Technical note: Capturing shape-Linear measurements and geometric morphometrics of the immature femora

OBJECTIVES

Growth and developmental studies have been a prominent theme in bioarchaeology. These works traditionally focus on metric measurements of long bone length and age-at-death or cross-sectional geometric studies with the use of computed tomography scans for questions on growth and mobility. However, teasing apart aspects of size and shape have been difficult due to the cylindrical nature of immature long bones. This research investigates the methodological use of surface geometries from linear measurements and geometric morphometric methods (GMM) to answer questions on mobility and allometry during childhood.

MATERIALS AND METHODS

Left femora were selected from 42 individuals ranging from fetal to 12 years of age from medieval St Gregory's Priory, Canterbury, UK. Femora were digitized with structured-light-scanning for auto3dgm analysis and measurements were obtained from physical caliper measurements. Individuals were put into age groups based on biomechanical milestones during this age range.

RESULTS

Ratio and GMM confirm hypotheses of allometry and biomechanical milestones. Geometric morphometrics, however, detects more subtle differences in mobility at each age group.

DISCUSSION

The findings of this preliminary study support the potential use of GMM of immature femora, while indicating that the extent in range of mobility that can occur varies at different biological milestones.

Immature porcine cortical bone mechanical properties and composition change with maturation and displacement rate

Computational models of mature bone have been used to predict fracture; however, analogous study of immature diaphyseal fracture has not been conducted due to sparse experimental mechanical data. A model of immature bone fracture may be used to aid in the differentiation of accidental and non-accidental trauma fractures in young, newly ambulatory children (0-3 years). The objective of this study was to characterize the evolution of tissue-level mechanical behavior, composition, and microstructure of maturing cortical porcine bone with uniaxial tension, Raman spectroscopy, and light microscopy as a function of maturation. We asked: 1) How do the monotonic uniaxial tensile properties change with maturation and displacement rate; 2) How does the composition and microstructure change with maturation; and 3) Is there a correlation between composition and tensile properties with maturation? Elastic modulus (p < 0.001), fracture stress (p < 0.001), and energy absorption (p < 0.014) increased as a function of maturation at the quasistatic rate by 110%, 86%, and 96%, respectively. Fracture stress also increased by 90% with maturation at the faster rate (p = 0.001). Fracture stress increased as a function of increasing displacement rate by 28% (newborn p = 0.048; 1-month p = 0.004; 3-month p= < 0.001), and fracture strain decreased by 68% with increasing displacement rate (newborn p = 0.002; 1-month p = 0.036; 3-month p < 0.001). Carbonate-to-phosphate ratio was positively linearly related to elastic modulus, and fracture stress was positively related to carbonate-to-phosphate ratio and matrix maturation ratio. The results of this study support that immature bone is strain-rate dependent and becomes more brittle at faster rates, contributing to the foundation upon which a computational model can be built to evaluate immature bone fracture.

Asymmetry in linear measurements and cross-sectional geometry in the humerus during ontogeny

OBJECTIVES

Adult upper limb asymmetry is used to reconstruct behavior. However, the developmental trajectory of asymmetry in bone length, cross-sectional geometry (CSG), and joint dimensions is poorly understood. This study examines the development trajectory of humeral asymmetry and if asymmetry in bone length, joint size, and CSG develop in concert.

MATERIALS AND METHODS

Linear measurements of bone length and metaphyseal/epiphyseal breadth, bending rigidity (Imax and Imin), and cross-sectional shape (Imax/Imin) at 30%, 50%, and 70% of bone length were acquired from 3D models of humeri from four skeletal samples of prehistoric hunter-gatherer populations (n = 82). Dental age cohorts were used to assess ontogenetic trends. Percent absolute (%AA) and directional (%DA) asymmetry were calculated for paired measures. Percentage of matching direction of asymmetry across variables and correlation analysis tested relationships between variables.

RESULTS

Within the total pooled sample, Imax shows the highest %AA and %DA, followed by shape and linear dimensions. Asymmetry is lowest in neonates and increases with age, particularly %DA of Imax in mid-proximal sections. Correlations among variables are low to moderate and strongest between Imax measures. Matching direction of asymmetry between variables is low and generally increases with age.

DISCUSSION

Higher correlations with age in CSG likely indicate greater responsiveness to mechanical loading. Low correlations in magnitude of asymmetry and side dominance suggest independence in the development of asymmetry between maximum rigidity, shape, and linear measures. Differences in how asymmetric loading affects the ontogeny of linear and CSG variables may account for the heterogeneous development of asymmetry.

Altered Osteoblast Metabolism with Aging Results in Lipid Accumulation and Oxidative Stress Mediated Bone Loss

Cellular aging is associated with dysfunction of numerous tissues affecting multiple organ systems. A striking example of this is related to age-related bone loss, or osteoporosis, increasing fracture incidence. Interestingly, the two compartments of bone, cortical and cancellous or trabecular, rely on different mechanisms for development and maintenance during 'normal' aging. At a cellular level, the aging process disturbs a multitude of intracellular pathways. In particular, alterations in cellular metabolic functions thereby impacting cellular bioenergetics have been implicated in multiple tissues. Therefore, this study aimed to characterize how metabolic processes were altered in bone forming osteoblasts in aged mice compared to young mice. Metabolic flux analyses demonstrated both stromal cells and mature, matrix secreting osteoblasts from aged mice exhibited mitochondrial dysfunction. This was also accompanied by a lack of adaptability or metabolic flexibility to utilize exogenous substrates compared to osteoblasts cultured from young mice. Additionally, lipid droplets accumulated in both early stromal cells and mature osteoblasts from aged mice, which was further depicted as increased lipid content within the bone cortex of aged mice. Global transcriptomic analysis of the bone further supported these metabolic data as enhanced oxidative stress genes were up-regulated in aged mice, while osteoblast-related genes were down-regulated when compared to the young mice. Collectively, these data suggest that aging results in altered osteoblast metabolic handling of both exogenous and endogenous substrates which could contribute to age-related osteoporosis.

The evolution of the adolescent growth spurt: Urinary biomarkers of bone turnover in wild chimpanzees (Pan troglodytes)

Life history theory addresses how organisms balance development and reproduction. Mammals usually invest considerable energy into growth in infancy, and they do so incrementally less until reaching adult body size, when they shift energy to reproduction. Humans are unusual in having a long adolescence when energy is invested in both reproduction and growth, including rapid skeletal growth around puberty. Although many primates, especially in captivity, experience accelerated growth in mass around puberty, it remains unclear whether this represents skeletal growth. Without data on skeletal growth in nonhuman primates, anthropologists have often assumed the adolescent growth spurt is uniquely human, and hypotheses for its evolution have focused on other uniquely human traits. The lack of data is largely due to methodological difficulties of assessing skeletal growth in wild primates. Here, we use two urinary markers of bone turnover-osteocalcin and collagen-to study skeletal growth in a large, cross-sectional sample of wild chimpanzees (Pan troglodytes) at Ngogo, Kibale National Park, Uganda. For both bone turnover markers, we found a nonlinear effect of age, which was largely driven by males. For male chimpanzees, values for osteocalcin and collagen peaked at age 9.4 years and 10.8 years, respectively, which corresponds to early and middle adolescence. Notably, collagen values increased from 4.5 to 9 years, suggesting faster growth during early adolescence compared to late infancy. Biomarker levels plateaued at 20 years in both sexes, suggesting skeletal growth continues until then. Additional data, notably on females and infants of both sexes, are needed, as are longitudinal samples. However, our cross-sectional analysis suggests an adolescent growth spurt in the skeleton of chimpanzees, especially for males. Biologists should avoid claiming that the adolescent growth spurt is uniquely human, and hypotheses for the patterns of human growth should consider variation in our primate relatives.

Pathology or expected morphology? Investigating patterns of cortical porosity and trabecularization during infancy and early childhood

Increased cortical porosity is associated with a heightened risk of skeletal fragility due to bone loss and structural decay in adults. However, few studies have examined the etiology of cortical porosity in infants and children. This study examines whether age-related changes in femoral growth and locomotor development influence femoral midshaft cortical porosity in a sample of 48 individuals (fetal to 3.99 years) from the 10th-13th century cemetery of St. Étienne de Toulouse, France. Histological sections were prepared and imaged using light microscopy. Midshaft geometric variables such as total area, cortical area, and pore area were calculated using BoneJ. Increased porosity and cortical trabecularization were found to be significantly associated with age, being almost exclusively present in individuals aged 0.5-1.99 years. At approximately 6 months of age infants typically begin engaging in regular femoral loading and experience an acceleration in growth. The observed increase in midshaft porosity and trabecularization, therefore, likely results from the reorganization and redistribution of cortical bone, stimulated by increased growth velocity and the onset of weight-bearing activities. The reduction in cortical porosity and trabecularization in individuals aged 2.0-3.99 years indicates that children are approaching some sort of homeostasis as growth velocity slows and their femora adapt to consistent loading. Understanding what expected skeletal development looks like is necessary when conducting bioarcheological studies and this study provides evidence for a pattern of transient midshaft porosity during infancy and early childhood.

The relationship between bipedalism and growth: A metric assessment in a documented modern skeletal collection (Certosa Collection, Bologna, Italy)

OBJECTIVES

Long bone variations during growth are susceptible to the combined action of nutritional, hormonal, and genetic factors that may modulate the mechanical forces acting upon growing individuals as they progressively acquire a mature gait. In this work, we explore diaphyseal length and breadth variations of tibia and fibula during ontogeny (a) to test the presence of changes in relation to early toddling, and (b) to further our understanding of developmental patterns in relation to sex.

MATERIALS AND METHODS

Lengths, breadths, and indices were analyzed on right and left leg bones of 68 subadult individuals (Human Identified Skeletal Collection of the University of Bologna, Italy). Analyses included intersex and age classes (1, 0-1 year; 2, 1.1-3 years; 3, 3.1-6 years) comparisons, linear regressions with age and assessment of correlation among tibial and fibular measurements, as well as principal component analysis.

RESULTS

A significant difference emerged among age class 1 and the others. Age class 1 and 3 differ between them, while age class 2 overlaps with the others. No sex dimorphism was detected. All measurements were strongly correlated with age. Tibial and fibular measurements correlated with each other.

CONCLUSIONS

Our results relate the progressive emergence of toddling attempts in growing individuals at the end of the first year of age. No significant sex differences were found, suggesting that tibial and fibula growth might diverge between sexes in later childhood. We provide quantitative data regarding tibial and fibular linear growth and its timing in a modern documented osteological sample from Italy.

Osteohistology of Greererpeton provides insight into the life history of an early Carboniferous tetrapod

The vertebrate transition to land is one of the most consequential, yet poorly understood periods in tetrapod evolution. Despite the importance of the water-land transition in establishing modern ecosystems, we still know very little about the life histories of the earliest tetrapods. Bone histology provides an exceptional opportunity to study the biology of early tetrapods and has the potential to reveal new insights into their life histories. Here, we examine the femoral bone histology from an ontogenetic series of Greererpeton, an early tetrapod from the Middle-Late Mississippian (early Carboniferous) of North America. Thin-sections and micro-CT data show a moderately paced rate of bone deposition with significant cortical thickening through development. An interruption to regular bone deposition, as indicated by a zone of avascular tissue and growth marks, is notable at the same late juvenile stage of development throughout our sample. This suggests that an inherent aspect to the life history of juvenile Greererpeton resulted in a temporary reduction in bone deposition. We review several possible life history correlates for this bony signature including metamorphosis, an extended juvenile phase, environmental stress, and movement (migration/dispersal) between habitats. We argue that given the anatomy of Greererpeton, it is unlikely that events related to polymorphism (metamorphosis, extended juvenile phase) can explain the bony signature observed in our sample. Furthermore, the ubiquity of this signal in our sample indicates a taxon-level rather than a population-level trait, which is expected for an environmental stress. We conclude that movement via dispersal represents a likely correlate, as such events are a common life history strategy of aquatically bound vertebrates.

Biomechanics of immature human cortical bone: A systematic review

The whole bone geometry, microstructure, and mechanical properties of mature human bone are widely reported; however, immature bone (0-18 years) has not been similarly robustly characterized. There is an interest in analyzing and predicting the mechanical loading conditions associated with long bone diaphyseal fractures attributed to trauma in children. Thus, understanding the mechanical properties of immature bone in a temporal reference frame is an essential first step to understand diaphyseal fractures of pediatric long bones. The purpose of this systematic review was to ask, what is the state of knowledge regarding the 1) evolution of whole bone geometry and microstructure of immature pediatric bone as a function of maturation and 2) cortical bone density and experimental quasi-static mechanical properties at the tissue level in the diaphyseal region of immature pediatric long bones? The systematic search yielded 36 studies of the whole bone geometry, microstructure, and mechanical properties of immature pediatric long bones. The elastic modulus, yield stress, and ultimate stress were shown to generally increase with maturation, whereas the yield strain was approximately invariant; however, the specific year-to-year progression of these properties could not be characterized from the limited studies available. The results of this systematic search indicate there is a dearth of knowledge associated with the biomechanics of cortical bone from immature pediatric long bones; it also provides a basis for computational studies of immature human long bones. Additional biomechanical studies of immature human bone are necessary to develop a robust catalogue, which can be used in broad applications to understand fracture mechanics, bone pathologies, and athletic injury in the pediatric setting.

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.

Age-related site-specific modifications in diaphyseal structural properties of the human fibula: Furrows and cross-sectional geometry

OBJECTIVES

Fibular structure is related to locomotor behavior, which allows an exploration of mobility in past human populations with diaphyseal cross-sectional geometry (CSG). However, bone structure depends on age-related changes. Nonmechanical alterations can affect biomechanical investigations. In this study, we examined how the cortical area and the variables used as functional markers in the fibular diaphysis (i.e., CSG and furrows) change with aging. We predict classic and specific modifications, and we discuss functional interpretations based on bone structure.

MATERIALS AND METHODS

The sample consisted of 124 individuals of known age in whom the fibular furrow depths were measured with calipers. Microcomputed tomography (micro-CT) scanning of 38 individuals provided CSG (e.g., cortical area, shape index, and robusticity) and fibular furrow indices. CSG was studied at five cross sections taken along the diaphysis. Linear regression analyses and age group comparisons were conducted.

RESULTS

The cross-sectional shape summary by fibular furrows and shape index and the total area did not change with aging; in contrast, the cortical area and the robusticity (Z_p-std) decreased with age.

DISCUSSION

The decrease in robusticity (Z_p-std) with aging is due to the maintenance of total area, which is related to the specific mechanical environment of the fibula, and to the loss of cortical bone and not to the decrease in mechanical stress. This finding is consistent with the lower bone modeling capacity in aged individuals, which also explains the lack of significant changes in the diaphyseal shape. Thus, fibular structure in older individuals is due to a combination of early bone adaptations to stress and aging effects.

Long bone histomorphogenesis of the naked mole-rat: Histodiversity and intraspecific variation

Lacking fur, living in eusocial colonies and having the longest lifespan of any rodent, makes naked mole-rats (NMRs) rather peculiar mammals. Although they exhibit a high degree of polymorphism, skeletal plasticity and are considered a novel model to assess the effects of delayed puberty on the skeletal system, scarce information on their morphogenesis exists. Here, we examined a large ontogenetic sample (n = 76) of subordinate individuals to assess the pattern of bone growth and bone microstructure of fore- and hindlimb bones by using histomorphological techniques. Over 290 undecalcified thin cross-sections from the midshaft of the humerus, ulna, femur, and tibia from pups, juveniles and adults were analyzed with polarized light microscopy. Similar to other fossorial mammals, NMRs exhibited a systematic cortical thickening of their long bones, which clearly indicates a conserved functional adaptation to withstand the mechanical strains imposed during digging, regardless of their chisel-tooth predominance. We describe a high histodiversity of bone matrices and the formation of secondary osteons in NMRs. The bones of pups are extremely thin-walled and grow by periosteal bone formation coupled with considerable expansion of the medullary cavity, a process probably tightly regulated and adapted to optimize the amount of minerals destined for skeletal development, to thus allow the female breeder to produce a higher number of pups, as well as several litters. Subsequent cortical thickening in juveniles involves high amounts of endosteal bone apposition, which contrasts with the bone modeling of other mammals where a periosteal predominance exists. Adults have bone matrices predominantly consisting of parallel-fibered bone and lamellar bone, which indicate intermediate to slow rates of osteogenesis, as well as the development of poorly vascularized lamellar-zonal tissues separated by lines of arrested growth (LAGs) and annuli. These features reflect the low metabolism, low body temperature and slow growth rates reported for this species, as well as indicate a cyclical pattern of osteogenesis. The presence of LAGs in captive individuals was striking and indicates that postnatal osteogenesis and its consequent cortical stratification most likely represents a plesiomorphic thermometabolic strategy among endotherms which has been suggested to be regulated by endogenous rhythms. However, the generalized presence of LAGs in this and other subterranean taxa in the wild, as well as recent investigations on variability of environmental conditions in burrow systems, supports the hypothesis that underground environments experience seasonal fluctuations that may influence the postnatal osteogenesis of animals by limiting the extension of burrow systems during the unfavorable dry seasons and therefore the finding of food resources. Additionally, the intraspecific variation found in the formation of bone tissue matrices and vascularization suggested a high degree of developmental plasticity in NMRs, which may help explaining the polymorphism reported for this species. The results obtained here represent a valuable contribution to understanding the relationship of several aspects involved in the morphogenesis of the skeletal system of a mammal with extraordinary adaptations.

Ontogenetic changes in femoral cross-sectional geometry during childhood locomotor development

OBJECTIVES

The femur is a major weight-bearing bone that is variably loaded throughout growth as children transition through locomotory states prior to the attainment of a mature bipedal gait. Here, we document ontogenetic trends in femoral cross-sectional geometry (CSG) and explore how changes in loading regime may impact the structural arrangement of cortical bone along the length of the developing diaphysis.

MATERIALS AND METHODS

Micro-CT scans of 110 immature femora were generated from a documented archaeological sample ranging in age from birth to 8.5 years old. CSG properties indicative of relative bone strength and bending rigidity were analyzed from cross-sections extracted at 35%, 50% and 65% of total intermetaphyseal length.

RESULTS

Infants experience a marked redistribution of cortical bone between birth and 7 months facilitating a more advantageous mechanical structure for early load bearing behaviors as bone is displaced further from the section centroid. Early walkers are characterized by a mediolaterally reinforced cross-section that becomes more circular as gait continues to develop.

DISCUSSION

During ontogeny the femur undergoes distinct morphological phases, which correspond with changes in loading regime. This study illustrates the importance of loading conditions in shaping immature bone morphology. Nonmechanical factors such as changes in hormonal environmental can also impact on this dynamic.

A Method to Interpolate Osteon Volume Designed for Histological Age Estimation Research

Aging adult skeletal material is a crucial component of building the biological profile of unknown skeletal remains, but many macro- and microscopic methods have challenges regarding accuracy, precision, and replicability. This study developed a volumetric method to visualize and quantify histological remodeling events in three dimensions, using a two-dimensional serialized approach that applied circular polarizing microscopy and geographic information systems protocols. This approach was designed as a tool to extend current histological aging methodologies. Three serial transverse sections were obtained from a human femoral midshaft. A total sample size of 6847 complete osteons from the three sections was identified; 1229 osteons connected between all sections. The volume of all connected osteons was interpolated using ArcGIS area calculations and truncated cone geometric functions. Each section was divided into octants, and two random samples of 100 and of 30 connected osteons from each octant were generated. Osteon volume was compared between the octants for each random sample using ANOVA. Results indicated that the medial aspect had relative uniformity in osteon volume, whereas the lateral aspect showed high variability. The anterolateral-lateral octant had significantly smaller osteon volume, whereas the posterior-posterolateral octant had significantly larger osteon volume. Results also indicated that a minimum of 100 osteons is statistically more robust and more representative of normal osteon distribution and volume; the use of 30 osteons is insufficient. This research has demonstrated that osteon volume can be interpolated using spatial geometry and GIS applications and may be a tool to incorporate into adult age-at-death estimation techniques.

Bone adaptation: Safety factors and load predictability in shaping skeletal form

Much is known about skeletal adaptation in relation to the mechanical functions that bones serve. This includes how bone adapts to mechanical loading during an individual's lifetime as well as over evolutionary time. Although controlled loading in animal models allows us to observe short-term bone adaptation (epigenetic mechanobiology), examining an assemblage of extant vertebrate bones or a group of fossils' bony structures can reveal the combined effects of long-term trends in loading history and the effects of natural selection. In this survey we examine adaptations that take place over both time scales and highlight a few of the extraordinary insights first published by John Currey. First, we provide a historical perspective on bone adaptation control mechanisms, followed by a discussion of safety factors in bone. We then summarize examples of structural- and material-level adaptations and mechanotransduction, and analyze the relationship between these structural- and material-level adaptations observed in situations where loading modes are either predictable or unpredictable. We argue that load predictability is a major consideration for bone adaptation broadly across an evolutionary timescale, but that its importance can also be seen during ontogenetic growth trajectories, which are subject to natural selection as well. Furthermore, we suggest that bones with highly predictable load patterns demonstrate more precise design with lower safety factors, while bones that experience less predictable loads or those that are less capable of repair and adaptation are designed with a higher safety factor. Finally, exposure to rare loading events with high potential costs of failure leads to design of structures with very high safety factor compared to everyday loading experience. Understanding bone adaptations at the structural and material levels, which take place over an individual's lifetime or over evolutionary time has numerous applications in translational and clinical research to understand and treat musculoskeletal diseases, as well as to permit the furthering of human extraterrestrial exploration in environments with altered gravity.

Relation between cross-sectional bone geometry and double zonal osteon frequency and morphology

OBJECTIVES

While double-zonal osteons (DZ) are characterized by a hyper-mineralized ring inside their lamellae, recent findings suggest that this ring is also defined by a change in the collagen fibers' orientation. Collagen and minerals are essential components to the maintenance of adequate bone strength and their alteration can modify the mechanical properties of the bone tissue. Consequently, the aim of this study is to explore the effect of past loads, as estimated from cross-sectional geometric properties, on the formation of DZ osteons compared to type I (common) osteons.

MATERIALS AND METHODS

The sample consists of paired humerus and femur midshaft sections (n = 23) of Eurocanadian settlers from the historical St. Matthew cemetery, Quebec City (1771-1860). Histomorphometric variables included in this study are osteon density for DZ and type I osteons (DZD; OPD), osteon area (DZOn.Ar; On. Ar), Haversian canal area (DZH.Ar; H.Ar), and the area within the hypermineralized ring (HR. Ar). Loading history is estimated from cross-sectional properties including the following variable: cortical and total area (CA, TA), maximum and minimum second moment of area (I_max , I_min ) and polar moment of area (J).

RESULTS

When the humerus and femur of the same individuals are compared, the femur has a higher OPD, DZD, and relative DZD (DZD/OPD). DZ osteons have a smaller area and Haversian canal area compared to type I osteons. The area within the hypermineralized ring in DZ is higher than the Haversian canal area of the type I osteons. Correlations between the residual scores of the regression of histomorphometric variables and cross-sectional properties of the humerus on the femur were not significant.

DISCUSSION

Based on the analysis of the entire cross-section, the lack of correlation between variations in cross-sectional properties and remodeling combined with the significant differences between humeri and femura suggests that the creation of DZ or type I osteons in the bone tissue might be due to a bone specific response, possibly related to differences in bone tissue age that needs to be further investigated. Definitive conclusion regarding biomechanical loads still seem to be premature as regional variations associated with mechanical properties remain to be explored.

Mapping Cheshire Cats' Leg: A histological approach of cortical bone tissue through modern GIS technology

The present study concerns the histological examination of the hind limb of a cat (Felis sp.), with an emphasis on Haversian bone. Acknowledging the variety of obstacles to be confronted, during histological studies, it was decided the documentation, description, and comparison of the longitudinal distribution of the main microstructural characteristics. To reveal what remains hidden from the sight of knowledge, the novel Geographical Information Systems (GIS) methodology was followed. In means to provide conclusive and credible results, it was analyzed the full spectrum of the resulted cross sections and not just a statistical acceptable number or a specific region of interest. In addition, having used the right femur and tibia from the same animal, species and age discrepancies were eliminated. More thoroughly, osteon and Haversian canal size and circularity were calculated and spatially analyzed. Absolute and relative osteon population densities (OPDs) and tissue-type distributions were also estimated. The use of GIS software constituted the core of the current research, since its application transformed cross sections into informative maps, where inter-skeletal, inter-cortical, and intra-cortical distributional patterns were directly recognized and accordingly correlated to strain and load regimes. As result, it is provided the histomorphological and histomorphometrical profile of the samples, under the prism of the existing biomechanical regime. Finally, having further deployed the potentials of GIS software, it is verified and promoted the feasibility of histological mapping as an indispensable procedure, aligned with the necessities of modern science, regardless of discipline or background.

Exploring conditions that make cortical bone geometry optimal for physiological loading

While physiological loading on lower long bones changes during bone development, the bone cross section either remains circular or slowly changes from nearly circular to other shapes such as oval and roughly triangular. Bone is said to be an optimal structure, where strength is maximized using the optimal distribution of bone mass (also called Wolff's law). One of the most appropriate mathematical validations of this law would be a structural optimization-based formulation where total strain energy is minimized against a mass and a space constraint. Assuming that the change in cross section during bone development and homeostasis after adulthood is direct result of the change in physiological loading, this work investigates what optimization problem formulation (collectively, design variables, objective function, constraints, loading conditions, etc.) results in mathematically optimal solutions that resemble bones under actual physiological loading. For this purpose, an advanced structural optimization-based computational model for cortical bone development and defect repair is presented. In the optimization problem, overall bone stiffness is maximized first against a mass constraint, and then also against a polar first moment of area constraint that simultaneously constrains both mass and space. The investigation is completed in two stages. The first stage is developmental stage when physiological loading on lower long bones (tibia) is a random combination of axial, bending and torsion. The topology optimization applied to this case with the area moment constraint results into circular and elliptical cross sections similar to that found in growing mouse or human. The second investigation stage is bone homeostasis reached in adulthood when the physiological loading has a fixed pattern. A drill hole defect is applied to the adult mouse bone, which would disrupt the homeostasis. The optimization applied after the defect interestingly brings the damaged section back to the original intact geometry. The results, however, show that cortical bone geometry is optimal for the physiological loading only when there is also a constraint on polar moment of area. Further numerical experiments show that application of torsion along with the gait-analysis-based physiological loading improves the results, which seems to indicate that the cortical bone geometry is optimal for some amount of torsion in addition to the gait-based physiological loading. This work has a potential to be extended to bone growth/development models and fracture healing models, where topology optimization and polar moment of area constraint have not been introduced earlier.

Changes in the microstructure of compact and trabecular bone tissues of mice subchronically exposed to alcohol

BACKGROUND

Alcohol is one of the most commonly consumed neurotoxins by humans. Its negative effect on bone health is known for a long time. However, its impact on qualitative and quantitative 2D characteristics of the compact bone is still unclear. Therefore, the aim of this study was to investigate in detail the effects of subchronic alcohol exposure on compact and trabecular bone tissues microstructure of laboratory mice using 2D and 3D imaging methods. Ten clinically healthy 12 weeks-old mice (males) were randomly divided into two groups. Animals from experimental group (group E; n = 5) drank a solution composed of 15% ethanol and water (1.7 g 100% ethanol kg-1 b.w. per day) for 8 weeks, while those from control group (group C; n = 5) drank only water.

RESULTS

Subchronic exposure to alcohol leads to several changes in qualitative 2D characteristics of the compact bone such as the presence of primary vascular radial bone tissue in pars anterior of endosteal border and a higher number of resorption lacunae (five times more) in the middle part of substantia compacta. Morphometrical 2D evaluations of the compact bone showed significantly increased sizes of primary osteons' vascular canals (p < 0.05) in mice from the experimental group (E group). Sizes of Haversian canals and secondary osteons were not affected by alcohol consumption. In mice from the E group, significantly lower values for relative bone volume and bone mineral density of the compact bone were observed. In the trabecular bone, decreased values for bone volume, trabecular number, trabecular thickness and bone surface (p < 0.05) were documented.

CONCLUSIONS

Alcohol decreased not only bone volume and density of the compact bone, but it also reduced trabecular bone volume and leads to trabecular thinning. It caused vasodilation of primary osteons' vascular canals and increased porosity in the compact bone.

The effect of hydrochloric acid on microstructure of porcine (Sus scrofa domesticus) cortical bone tissue

We evaluated the degradation of cortical bone tissue by hydrochloric acid (HCl) since intentional bone decalcification in a forensic context has not been studied on a histomorphological level. We used 70 pig metatarsal bones split into subsamples and immersed in one of three concentrations of acidic solutions (0.5M, 1M, 2M HCl) for two and four hours. We analyzed the cortical thicknesses on transversal cross-sections, thicknesses of the three histomorphologically distinct zones present in acid-immersed bones, and number and area of crystals present in one of the zones. Furthermore, we analyzed the ratio of calcium to phosphorus (Ca:P). We observed a division of the cortical bone cross section into three distinctive zones: demineralized matrix (DM) in the periosteal part of bone, middle contact zone (CZ), and mineralized matrix (MM) in the endosteal part of bone. With increasing acid concentration and time of immersion (from 0.5M HCl for 2h to 2M HCl for 4h), the thickness of DM increased by 67%, the thickness of CZ increased by 56%, and the thickness of MM decreased by 32%. The Ca:P ratio in the contact zone of acid-treated samples did not change significantly with changing acid concentration and time of immersion. The Ca:P ratio of the CZ decreased by 10% when compared to the Ca:P ratio of MM in acid-treated samples. Moreover, we observed crystals on the outer periosteal border of the DM zone, in the CZ, and in the MM Haversian/Volkmann's canals. The size and number of the crystals in the CZ of acid-treated bones increased with acid concentration and time of acid immersion. Moreover, we also observed significant differences in all analyzed properties between anatomical regions. Due to varying reactions to acid immersion among anatomical regions, bone micro-degradation should be observed separately for each region.

Digging the compromise: investigating the link between limb bone histology and fossoriality in the aardvark (Orycteropus afer)

Bone microstructure has long been known as a powerful tool to investigate lifestyle-related biomechanical constraints, and many studies have focused on identifying such constraints in the limb bones of aquatic or arboreal mammals in recent years. The limb bone microstructure of fossorial mammals, however, has not been extensively described. Furthermore, so far, studies on this subject have always focused on the bone histology of small burrowers, such as subterranean rodents or true moles. Physiological constraints associated with digging, however, are known to be strongly influenced by body size, and larger burrowers are likely to exhibit a histological profile more conspicuously influenced by fossorial activity. Here, we describe for the first time the limb bone histology of the aardvark (Orycteropus afer), the largest extant burrowing mammal. The general pattern is very similar for all six sampled limb bones (i.e., humerus, radius, ulna, femur, tibia, and fibula). Most of the cortex at midshaft is comprised of compacted coarse cancellous bone (CCCB), an endosteal tissue formed in the metaphyses through the compaction of bony trabeculae. Conversely, the periosteal bone is highly resorbed in all sections, and is reduced to a thin outer layer, suggesting a pattern of strong cortical drift. This pattern contrasts with that of most large mammals, in which cortical bone is of mostly periosteal origin, and CCCB, being a very compliant bone tissue type, is usually resorbed or remodeled during ontogeny. The link between histology and muscle attachment sites, as well as the influence of the semi-arid environment and ant-eating habits of the aardvark on its bone microstructure, are discussed. We hypothesize that the unusual histological profile of the aardvark is likely the outcome of physiological constraints due to both extensive digging behavior and strong metabolic restrictions. Adaptations to fossoriality are thus the result of a physiological compromise between limited food availability, an environment with high temperature variability, and the need for biomechanical resistance during digging. These results highlight the difficulties of deciphering all factors potentially involved in bone formation in fossorial mammals. Even though the formation and maintaining of CCCB through ontogeny in the aardvark cannot be unambiguously linked with its fossorial habits, a high amount of CCCB has been observed in the limb bones of other large burrowing mammals. The inclusion of such large burrowers in future histological studies is thus likely to improve our understanding of the functional link between bone growth and fossorial lifestyle in an evolutionary context.

Cortical bone adaptation and mineral mobilization in the subterranean mammal Bathyergus suillus (Rodentia: Bathyergidae): effects of age and sex

The patterns of bone modeling and mineral mobilization (skeletal homeostasis) among mammals other than humans and laboratory rodents are still poorly known. In this study we assessed the pattern of bone formation and bone resorption in the femur of a wild population of Cape dune molerats, Bathyergus suillus (n = 41) (Bathyergidae), a solitary subterranean mammal with a marked extended longevity among rodents, and which also lives in a naturally deficient state of vitamin D. In order to determine ontogenetic and sex effects on histomorphometric parameters of transversal undecalcified bone sections, two-way ANOVA, linear mixed-effects model and regression statistical analyses were performed. During ontogeny, B. suillus increased their cross sectional area, cortical area and cortical thickness, and most importantly, they showed scarce endosteal bone resorption which resulted in a retained medullary cavity size during ontogeny. This resulted in a positively imbalanced bone modeling, where bone formation considerably surpasses bone loss by almost 100-fold in adulthood. This differs markedly from other terrestrial mammals with relatively thin cortical walls. Regarding bone loss and remodeling, three main processes involving intracortical resorption were observed: modeling-related bone loss in early postnatal growth; secondary osteon formation occurring in both sexes; and subendosteal secondary reconstruction observed only in females. The latter is accompanied by females having six-fold more relative bone loss than males, which is evidenced by the development of enlarged resorption cavities (RCs) distributed circumferentially around the medullary cavity. Males have smaller, more circular and randomly distributed RCs. In general, our data indicate no age-related decline in mineral content in B. suillus, and provides strong support for a pattern of sexual dimorphism in skeletal homeostasis, similar to that occurring in humans and other mammals, with females losing more bone throughout aging as compared to males due to reproductive factors. Interestingly as well, despite the high mechanical loads experienced during burrow construction, bone remodeling in B. suillus is kept at very low levels throughout their lifespan, and dense Haversian tissue never forms. This study represents the first comprehensive assessment of skeletal homeostasis in a subterranean mammal, and it enables a better understanding of the complex processes governing the acquisition and maintenance of bone properties in this species with extraordinary fossorial adaptations.

Age-related mechanical strength evolution of trabecular bone under fatigue damage for both genders: Fracture risk evaluation

Bone tissue is a living composite material, providing mechanical and homeostatic functions, and able to constantly adapt its microstructure to changes in long term loading. This adaptation is conducted by a physiological process, known as "bone remodeling". This latter is manifested by interactions between osteoclasts and osteoblasts, and can be influenced by many local factors, via effects on bone cell differentiation and proliferation. In the current work, age and gender effects on damage rate evolution, throughout life, have been investigated using a mechanobiological finite element modeling. To achieve the aim, a mathematical model has been developed, coupling both cell activities and mechanical behavior of trabecular bone, under cyclic loadings. A series of computational simulations (ABAQUS/UMAT) has been performed on a 3D human proximal femur, allowing to investigate the effects of mechanical and biological parameters on mechanical strength of trabecular bone, in order to evaluate the fracture risk resulting from fatigue damage. The obtained results revealed that mechanical stimulus amplitude affects bone resorption and formation rates, and indicated that age and gender are major factors in bone response to the applied loadings.

Physiology of ageing of the musculoskeletal system

This review aims to provide a summary of current concepts of ageing in relation to the musculoskeletal system, highlighting recent advances in the understanding of the mechanisms involved in the development of age-related changes in bone, skeletal muscle, chondroid and fibrous tissues. The key components of the musculoskeletal system and their functions are introduced together with a general overview of the molecular hallmarks of ageing. A brief description of the normal architecture of each of these tissue types is followed by a summary of established and developing concepts of mechanisms contributing to the age-related alterations in each. Extensive detailed description of these changes is beyond the scope of this review; instead, we aim to highlight some of the most significant processes and, where possible, the molecular changes underlying these and refer the reader to in-depth, subspecialist reviews of the individual components for further details.

Cortical Histomorphometry of the Human Humerus During Ontogeny

Modeling and remodeling are two key determinants of human skeletal growth though little is known about the histomorphometry of cortical bone during ontogeny. In this study, we examined the density and geometric properties of primary and secondary osteons (osteon area and diameter, vascular canal area and diameter) in subperiosteal cortical bone from the human humerus (n = 84) between birth and age 18 years. Sections were removed from the anterior midshaft aspect of humeri from skeletons. Age-at-death was reconstructed using standard osteological techniques. Analyses revealed significant correlation between the histomorphometric variables and age. Higher densities of primary osteons occurred between infancy and 7 years of age but were almost completely replaced by secondary osteons after 14 years of age. The geometry of primary osteons was less clearly related to age. Secondary osteons were visible after 2 years of age and reached their greatest densities in the oldest individuals. Osteon size was positively but weakly influenced by age. Our data imply that modeling and remodeling are age-dependent processes that vary markedly from birth to adulthood in the human humerus.

Collagen Fiber Orientation in Primate Long Bones

Studies of variation in orientation of collagen fibers within bone have lead to the proposition that these are preferentially aligned to accommodate different kinds of load, with tension best resisted by fibers aligned longitudinally relative to the load, and compression best resisted by transversely aligned fibers. However, previous studies have often neglected to consider the effect of developmental processes, including constraints on collagen fiber orientation (CFO), particularly in primary bone. Here we use circularly polarized light microscopy to examine patterns of CFO in cross-sections from the midshaft femur, humerus, tibia, radius, and ulna in a range of living primate taxa with varied body sizes, phylogenetic relationships and positional behaviors. We find that a preponderance of longitudinally oriented collagen is characteristic of both periosteal primary and intracortically remodeled bone. Where variation does occur among groups, it is not simply understood via interpretations of mechanical loads, although prioritized adaptations to tension and/or shear are considered. While there is some suggestion that CFO may correlate with body size, this relationship is neither consistent nor easily explicable through consideration of size-related changes in mechanical adaptation. The results of our study indicate that there is no clear relationship between CFO and phylogenetic status. One of the principle factors accounting for the range of variation that does exist is primary tissue type, where slower depositing bone is more likely to comprise a larger proportion of oblique to transverse collagen fibers. Anat Rec, 300:1189-1207, 2017. © 2017 Wiley Periodicals, Inc.

Intracranial and hierarchical perspective on dietary plasticity in mammals

The effect of dietary properties on craniofacial form has been the focus of numerous functional studies, with increasingly more work dedicated to the importance of phenotypic plasticity. As bone is a dynamic tissue, morphological variation related to differential loading is well established for many masticatory structures. However, the adaptive osteogenic response of several cranial sites across multiple levels of bony organization remains to be investigated. Here, rabbits were obtained at weaning and raised for 48 weeks until adulthood in order to address the naturalistic influence of altered loading on the long-term development of masticatory and non-masticatory elements. Longitudinal data from micro-computed tomography (μCT) scans were used to test the hypothesis that variation in cortical bone formation and biomineralization in masticatory structures is linked to increased stresses during oral processing of mechanically challenging foods. It was also hypothesized that similar parameters for neurocranial structures would be minimally affected by varying loads as this area is characterized by low strains during mastication and reduced hard-tissue mechanosensitivity. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements, though biomineralization trends of masticatory structures did not mirror macroscale findings. Varying osteogenic responses in masticatory elements suggest that physiological adaptation, and corresponding variation in skeletal performance, may reside differentially at one level of bony architecture, potentially affecting the accuracy of behavioral and in silico reconstructions. Together, these findings underscore the complexity of bone adaptation and highlight functional and developmental variation in determinants of skull form.

Long bone histology of the subterranean rodent Bathyergus suillus (Bathyergidae): ontogenetic pattern of cortical bone thickening

Patterns of bone development in mammals are best known from terrestrial and cursorial groups, but there is a considerable gap in our understanding of how specializations for life underground affect bone growth and development. Likewise, studies of bone microstructure in wild populations are still scarce, and they often include few individuals and tend to be focused on adults. For these reasons, the processes generating bone microstructural variation at intra- and interspecific levels are not fully understood. This study comprehensively examines the bone microstructure of an extant population of Cape dune molerats, Bathyergus suillus (Bathyergidae), the largest subterranean mammal endemic to the Western Cape of South Africa. The aim of this study is to investigate the postnatal bone growth of B. suillus using undecalcified histological sections (n = 197) of the femur, humerus, tibia-fibula, ulna and radius, including males and females belonging to different ontogenetic and reproductive stages (n = 42). Qualitative histological features demonstrate a wide histodiversity with thickening of the cortex mainly resulting from endosteal and periosteal bone depositions, whilst there is scarce endosteal resorption and remodeling throughout ontogeny. This imbalanced bone modeling allows the tissues deposited during ontogeny to remain relatively intact, thus preserving an excellent record of growth. The distribution of the different bone tissues observed in the cortex depends on ontogenetic status, anatomical features (e.g. muscle attachment structures) and location on the bone (e.g. anterior or lateral). The type of bone microstructure and modeling is discussed in relation to digging behavior, reproduction and physiology of this species. This study is the first histological assessment describing the process of cortical thickening in long bones of a fossorial mammal.

Mechanical and metabolic interactions in cortical bone development

OBJECTIVES

Anthropological studies of cortical bone often aim to reconstruct either habitual activities or health of past populations. During development, mechanical loading and metabolism simultaneously shape cortical bone structure; yet, few studies have investigated how these factors interact. Understanding their relative morphological effects is essential for assessing human behavior from skeletal samples, as previous studies have suggested that interaction effects may influence the interpretation from cortical structure of physical activity or metabolic status.

MATERIAL AND METHODS

This study assesses cross-sectional geometric and histomorphometric features in bones under different loading regimes (femur, humerus, rib) and compares these properties among individuals under different degrees of metabolic stress. The study sample consists of immature humans from a late medieval Lithuanian cemetery (Alytus, 14th-18th centuries AD). Analyses are based on the hypothesis that metabolic bone loss is distributed within the skeleton in a way that optimizes mechanical competency.

RESULTS

Results suggest mechanical compensation for metabolic bone loss in the cross-sectional properties of all three bones (especially ribs), suggesting a mechanism for conserving adequate bone strength for different loads across the skeleton. Microscopic bone loss is restricted to stronger bones under high loads, which may mitigate fracture risk in areas of the skeleton that are more resistive to loading, although alternative explanations are examined.

DISCUSSION

Distributions of metabolic bone loss and subsequent structural adjustments appear to preserve strength. Nevertheless, both mechanics and metabolism have a detectable influence on morphology, and potential implications for behavioral interpretations in bioculturally stressed samples due to this interaction are explored. Am J Phys Anthropol 160:317-333, 2016. © 2016 Wiley Periodicals, Inc.

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.

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.

Methods and theory in bone modeling drift: comparing spatial analyses of primary bone distributions in the human humerus

This study compares two novel methods quantifying bone shaft tissue distributions, and relates observations on human humeral growth patterns for applications in anthropological and anatomical research. Microstructural variation in compact bone occurs due to developmental and mechanically adaptive circumstances that are 'recorded' by forming bone and are important for interpretations of growth, health, physical activity, adaptation, and identity in the past and present. Those interpretations hinge on a detailed understanding of the modeling process by which bones achieve their diametric shape, diaphyseal curvature, and general position relative to other elements. Bone modeling is a complex aspect of growth, potentially causing the shaft to drift transversely through formation and resorption on opposing cortices. Unfortunately, the specifics of modeling drift are largely unknown for most skeletal elements. Moreover, bone modeling has seen little quantitative methodological development compared with secondary bone processes, such as intracortical remodeling. The techniques proposed here, starburst point-count and 45° cross-polarization hand-drawn histomorphometry, permit the statistical and populational analysis of human primary tissue distributions and provide similar results despite being suitable for different applications. This analysis of a pooled archaeological and modern skeletal sample confirms the importance of extreme asymmetry in bone modeling as a major determinant of microstructural variation in diaphyses. Specifically, humeral drift is posteromedial in the human humerus, accompanied by a significant rotational trend. In general, results encourage the usage of endocortical primary bone distributions as an indicator and summary of bone modeling drift, enabling quantitative analysis by direction and proportion in other elements and populations.

Childhood cortical porosity is related to microstructural properties of the bone-muscle junction

Childhood cortical porosity is attributable to giant asymmetrical drifting osteonal canals that arise predominantly along the primary-secondary bone interface (PSBI). Bone from the external iliac crest cortex of 92 subjects aged 0 to 25 years was examined histomorphometrically for differences in microstructural properties between primary and secondary bone that might account for features of drifting osteonal canals. Primary compared with secondary bone showed greater numbers of osteocyte lacunae, thinner collagen lamellae, and a scaffold of elastic perforating fibers (PFs). The greater number of osteocyte lacunae compounded by known perilacunar strain amplification and the presence of elastic PFs are expected to be associated with greater bone tissue strain in primary than in secondary bone and thus with strain gradients at the PSBI. Strain gradients may lead local osteocytes to originate resorption canals and to promote transverse drift of the resorption front into lower-strain secondary bone, thus creating giant asymmetrical drifting osteonal canals that remodel primary to secondary bone. PFs extended from muscle fibers through periosteum and primary bone to the PSBI, where they were resorbed by origination of drifting canals. Growth modeling by periosteal osteoblasts proceeds in the gaps between PFs. Through the direct connection between muscle and the PSBI via PFs, muscle forces may influence not only modeling by raising strain but also remodeling of primary to secondary bone by increasing strain gradients at the PSBI. With reduction in primary bone width after the mid-teens, numbers of drifting canals and porosity declined. Differences in microstructural properties between primary and secondary bone are expected to generate strain gradients at the PSBI that contribute to site, transverse drift, asymmetry and large size of drifting canals, and, hence, to cortical porosity. Cortical porosity in children is a physiological feature of bone growth in width. Advisability of therapeutic intervention remains to be defined.

Intracortical remodeling parameters are associated with measures of bone robustness

Prior work identified a novel association between bone robustness and porosity, which may be part of a broader interaction whereby the skeletal system compensates for the natural variation in robustness (bone width relative to length) by modulating tissue-level mechanical properties to increase stiffness of slender bones and to reduce mass of robust bones. To further understand this association, we tested the hypothesis that the relationship between robustness and porosity is mediated through intracortical, BMU-based (basic multicellular unit) remodeling. We quantified cortical porosity, mineralization, and histomorphometry at two sites (38% and 66% of the length) in human cadaveric tibiae. We found significant correlations between robustness and several histomorphometric variables (e.g., % secondary tissue [R(2)  = 0.68, P < 0.004], total osteon area [R(2)  = 0.42, P < 0.04]) at the 66% site. Although these associations were weaker at the 38% site, significant correlations between histological variables were identified between the two sites indicating that both respond to the same global effects and demonstrate a similar character at the whole bone level. Thus, robust bones tended to have larger and more numerous osteons with less infilling, resulting in bigger pores and more secondary bone area. These results suggest that local regulation of BMU-based remodeling may be further modulated by a global signal associated with robustness, such that remodeling is suppressed in slender bones but not in robust bones. Elucidating this mechanism further is crucial for better understanding the complex adaptive nature of the skeleton, and how interindividual variation in remodeling differentially impacts skeletal aging and an individuals' potential response to prophylactic treatments.

An approach to the histomorphological and histochemical variations of the humerus cortical bone through human ontogeny

For many years, clinical and non-clinical investigations have investigated cortical bone structure in an attempt to address questions related to normal bone development, mineralisation, pathologies and even evolutionary trends in our lineage (adaptations). Research in the fields of medicine, materials science, physical anthropology, palaeontology, and even archaeobiology has contributed interesting data. However, many questions remain regarding the histomorphological and histochemical variations in human cortical bone during different stages of life. In the present work, we describe a study of long bone cortex transformations during ontogeny. We analysed cross-sections of 15 human humeri histomorphologically and histochemically from perinatal to adult age, marking and quantifying the spatial distribution of bone tissue types using GIS software and analysing the mineral composition and crystallinity of the mineralised cortex using Raman spectroscopy and X-ray diffraction. Our results allowed us to propose that human cortical bone undergoes three main 'events' through ontogeny that critically change the proportions and structure of the cortex. In early development, bone is not well mineralised and proportionally presents a wide cortex that narrows through the end of childhood. Before reaching complete maturity, the bone mineral area increases, allowing the bone to nearly reach the adult size. The medullary cavity is reduced, and the mineral areas have a highly ordered crystalline structure. The last event occurs in adulthood, when the 'oldest' individuals present a reduced mineralised area, with increasing non-mineralised cavities (including the medullary cavity) and reduced crystalline organisation.

Three-dimensional microarchitecture of adolescent cancellous bone

This study investigated microarchitectural, mechanical, collagen and mineral properties of normal adolescent cancellous bone, and compared them with adult and aging cancellous bone, to obtain more insight into the subchondral bone adaptations during development and growth. Twenty-three human proximal tibiae were harvested and divided into 3 groups according to their ages: adolescence (9 to 17 years, n=6), young adult (18 to 24 years, n=9), and adult (25 to 30 years, n=8). Twelve cubic cancellous bone samples with dimensions of 8×8×8 mm(3) were produced from each tibia, 6 from each medial and lateral condyle. These samples were micro-CT scanned (vivaCT 40, Scanco Medical AG, Switzerland) resulting in cubic voxel sizes of 10.5*10.5*10.5 μm(3). Microarchitectural properties were calculated. The samples were then tested in compression followed by collagen and mineral determination. Interestingly, the adolescent cancellous bone had similar bone volume fraction (BV/TV), structure type (plate, rod or mixtures), and connectivity (3-D trabecular networks) as the adult cancellous bone. The adolescent cancellous bone had significantly lower bone surface density (bone surface per total volume of specimen) but higher collagen concentration (collagen weight per dry weight of specimen) than the adult cancellous bone; and significant greater trabecular separation (mean distance between trabeculae), significant lower trabecular number (number of trabeculae per volume), tissue density (dry weight per volume of bone matrix excluding marrow space) and mineral concentration (ash weight per dry weight of specimen) than the young adult and adult cancellous bones. Despite these differences, ultimate stress and failure energy were not significantly different among the three groups, only the Young's modulus in anterior-posterior direction was significantly lower in adolescence. Apparent density appears to be the single best predictor of mechanical properties. In conclusion, adolescent cancellous bone has similar bone volume fraction, structure type, and connectivity as the young adult and adult cancellous bones, and significant lower tissue density, bone surface density and mineral concentration but higher collagen concentration than in the young adult and adult bone. Despite these differences, the mechanical properties did not show significant difference among the three groups except less stiffness in anterior-posterior direction in the adolescents.

Mapping human long bone compartmentalisation during ontogeny: a new methodological approach

Throughout ontogeny, human bones undergo differentiation in terms of shape, size and tissue type; this is a complex scenario in which the variations in the tissue compartmentalisation of the cortical bone are still poorly understood. Currently, compartmentalisation is studied using methodologies that oversimplify the bone tissue complexity. Here, we present a new methodological approach that integrates a histological description and a mineral content analysis to study the compartmentalisation of the whole mineralised and non-mineralised tissues (i.e., spatial distribution in long bone sections). This new methodology, based on Geographical Information System (GIS) software, allows us to draw areas of interest (i.e., tracing vectorial shapes which are quantifiable) in raw images that are extracted from microscope and compared them spatially in a semi-automatic and quantitative fashion. As an example of our methodology, we have studied the tibiae from individuals with different age at death (infant, juvenile and adult). The tibia's cortical bone presents a well-formed fibrolamellar bone, in which remodelling is clearly evidenced from early ontogeny, and we discuss the existence of "lines of arrested growth". Concurrent with the histological variation, Raman and FT-IR spectroscopy analyses corroborate that the mineral content in the cortical bone changes differentially. The anterior portion of the tibia remains highly pierced and is less crystalline than the rest of the cortex during growth, which is evidence of more active and continuous remodelling. Finally, while porosity and other "non-mineralised cavities" are largely modified, the mineralised portion and the marrow cavity size persist proportionally during ontogeny.