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.     

Morphometric analysis of osteonal architecture in bones from healthy young human male subjects using scanning electron microscopy

The shape and structure of bones is a topic that has been studied for a long time by morphologists and biologists with the goal of explaining the laws governing their development, aging and pathology. The osteonal architecture of tibial and femoral mid‐diaphyses was examined morphometrically with scanning electron microscopy in four healthy young male subjects. In transverse sections of the mid‐diaphysis, the total area of the anterior, posterior, lateral and medial cortex sectors was measured and analysed for osteonal parameters including osteon number and density, osteon total and bone area and vascular space area. Osteons were grouped into four classes including cutting heads (A), transversely cut osteons (B), longitudinally cut osteons (C) and sealed osteons (D). The morphometric parameters were compared between the inner (endosteal) and outer (periosteal) half of the cortex. Of 5927 examined osteons, 24.4% cutting heads, 71.1% transversely cut osteons, 2.3% longitudinally cut osteons and 2.2% sealed osteons were found. The interosteonic bone (measured as the area in a lamellar system that has lost contact with its own central canal) corresponded to 51.2% of the endosteal and 52.4% of the periosteal half‐cortex. The mean number of class A cutting heads and class B osteons was significantly higher in the periosteal than in the endosteal half‐cortex (P < 0.001 and P < 0.05, respectively), whereas there was no significant difference in density. The mean osteon total area, osteon bone area and vascular space area of both classes A and B were significantly higher (P < 0.001 for all three parameters) in the endosteal than in the periosteal half‐cortex. The significant differences between the two layers of the cortex suggest that the osteoclast activity is distributed throughout the whole cortical thickness, with more numerous excavations in the external layer, but larger resorption lacunae closer to the marrow canal. A randomly selected population of 109 intact class B osteons was examined at higher magnification (350×) to count osteocyte lacuna and to analyse their relationship with osteon size parameters. The distribution frequency of the mean number of osteocyte lacunae increased with the increment in the sub‐classes of osteon bone area, whereas the density did not show significant differences. The number of osteocyte lacunae had a direct correlation with the osteon bone area and the mean osteon wall thickness, as well as the mean number of lamellae. The osteocyte lacunae density showed an inverse relationship. These data suggest a biological regulation of osteoblast activity with a limit to the volume of matrix produced by each cell and proportionality with the number of available cells in the space of the cutting cone (total osteon area). The collected data can be useful as a set of control parameters in healthy human bone for studies on bone aging and metabolic bone diseases.     

Analysis of the Effect of Osteon Diameter on the Potential Relationship of Osteocyte Lacuna Density and Osteon Wall Thickness

An important hypothesis is that the degree of infilling of secondary osteons (Haversian systems) is controlled by the inhibitory effect of osteocytes on osteoblasts, which might be mediated by sclerostin (a glycoprotein produced by osteocytes). Consequently, this inhibition could be proportional to cell number: relatively greater repression is exerted by progressively greater osteocyte density (increased osteocytes correlate with thinner osteon walls). This hypothesis has been examined, but only weakly supported, in sheep ulnae. We looked for this inverse relationship between osteon wall thickness (On.W.Th) and osteocyte lacuna density (Ot.Lc.N/B.Ar) in small and large osteons in human ribs, calcanei of sheep, deer, elk, and horses, and radii and third metacarpals of horses. Analyses involved: (1) all osteons, (2) smaller osteons, either ≤150 μm diameter or less than or equal to the mean diameter, and (3) larger osteons (>mean diameter). Significant, but weak, correlations between Ot.Lc.N/B.Ar and On.W.Th/On.Dm (On.Dm = osteon diameter) were found when considering all osteons in limb bones (r values ?0.16 to ?0.40, P < 0.01; resembling previous results in sheep ulnae: r = ?0.39, P < 0.0001). In larger osteons, these relationships were either not significant (five/seven bone types) or very weak (two/seven bone types). In ribs, a negative relationship was only found in smaller osteons (r = ?0.228, P < 0.01); this inverse relationship in smaller osteons did not occur in elk calcanei. These results do not provide clear or consistent support for the hypothesized inverse relationship. However, correlation analyses may fail to detect osteocyte‐based repression of infilling if the signal is spatially nonuniform (e.g., increased near the central canal). Anat Rec,, 2011. © 2011 Wiley‐Liss, Inc.     

Does 3D orientation account for variation in osteon morphology assessed by 2D histology?

The primary microstructural unit of cortical bone, the secondary osteon or Haversian system, is widely assumed to have a cylindrical shape. It is generally accepted that osteons are roughly circular in cross-section and deviations from circularity have been attributed to deviations from longitudinal orientation. To our knowledge this idealized geometric relationship, which assumes osteons are perfect cylinders, has not been rigorously explored. As such, we sought to explore two research questions: (i) Does the orientation of osteons in 3D explain variation in shapes visualized in 2D? (ii) Can differences in osteon 3D orientation explain previously reported age-related differences observed in their 2D cross-sectional shape (e.g. more circular shape and decreased area with age)? To address these questions we utilized a combination of 2D histology to identify osteon shape and superimposed micro-computed tomography data to assess osteon orientation in 3D based upon the osteonal canal. Shape was assessed by the inverse of Aspect Ratio (On.AspR⁻¹, based on a fitted ellipse) – which ranged from 0 (infinitely elongated shape) to 1 (perfectly circular). A sample (n = 27) of human female anterior femoral cortical bone samples from across the human lifespan (20–87 years) were included in the analysis, which involved 1418 osteons. The overall mean measure of On.AspR⁻¹ was 0.703 (1.42 Aspect Ratio). Mean osteon orientation was 79.1° (90° being longitudinal). While we anticipated a positive relation between orientation and On.AspR⁻¹, we found the opposite – a weak negative correlation (with more oblique 3D osteon alignment, the 2D shape became more circular as reflected by increased On.AspR⁻¹). When analysis of covariance was performed with age and orientation as covariates, the negative relation with orientation was replaced by a significant relation with age alone. This relation with age accounted for 41% of the variation of On.AspR⁻¹. The results revealed that osteons, on average, are not circular in cross-section and that 3D orientation cannot account for deviation from circular shape. Osteons thus are strictly speaking not cylinders, as they tend to have elliptical cross-sections. We observed that osteons did become less elliptical in cross-section with age independent of orientation – suggesting this is a real change in morphology.     

Histomorphometric assessment of Haversian canal and osteocyte lacunae in different-sized osteons in human rib

There is no detailed information available concerning the variations in bone, the Haversian canal, and osteocyte populations in different-sized osteons. In this study a total of 398 secondary osteons were measured in archived rib sections from nine white men (20-25 years old). The sections were stained with basic fuchsin. The parameters included the osteon area (On.Ar), Haversian canal area (HC.Ar) and perimeter (HC.Pm), bone area (B.Ar), and osteocyte lacunar number (Lc.N). From these primary measurements the following indices were deduced: 1) lacunar number per bone area (Lc.N/B.Ar) and per osteon (Lc.N/On); 2) the ratio between Haversian canal perimeter and bone area (HC.Pm/B.Ar); and 3) the fraction of Haversian canal area (HC.Ar/On.Ar) and its complement, the fraction of bone area (B.Ar/On.Ar). The results showed that the osteons varied greatly in size, but very little in the fraction of bone area. Regression analyses showed that HC.Ar, HC.Pm, and Lc.N/On were positively associated with On.Ar (P < 0.001 for all). A significant negative correlation was found between On.Ar and Lc.N/B.Ar (P < 0.05) and HC.Pm/B.Ar (P < 0.0001). HC.Ar and HC.Pm increased significantly with increasing Lc.N/On (both P < 0.0001) rather than Lc.N/B.Ar. Lc.N/B.Ar had a significant positive correlation with HC.Ar/On.Ar (P < 0.05) and HC.Pm/B.Ar (P < 0.01). We conclude that: 1) the size of the osteon is determined by the quantum of bone removed by osteoclasts, 2) the osteon is well designed for molecular exchange, and 3) a well designed osteon may be produced via the regulation of bone apposition by osteocytes during the process of osteon refilling.     

Three‐dimensional reconstruction of Haversian systems in human cortical bone using synchrotron radiation‐based micro‐CT: morphology and quantification of branching and transverse connections across age

This study uses synchrotron radiation‐based micro‐computed tomography (CT) scans to reconstruct three‐dimensional networks of Haversian systems in human cortical bone in order to observe and analyse interconnectivity of Haversian systems and the development of total Haversian networks across different ages. A better knowledge of how Haversian systems interact with each other is essential to improve understanding of remodeling mechanisms and bone maintenance; however, previous methodological approaches (e.g. serial sections) did not reveal enough detail to follow the specific morphology of Haversian branching, for example. Accordingly, the aim of the present study was to identify the morphological diversity of branching patterns and transverse connections, and to understand how they change with age. Two types of branching morphologies were identified: lateral branching, resulting in small osteon branches bifurcating off of larger Haversian canals; and dichotomous branching, the formation of two new osteonal branches from one. The reconstructions in this study also suggest that Haversian systems frequently target previously existing systems as a path for their course, resulting in a cross‐sectional morphology frequently referred to as ‘type II osteons’. Transverse connections were diverse in their course from linear to oblique to curvy. Quantitative assessment of age‐related trends indicates that while in younger human individuals transverse connections were most common, in older individuals more evidence of connections resulting from Haversian systems growing inside previously existing systems was found. Despite these changes in morphological characteristics, a relatively constant degree of overall interconnectivity is maintained throughout life. Altogether, the present study reveals important details about Haversian systems and their relation to each other that can be used towards a better understanding of cortical bone remodeling as well as a more accurate interpretation of morphological variants of osteons in cross‐sectional microscopy. Permitting visibility of reversal lines, synchrotron radiation‐based micro‐CT is a valuable tool for the reconstruction of Haversian systems, and future analyses have the potential to further improve understanding of various important aspects of bone growth, maintenance and health.     

Examination of Factors Potentially Influencing Osteon Size in the Human Rib

Previous research demonstrates that the size of secondary osteons varies considerably between individuals, though what factors act in the delineation of osteon size remain uncertain. This study explores the influence of age, sex, percent cortical area (%Ct.Ar), percent cortical porosity (%Po.Ar), and loading environment on osteon area (On.Ar) in human ribs. The sample consisted of midshaft 6^th ribs from 80 individuals, 6–94 years of age. T‐tests demonstrated no significant differences in On.Ar between the sexes (P=0.383). Age showed a significant correlation with both %Ct.Ar and %Po.Ar, so a hierarchical regression model was used to control for the effects of age on the other variables. Results indicate that age is the most significant factor of those tested in this study (P=0.004), with %Ct.Ar playing a much smaller but still significant role (P=0.014), while %Po.Ar had no significant influence on On.Ar (P=0.443). Age demonstrates an inverse relationship with On.Ar, while %Ct.Ar has a direct relationship with On.Ar. Significant differences in On.Ar between the pleural and cutaneous cortices are attributed to variation in %Ct.Ar of each cortex. Therefore, age and %Ct.Ar account for the majority of osteon size variability in this study, although it is likely genetics play an important role as well. Understanding the biological mechanisms that act in remodeling and determine osteon size is essential for accurately addressing and interpreting histological findings, work that is invaluable in its implications for bone biology. Anat Rec, 299:313–324, 2016. © 2016 Wiley Periodicals, Inc.     

A model of osteoblast-osteocyte kinetics in the development of secondary osteons in rabbits

The kinetics of osteogenic cells within secondary osteons have been examined within a 2-D model. The linear osteoblast density of the osteons and the osteocyte lacunae density were compared with other endosteal lamellar systems of different geometries. The cell density was significantly greater in the endosteal appositional zone and was always flatter than the central osteonal canals. Fully structured osteons compared with early structuring (cutting cones) did not show any significant differences in density. The osteoblast density may remain constant because some of them leave the row and become embedded within matrix. The overall shape of the Haversian system represented a geometrical restraint and it was thought to be related to osteoblast-osteocyte transformation. To test this hypothesis of an early differentiation and recruitment of the osteoblast pool which completes the lamellar structure of the osteon, the number and density of osteoblasts and osteocyte lacunae were evaluated. In the central canal area, the mean osteoblast linear density and the osteocyte lacunae planar density were not significantly different among sub-classes (with the exclusion of the osteocyte lacunae of the 300-1000 μm(2) sub-class). The mean number of osteoblasts compared with osteocyte lacunae resulted in significantly higher numbers in the two sub-classes, no significant difference was seen in the two middle sub-classes with the larger canals, and there were significantly lower levels in the smallest central canal sub-class. The TUNEL technique was used to identify the morphological features of apoptosis within osteoblasts. It was found that apoptosis occurred during the late phase of osteon formation but not in osteocytes. This suggests a regulatory role of apoptosis in balancing the osteoblast-osteocyte equilibrium within secondary osteon development. The position of the osteocytic lacunae did not correlate with the lamellar pattern and the lacunae density in osteonal radial sectors was not significantly different. These findings support the hypothesis of an early differentiation of the osteoblast pool and the independence of the fibrillar lamellation from osteoblast-osteocyte transformation.     

Differences in osteon structure histomorphometry between puppyhood and adult stages in the Golden Retriever

Osteon structure has been widely studied in mammals, but osteon structure in dogs has received relatively little attention, especially in terms of whether aging has any effect on osteon structure. The aim of this study was to compare the osteon structure of both flat (scapula and os coxae) and long bones (humerus, radius, ulna, metacarpus, femur and tibia) of male puppy and adult Golden Retrievers. We examined five parameters: Haversian canal diameter, Haversian canal area, osteon diameter, osteon area, and number of lacunae per osteon. Our results show that the values for Haversian canal diameter were significantly higher in the os coxae and tibia, but significantly lower in the femur of adult dogs as compared to those of puppies. The Haversian canal diameter of the other bones investigated did not show any significant differences between puppies and adult dogs. The Haversian canal area was significantly greater in the os coxae, radius and femur of adult dogs than in those of puppies. The osteon diameter and area of every bone examined were significantly smaller in puppies than in adult dogs. Lastly, the number of lacunae per osteon showed the same trend as osteon diameter and area. Plexiform bone could be found in three bones in puppies, i.e. the femur, humerus and tibia. Overall, the results of this study should provide basic knowledge on the microanatomy of cortical bone in dogs and on the possible influence age.     

Ancient Human Bone Microstructure in Medieval England: Comparisons between Two Socio‐Economic Groups

Understanding the links between bone microstructure and human lifestyle is critical for clinical and anthropological research into skeletal growth and adaptation. The present study is the first to report correspondence between socio‐economic status and variation in bone microstructure in ancient humans. Products of femoral cortical remodeling were assessed using histological methods in a large human medieval sample (N = 450) which represented two distinct socio‐economic groups. Osteonal parameters were recorded in posterior midshaft femoral sections from adult males (N = 233) and females (N = 217). Using univariate and multivariate statistics, intact, fragmentary, and osteon population densities, Haversian canal area and diameter, and osteon area were compared between the two groups, accounting for sex, age, and estimated femoral robusticity. The size of osteons and their Haversian canals, as well as osteon density, varied significantly between the socio‐economic groups, although minor inconsistencies were observed in females. Variation in microstructure was consistent with historical textual evidence that describes differences in mechanical loading and nutrition between the two groups. Results demonstrate that aspects of ancient human lifestyle can be inferred from bone microstructure. Anat Rec, 299:42–59, 2016. © 2015 Wiley Periodicals, Inc.     

Sealed osteons in animals and humans: low prevalence and lack of relationship with age

Sealed osteons are unusual variants of secondary osteons that have received little attention, especially in non‐human bones. Sealed osteons are characterized by central canals that are plugged with bone tissue. As with other variants of secondary osteons (e.g. drifting, dumbbell, multi‐canal), understanding how and why sealed osteons form can shed light on the mechanisms that regulate normal bone remodeling and how this process can be perturbed with aging and some diseases. In a recent microscopic evaluation of human tibiae obtained after traumatic amputations, 4–5% of the osteons were sealed. It is suggested that this high prevalence reflects occasional localized microscopic ischemia from normal osteonal remodeling; hence sealed osteons are implicated in human skeletal fragility. Therefore, osteon prevalence would be expected to correlate with the bone remodeling seen with aging; for example, showing positive relationships between sealed osteons and the population density of typical secondary osteons (OPD). We evaluated the prevalence of partially sealed (80–99% sealed) and fully sealed osteons with respect to age and variations in OPD in 10 adult human femora (34–71 years) and in various non‐human appendicular bones of mature animals that were not of advanced age, including deer calcanei, equine radii and equine third metacarpals. An additional sample of 10 bilateral human femora with unilateral non‐cemented total hip replacements (F,+HR) and non‐implanted contralateral femora (F,?HR) were evaluated (10 patients; 52–94 years). In non‐human bones, sealed + partially sealed osteons were rare (~0.1%) even when having relatively high OPD. When considering sealed + partially sealed osteons in femora from patients without any HR, results showed that 1.6% of the osteons were sealed or partially sealed, which was much lower than anticipated, but this is 10‐ to 20‐fold more than in any of the non‐human bones. Additionally, in all bones, sealed + partially sealed osteons were significantly smaller than typical secondary osteons (mean diameters: 125 vs. 272 μm; P  < 0.005). In the patients with HR, the percentage of sealed + partially sealed osteons: (i) did not correlate with age, (ii) showed no significant difference between F,?HR and F,+HR (1.9 vs. 2.1%; P  = 0.2), and (iii) was positively correlated with OPD (r  = 0.67, P  = 0.001), which differs from the very weak or lack of correlations in the non‐human bones and the other human femur sample. The lack of an age‐related relationship, in addition to the very low prevalence of sealed + partially sealed osteons are inconsistent with the idea that they contribute to reduced bone quality seen in aging humans. The small size of sealed and partially sealed osteons, regardless of species affiliation, suggests that they represent closing cones at the termini of some osteons. Available evidence suggests that osteons of primates might have a greater capacity for branching that is associated with closing cones, which might explain the 10–20 times higher prevalence of sealed + partially sealed osteons in the human bones examined in this study.     

Ontogenetic changes to bone microstructure in an archaeologically derived sample of human ribs

There is considerable variation in the gross morphology and tissue properties among the bones of human infants, children, adolescents, and adults. Using 18 known-age individuals (n_female = 8, n_male = 9, n_unknown = 1; birth to 21 years old), from a well-documented cemetery collection, Spitalfields Christ Church, London, UK, this study explores growth-related changes in cortical and trabecular bone microstructure. Micro-CT scans of mid-shaft middle thoracic ribs are used for quantitative analysis. Results are then compared to previously quantified conventional histomorphometry of the same sample. Total area (Tt.Ar), cortical area (Ct.Ar), cortical thickness (Ct.Th), and the major (Maj.Dm) and minor (Min.Dm) diameters of the rib demonstrate positive correlations with age. Pore density (Po.Dn) increases, but age-related changes to cortical porosity (Ct.Po) appear to be non-linear. Trabecular thickness (Tb.th) and trabecular separation (Tb.Sp) increase with age, whereas trabecular bone pattern factor (Tb.Pf), structural model index (SMI), and connectivity density (Conn.D) decrease with age. Sex-based differences were not identified for any of the variables included in this study. Some samples display clear evidence of diagenetic alteration without corresponding changes in radiopacity, which compromises the reliability of bone mineral density (BMD) data in the study of past populations. Cortical porosity data are not correlated with two-dimensional measures of osteon population density (OPD). This suggests that unfilled resorption spaces contribute more significantly to cortical porosity than do the Haversian canals of secondary osteons. Continued research using complementary imaging techniques and a wide array of histological variables will increase our understanding of age- and sex-specific ontogenetic patterns within and among human populations.     

Spatial variation in osteon population density at the human femoral midshaft: histomorphometric adaptations to habitual load environment

Intracortical remodeling, and the osteons it produces, is one aspect of the bone microstructure that is influenced by and, in turn, can influence its mechanical properties. Previous research examining the spatial distribution of intracortical remodeling density across the femoral midshaft has been limited to either considering only small regions of the cortex or, when looking at the entirety of the cortex, considering only a single individual. This study examined the spatial distribution of all remodeling events (intact osteons, fragmentary osteons, and resorptive bays) across the entirety of the femoral midshaft in a sample of 30 modern cadaveric donors. The sample consisted of 15 males and 15 females, aged 21–97 years at time of death. Using geographic information systems software, the femoral cortex was subdivided radially into thirds and circumferentially into octants, and the spatial location of all remodeling events was marked. Density maps and calculation of osteon population density in cortical regions of interest revealed that remodeling density is typically highest in the periosteal third of the bone, particularly in the lateral and anterolateral regions of the cortex. Due to modeling drift, this area of the midshaft femur has some of the youngest primary tissue, which consequently reveals that the lateral and anterolateral regions of the femoral midshaft have higher remodeling rates than elsewhere in the cortex. This is likely the result of tension/shear forces and/or greater strain magnitudes acting upon the anterolateral femur, which results in a greater amount of microdamage in need of repair than is seen in the medial and posterior regions of the femoral midshaft, which are more subject to compressive forces and/or lesser strain magnitudes.     

Microanatomical Record of Cortical Bone Remodeling and High Vascularity in a Fossil Giant Rat Midshaft Femur

Rat cortical bone does not typically undergo secondary (Haversian) remodeling. Haversian organization of rat bone has been mainly observed in experimental settings following biomechanical or dietary manipulation. Here, we report an observation of cortical secondary osteons within a histological femur cross-section from an extinct (late Quaternary) form of Timorese giant rat (Murinae gen. et sp. indet). The medio-lateral midshaft diameter of its femur, used as a measure of bone size, is 6.15 mm and indicates a heavier than normal skeletal frame. We compare this sample to bone histology in a small rat's midshaft femur of 2.33 mm diameter. A complete lack of Haversian bone remodeling characteristics is noted for the smaller sample, which is dominated by radial vascular canals. The giant rat shows clear secondary osteons and diffuse vascularity mainly composed of tightly packed longitudinal canals across its cortex. It appears that rat cortical bone can undergo bone remodeling, and is organized in a highly vascularized manner, in insular giant cases. Our findings from Timor align with results reported in experimental rat model skeletal biology literature and other insular fossil rat material. Where macroanatomical examination is limited, histological observations on fossil rat limb bones have the potential to aid reconstructions of life history and skeletal growth aspects in these rodents. Anat Rec, 302:1934–1940, 2019. © 2019 American Association for Anatomy     

Differences and relationships between the physical properties and the microscopic structure of human femoral,tibial and fibular cortical bone

Coefficients of correlation between certain physical properties and the histological components of the break area were calculated on an IBM 7090 computer for 56 femoral, 79 tibial and 37 fibular specimens of embalmed cortical bone of standardized size and shape. Strong positive correlations (0.01–0.02 significance level) were found between tensile strength and the percentage of interstitial lamellae in the break area; between hardness and the number of osteons/mm²; and between hardness and the percentage of osteons in the break area. Equally high negative correlations were found between tensile strength and percentage of osteons in the break area; between shearing strength and average area/osteon remnant; between elastic modulus and percentage of spaces in the break area; and an even higher correlation (0.001) between hardness and percentage of spaces in the break area. Negative correlations (at slightly more than 0.05 significance level) were found between shearing strength and modulus and average area/osteon. Osteons tend to reduce the tensile strength and elastic modulus of bone while interstitial lamellae tend to increase them. The probable reason is the relatively greater amount of cement lines, which are sites of weakness where failure can occur, in Haversian bone as compared with lamellar bone. The predominant orientation of collagen fibers and the amount and distribution of calcium may also be involved. These factors are now being investigated.     

Super‐osteons (remodeling clusters) in the cortex of the femoral shaft: Influence of age and gender

Previous studies of cortical remodeling in the fractured femoral neck indicated that the merging of spatially clustered remodeling osteons could result in the formation of deleteriously large cavities associated with femoral neck fracture. This study aimed to identify whether remodeling osteons in the femoral shaft were also clustered and to assess the influence of age and gender. Microradiographic images of femoral mid‐shaft cross‐sections from 66 subjects over 21 years of age were analyzed to determine the number, size and location of all Haversian canals. Those most recently remodeled were identified using an edge‐detection algorithm highlighting the most marked differential gradients in grey levels. Cluster analysis (JMP software) of these osteons identified the proportion of recently remodeled osteons that were within 0.75mm clusters. As in the femoral neck, remodeling osteons were significantly more clustered than could occur by chance (real, 59.4%; random, 39.4%; P < 0.0001). The density of these clusters (number/mm²) was not significantly associated with subject age or gender but was greatest near the periosteum and decreased toward the marrow cavity (periosteal 0.043 ± 0.004; mid‐cortex 0.028 ± 0.003; endosteal 0.017 ± 0.002). Cortical porosity increased with age. The presence of giant canals (diameter >385μm) was inversely related to the presence of clusters (R² = 0.237, P < 0.0001). This data suggest that remodeling osteons tend to be spatially colocalized in the shaft as they are in the neck of the femur and their presence is independent of age or gender. We propose that these remodeling clusters be termed super‐osteons. The negative relationship between super‐osteons and giant canals raises the intriguing possibility that loss of the control of remodeling depth results in the merging of osteonal systems to form deleteriously large cortical cavities with a marked reduction in bone strength. Anat Rec 264:378–386, 2001. © 2001 Wiley‐Liss, Inc.     

Accumulation of type VI collagen in the primary osteon of the rat femur during postnatal development

In rodents, the long bone diaphysis is expanded by forming primary osteons at the periosteal surface of the cortical bone. This ossification process is thought to be regulated by the microenvironment in the periosteum. Type VI collagen (Col VI), a component of the extracellular matrix (ECM) in the periosteum, is involved in osteoblast differentiation at early stages. In several cell types, Col VI interacts with NG2 on the cytoplasmic membrane to promote cell proliferation, spreading and motility. However, the detailed functions of Col VI and NG2 in the ossification process in the periosteum are still under investigation. In this study, to clarify the relationship between localization of Col VI and formation of the primary osteon, we examined the distribution of Col VI and osteoblast lineages expressing NG2 in the periosteum of rat femoral diaphysis during postnatal growing periods by immunohistochemistry. Primary osteons enclosing the osteonal cavity were clearly identified in the cortical bone from 2 weeks old. The size of the osteonal cavities decreased from the outer to the inner region of the cortical bone. In addition, the osteonal cavities of newly formed primary osteons at the outermost region started to decrease in size after rats reached the age of 4 weeks. Immunohistochemistry revealed concentrated localization of Col VI in the ECM in the osteonal cavity. Col VI-immunoreactive areas were reduced and they disappeared as the osteonal cavities became smaller from the outer to the inner region. In the osteonal cavities of the outer cortical regions, Runx2-immunoreactive spindle-shaped cells and mature osteoblasts were detected in Col VI-immunoreactive areas. The numbers of Runx2-immunoreactive cells were significantly higher in the osteonal cavities than in the osteogenic layers from 2 to 4 weeks. Most of these Runx2-immunoreactive cells showed NG2-immunoreactivity. Furthermore, PCNA-immunoreactivity was detected in the Runx2-immunoreactive spindle cells in the osteonal cavities. These results indicate that Col VI provides a characteristic microenvironment in the osteonal cavity of the primary osteon, and that differentiation and proliferation of the osteoblast lineage occur in the Col VI-immunoreactive area. Interaction of Col VI and NG2 may be involved in the structural organization of the primary osteon by regulating osteoblast lineages.     

Bone remodeling in the longest living rodent,the naked mole-rat: Interelement variation and the effects of reproduction

The pattern of bone remodeling of one of the most peculiar mammals in the world, the naked mole-rat (NMR), was assessed. NMRs are known for their long lifespans among rodents and for having low metabolic rates. We assessed long-term in vivo bone labeling of subordinate individuals, as well as the patterns of bone resorption and bone remodeling in a large sample including reproductive and non-reproductive individuals (n = 70). Over 268 undecalcified thin cross-sections from the midshaft of humerus, ulna, femur and tibia were analyzed with confocal fluorescence and polarized light microscopy. Fluorochrome analysis revealed low osteogenesis, scarce bone resorption and infrequent formation of secondary osteons (Haversian systems) (i.e., slow bone turnover), thus most likely reflecting the low metabolic rates of this species. Secondary osteons occurred regardless of reproductive status. However, considerable differences in the degree of bone remodeling were found between breeders and non-breeders. Pre-reproductive stages (subordinates) exhibited quite stable skeletal homeostasis and bone structure, although the attainment of sexual maturity and beginning of reproductive cycles in female breeders triggered a series of anabolic and catabolic processes that up-regulate bone turnover, most likely associated with the increased metabolic rates of reproduction. Furthermore, bone remodeling was more frequently found in stylopodial elements compared to zeugopodial elements. Despite the limited bone remodeling observed in NMRs, the variation in the pattern of skeletal homeostasis (interelement variation) reported here represents an important aspect to understand the skeletal dynamics of a small mammal with low metabolic rates. Given the relevance of the remodeling process among mammals, this study also permitted the comparison of such process with the well-documented histomorphology of extinct therapsids (i.e., mammalian precursors), thus evidencing that bone remodeling and its endocortical compartmentalization represent ancestral features among the lineage that gave rise to mammals. It is concluded that other factors associated with development (and not uniquely related to biomechanical loading) can also have an important role in the development of bone remodeling.     

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.