygathrix scapula resembles obligate brachiators (gibbons) or obligate arboreal quadrupeds (other cercopithecoids). Using a MicroScribe G2X 3D digitizer, the positions of 17 landmarks were recorded on 15 different species of nonhuman primates (n = 100) from three categories of locomotor behavior: brachiator, arboreal quadruped, and unknown (Pygathrix). All analyses were conducted in the R package geomorph. A Procrustes analysis uniformly scaled the shape data and placed specimens into the same morphospace. A Principal Component Analysis was used to examine scapular shape and a Procrustes ANOVA was conducted to test for shape difference in the scapulae. A pairwise analysis was used to compare the means of the locomotor categories and identify any statistically significant differences. A phylogenetically controlled Procrustes ANOVA was also conducted using a phylogeny from 10kTrees. Results show Pygathrix scapular morphology is significantly different from both arboreal colobine quadrupeds (p < 0.01) and hylobatid brachiators (p < 0.01). It does, however, share some features with each including a long vertebral border, like other cercopithecoids, and a more laterally projecting acromion process, like the hylobatids. The principal difference segregating Pygathrix from both the arboreal quadrupeds and the brachiators is the more medially placed superior angle. These nuanced morphological characteristics associated with suspensory behaviors may be useful for inferring suspensory locomotion in the primate fossil record.

Bone remodeling is reduced in high stress regions of the cercopithecoid mandible.

OBJECTIVES: Independent lines of evidence suggest that osteonal bone remodeling is a function of both mechanical (i.e., changes in stress) and non-mechanical (i.e., metabolic needs related to calcium liberation) factors. The degree to which secondary bone reflects mechanical factors, however, is incompletely understood despite the common assumption that the stress environment mediates remodeling activity. Here, we investigate whether there are remodeling differences between regions of primate mandibular bone known to have distinct stress environments. METHODS: Osteon density, osteon fragment density, and relative osteonal area are measured as indicators of remodeling activity from postcanine and symphyseal thin sections of four sympatric monkey species (N = 20 total) from Taï Forest, Côte d'Ivoire: Piliocolobus badius, Colobus polykomos, Cercocebus atys, Cercopithecus diana. Subfamily and regional effects were assessed by two-way ANOVA. RESULTS: Symphyseal bone has lower osteonal density, fragment density and relative osteonal area than postcanine bone in all species, indicating relatively low remodeling activity in symphyseal bone, despite the likelihood of relatively high stresses in its lingual cortex. Subfamily differences in postcanine remodeling are significant in that colobines exhibit greater remodeling than cercopithecines. DISCUSSION: The data suggest that high strains are not requisite for bone remodeling. How the lingual symphysis negotiates a high strain environment without the reparative benefit of remodeling is yet to be elucidated, but the data prompt reevaluation of the relationship of targeted remodeling to stress histories. Differences in osteonal bone density between taxa might reflect feeding or dietary influences on remodeling activity, but sample composition prevents parsing behavioral from other taxon-specific effects.