Functional associations between collagen fibre orientation and locomotor strain direction in cortical bone of the equine radius

A novel technique for determining the collagen fibre orientation pattern of cross-sections of cortical bone was used to study mid-diaphyseal sections from the equine radius. Several in vivo strain gauge studies have demonstrated that this bone is loaded in bending during locomotion in such a way that the cranial cortex is consistently subjected to longitudinal tensile strains and the caudal cortex to longitudinal compressive strains. Twenty-three radii from 17 horses were studied. All the bones obtained from adult horses exhibited a consistent pattern of collagen fibre orientation across the cortex. The cranial cortex, subjected to intermittent tension, and the lateral and medial cortices, through which the neutral axis passes, contained predominantly longitudinally oriented collagen fibres. The caudal cortex, subjected to longitudinal compression during life, contained predominantly oblique/transverse collagen. This pattern was less evident in bones from foals. Microscopic analysis of the bones studied showed that primary lamellar bone was composed of predominantly longitudinal collagen fibres, irrespective of cortex. However, there was a strong relationship between cortical location and fibre orientation within remodelled bone. Secondary osteons which formed in the caudal (compressive) cortex contained predominantly oblique/transverse collagen, while those which formed elsewhere contained longitudinal collagen. This observation explained the developmental appearance of the characteristic macroscopic pattern of collagen fibre orientation across the whole cortex in the adult. These findings provide evidence for the existence of a relationship between the mechanical function of a bone with its architecture, and now demonstrate that it extends to the molecular level.     

Does the degree of laminarity correlate with site‐specific differences in collagen fibre orientation in primary bone? An evaluation in the turkey ulna diaphysis

de Margerie hypothesized that preferred orientations of primary vascular canals in avian primary cortical bone mediate important mechanical adaptations. Specifically, bones that receive habitual compression, tension or bending stresses typically have cortices with a low laminarity index (LI) (i.e. relatively lower cross-sectional areas of circularly (C) orientated primary vascular canals, and relatively higher areas of canals with radial (R), oblique (O) or longitudinal (L) orientations. By contrast, bones subject to habitual torsion have a high LI (i.e. relatively higher C-orientated canal area) [LI, based on percentage vascular canal area, = C/(C + R + O + L)]. Regional variations in predominant collagen fibre orientation (CFO) may be the adaptive characteristic mediated by LI. Using turkey ulnae, we tested the hypothesis that site-specific variations in predominant CFO and LI are strongly correlated. Mid-diaphyseal cross-sections (100 +/- 5 micro m) from subadult and adult bones were evaluated for CFO and LI using circularly polarized light images of cortical octants. Results showing significant differences between mean LI of subadult (40.0% +/- 10.7%) and adult (50.9% +/- 10.4%) (P < 0.01) bones suggest that adult bones experience more prevalent/predominant torsion. Alternatively, this relationship may reflect differences in growth rates. High positive correlations between LI and predominant CFO (subadults: r = 0.735; adults: r = 0.866; P < 0.001) suggest that primary bone can exhibit potentially adaptive material variations that are independent of secondary osteon formation.     

Modeling and remodeling in a developing artiodactyl calcaneus: a model for evaluating Frost's Mechanostat hypothesis and its corollaries

The artiodactyl (mule deer) calcaneus was examined for structural and material features that represent regional differences in cortical bone modeling and remodeling activities. Cortical thickness, resorption and formation surfaces, mineral content (percent ash), and microstructure were quantified between and within skeletally immature and mature bones. These features were examined to see if they are consistent with predictions of Frost's Mechanostat paradigm of mechanically induced bone adaptation in a maturing "tension/compression" bone (Frost, 1990a,b, Anat Rec 226:403-413, 414-422). Consistent with Frost's hypothesis that surface modeling activities differ between the "compression" (cranial) and "tension" (caudal) cortices, the elliptical cross-section of the calcaneal diaphysis becomes more elongated in the direction of bending as a result of preferential (> 95%) increase in thickness of the compression cortex. Regional differences in mineral content and population densities of new remodeling events (NREs = resorption spaces plus newly forming secondary osteons) support Frost's hypothesis that intracortical remodeling activities differ between the opposing cortices: 1.) in immature and mature bones, the compression cortex had attained a level of mineralization averaging 8.9 and 6.8% greater (P < 0.001), respectively, than that of the tension cortex, and 2.) there are on average 350 to 400% greater population densities of NREs in the tension cortices of both age groups (P < 0.0003). No significant differences in cortical thickness, mineral content, porosity, or NREs were found between medial and lateral cortices of the skeletally mature bones, suggesting that no modeling or remodeling differences exist along a theoretical neutral axis. However, in mature bones these cortices differed considerably in secondary osteon cross-sectional area and population density. Consistent with Frost's hypothesis, remodeling in the compression cortex produced bone with microstructural organization that differs from the tension cortex. However, the increased remodeling activity of the tension cortex does not appear to be related to a postulated low-strain environment. Although most findings are consistent with predictions of Frost's Mechanostat paradigm, there are several notable inconsistencies. Additional studies are needed to elucidate the nature of the mechanisms that govern the modeling and remodeling activities that produce and maintain normal bone. It is proposed that the artiodactyl calcaneus will provide a useful experimental model for these studies.     

Differences in osteonal micromorphology between tensile and compressive cortices of a bending skeletal system: Indications of potential strain-specific differences in bone microstructure

Background: It has been hypothesized that bone has the capacity to accommodate regional differences in tension and compression strain mode and/or magnitude by altering its osteonal microstructure. We examined a simple cantilevered bone to determine whether regional differences in particular strain-related features are reflected in the microstructural organization of compact bone. Methods & Results: The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition which is typified by prevailing compressive and tensile strains on opposite cortices, and variations in strain magnitudes across each of these cortices. Microscopic examination showed osteon density and cortical porosity differences between tension (caudal) and compression (cranial) cortices, averaging 11.4% more osteons in the compression cortex (P < 0.01) and 80.2% greater porosity in the tension cortex (P < 0.01). There is 43.5% more interstitial bone in the compression cortex (P < 0.01). Osteons in the compression cortex also have smaller areas in contrast to the larger area per osteon in the tension cortex. Although no definite transcortical gradient in osteonal density or cortical porosity is found, fractional area of interstitial bone is largest and osteon population density is lowest in the endocortical regions of both tension and compression cortices. The endocortical regions also have greater porosity than their corresponding middle and pericortical regions (P < 0.01). Conclusions: These osteonal microstructure and cortical porosity differences may be adaptations related to regional differences in strain mode and/or strain magnitude. This may be related to the disparity in mechanical properties of compact bone in tension vs. compression. These differences may reflect a capacity of bone to process local and regional strain-related information. © 1994 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public dotmain in the United states of America

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Osteocyte lacuna population densities in sheep, elk and horse calcanei

Osteocytes, the most prevalent cell type in bone, appear to communicate via gap junctions. In limb-bone diaphyses, it has been hypothesized that these cellular networks have the capacity to monitor habitual strains, which can differ significantly between cortical locations of the same bone. Regional differences in microdamage associated with prevalent/predominant strain mode (tension, compression, or shear) and/or magnitude may represent an important "variable" detected by this network. This hypothesis was indirectly addressed by examining bones subjected to habitual bending for correlations of osteocyte lacuna population densities (n/mm(2) bone area, Ot.Lc.N/B.Ar) with locations experiencing high and low strain, and/or prevalent/predominant tension, compression, and shear. We examined dorsal ("compression"), plantar ("tension"), and medial/lateral ("shear" or neutral axis) cortices of mid-diaphyseal sections of calcanei of adult sheep, elk, and horses. Ot.Lc.N/B.Ar data, quantified in backscattered electron images, were also evaluated in a context of various additional structural and material variables (e.g. % ash, cortical thickness, porosity, and secondary osteon population). Results showed significant differences in dorsal versus plantar comparisons with the highest Ot.Lc.N/B.Ar in dorsal cortices of sheep and elk (p < 0.0001); but this was a statistical trend in the equine calcanei (p = 0.14). There were no consistent transcortical (pericortical to endocortical) differences, and Ot.Lc.N/B.Ar in neutral axes was not consistently different from dorsal/plantar cortices. Correlations of Ot.Lc.N/B.Ar with structural and material parameters were also poor and/or inconsistent within or between species. These results provide little or no evidence that the number of osteocyte lacunae has a functional role in mechanotransduction pathways that are typically considered in bone adaptation. Although dorsal/plantar differences may be adaptations for prevalent/predominant strain modes and/or associated microdamage, it is also plausible that they are strongly influenced by differences in the bone formation rates that produced the tissue in these locations.     

Regional differences in cortical bone organization and microdamage prevalence in Rocky Mountain mule deer

The limb bones of cursorial mammals may exhibit regional structural/material variations for local mechanical requirements. For example, it has been hypothesized that mineral content (%ash) and secondary osteon population density (OPD) progressively change from proximal (e.g., humerus) to distal (e.g., phalanx), in accordance with corresponding progressive changes in stress and mechanical/metabolic cost of functional use (both greatest in the distal limb). We tested this hypothesis in wild-shot Rocky Mountain mule deer by examining transverse segments from mid-diaphyses of medial proximal phalanges, principal metacarpals, radii, and humeri, as well as the lateral aspects of sixth ribs from each of 11 mature males. Quantified structural parameters included the section modulus (Z), polar moment of inertia (J), cortical area/total area ratio (CA/TA), bone girth, and cortical thickness. In addition, %ash and the prevalence of in vivo microcracks were measured in each bone. Thin sections from seven animals were further examined for OPD and population densities of new remodeling events (NREs). Results showed a significant progressive decrease in %ash from the humerus (75.4% +/- 0.9%) to the phalanx (69.4% +/- 1.1%) (P < 0.0001), with general proximal-to-distal increases in OPD and general decreases in J and Z. Thirteen microcracks were identified in the rib sections, and only two were observed in the limb bones. Although the ribs had considerably greater NREs, no significant differences in NREs were found between the limb bones, indicating that they had similar remodeling rates. Equivalent microcrack prevalence, but nonequivalent structural/material organization, suggests that there are regional adaptations that minimize microcrack production in locations with differences in loading conditions. The progressive proximal-to-distal decrease in %ash (up to 6%); moderate-to-high correlations between OPD, %ash, J, and CA/TA; and additional moderate-to-high correlations of these parameters with each bone's radius of gyration support the possibility that these variations are adaptations for regional loading conditions.     

Collagen fiber orientation pattern,osteon morphology and distribution,and presence of laminar histology do not distinguish torsion from bending in bat and pigeon wing bones

Bone can adapt to its habitual load history at various levels of its hierarchical structural and material organization. However, it is unclear how strongly a bone's structural characteristics (e.g. cross‐sectional shape) are linked to microstructural characteristics (e.g. distributions of osteons and their vascular canals) or ultrastructural characteristics [e.g. patterns of predominant collagen fiber orientation (CFO)]. We compared the cross‐sectional geometry, microstructure and ultrastructure of pigeon (Columba livia domestica) humeri, and third metacarpals (B3M) and humeri of a large bat (Pteropus poliocephalus). The pigeon humerus is habitually torsionally loaded, and has unremodeled (‘primary’) bone with vessels (secondary osteons are absent) and high ‘laminarity’ because a large majority of these vessels course circularly with respect to the bone's external surface. In vivo data show that the bat humerus is also habitually torsionally loaded; this contrasts with habitual single‐plane bending of the B3M, where in vivo data show that it oscillates back and forth in the same direction. In contrast to pigeon humeri where laminar bone is present, the primary tissue of these bat bones is largely avascular, but secondary osteons are present and are usually in the deeper cortex. Nevertheless, the load history of humeri of both species is prevalent/predominant torsion, producing diffusely distributed shear stresses throughout the cross‐section. We tested the hypothesis that despite microstructural/osteonal differences in these pigeon and bat bones, they will have similar characteristics at the ultrastructural level that adapt each bone for its load history. We postulate that predominant CFO is this characteristic. However, even though data reported in prior studies of bones of non‐flying mammals suggest that CFO would show regional variations in accordance with the habitual ‘tension regions’ and ‘compression regions’ in the direction of unidirectional habitual bending, we hypothesized that alternating directions of bending within the same plane would obviate these regional/site‐specific adaptations in the B3M. Similarly, but for other reasons, we did not expect regional variations in CFO in the habitually torsionally loaded bat and pigeon humeri because uniformly oblique‐to‐transverse CFO is the adaptation expected for the diffusely distributed shear stresses produced by torsion/multidirectional loads. We analyzed transverse sections from mid‐diaphyses of adult bones for CFO, secondary osteon characteristics (size, shape and population density), cortical thickness in quadrants of the cortex, and additional measures of cross‐sectional geometry, including the degree of circular shape that can help distinguish habitual torsion from bending. Results showed the expected lack of regional CFO differences in quasi‐circular shaped, and torsionally loaded, pigeon and bat humeri. As expected, the B3M also lacked CFO variations between the opposing cortices along the plane of bending, and the quasi‐elliptical cross‐sectional shape and regional microstructural/osteonal variations expected for bending were not found. These findings in the B3M show that uniformity in CFO does not always reflect habitual torsional loads. Osteon morphology and distribution, and presence of laminar histology also do not distinguish torsion from bending in these bat and pigeon wing bones.     

A scanning electron microscopic study of the development of the shoulder,visceral arches,and the region ventral to the cervical somites of the chick embryo

Scanning electron microscopy (SEM) was used to investigate the development of the shoulder in the chick embryo. Initially the wing grows on an axis perpendicular to the dorso-ventral and cranial-caudal axes of the embryo, but soon begins to grow in a ventral and partially caudal direction. The change in axis of outgrowth occurs while the shoulder forms at the cranial proximal portion of the wing. Analysis of SEM observations, together with an analysis of serially sectioned embryos and photographs of live embryos in ovo has demonstrated that the shoulder continues to grow out on an axis perpendicular to the dorso-ventral axis of the embryo, while the caudal and distal portions of the wing grow ventrally. The change in axis of outgrowth seems to be due to (1) the formation of the viscera under the wing, (2) the closing of the membrana reuniens to form a continuous sheet covering the viscera under the wing, (3) caudal movement of the duct of Cuvier and the cranial margin of the pleural coelom, and (4) ventral movement of the lateral body fold caudal to the wing. Although the visceral arches undergo major morphogenetic changes during this period, the visceral arches do not appear to have an influence on shoulder development.     

Mechanical implications of collagen fibre orientation in cortical bone of the equine radius

Mechanical test specimens were prepared from the cranial and caudal cortices of radii from eight horses. These were subjected to destructive tests in either tension or compression. The ultimate stress, elastic modulus and energy absorbed to failure were calculated in either mode of loading. Analysis was performed on the specimens following mechanical testing to determine their density, mineral content, mineral density distribution and histological type. A novel technique was applied to sections from each specimen to quantify the predominant collagen fibre orientation of the bone near the plane of fracture. The collagen map for each bone studied was in agreement with the previously observed pattern of longitudinal orientation in the cranial cortex and more oblique to transverse collagen in the caudal cortex. Bone from the cranial cortex had a significantly higher ultimate tensile stress (UTS) than that from the caudal cortex (160 MPa vs 104 MPa; P<0.001) though this trend was reversed in compression, the caudal cortex becoming relatively stronger (185 MPa vs 217 MPa; P<0.01). Bone from the cranial cortex was significantly suffer than that from the caudal cortex both in tension (22 GPa vs 15 GPa; P<0.001) and compression (19 GPa vs 15 GPa; P<0.01). Of all the histo-compositional variables studied, collagen fibre orientation was most closely correlated with mechanical properties, accounting for 71% of variation in ultimate tensile stress and 58% of variation in the elastic modulus. Mineral density and porosity were the only other variables to show any significant correlation with either UTS or elastic modulus. The variations in mechanical properties around the equine radius, which occur in close association with the different collagen fibre orientations, provide maximal safety factors in terms of ultimate stress, yet contribute to greater bending of the bone as it is loaded during locomotion, and thus lower safety factors through the higher strains this engenders.     

Analysis of a tension/compression skeletal system: Possible strain-specific differences in the hierarchical organization of bone

Background: Examination of a simple skeletal cantilevered beam-like bone (artiodactyl calcaneus) suggests that regional differences in strain magnitude and mode (tension vs. compression) reflect regional adaptation in the structural/material organization of bone. The artiodactyl (e.g., sheep and deer) calcaneus has a predominant loading condition typified by the unambiguous presence of prevailing compressive and tensile strains on opposite cortices. Bone habitually loaded in bending may accommodate regional disparities in loading conditions through modifications of various aspects of its organization. These include overall bone build (gross size and shape), cross-sectional shape, cortical thickness, and mineral content. Methods & Results: Cross-sections taken along the calcaneal body exhibited cranial-caudal elongation with the compression (cranial) cortex thicker than the tension cortex (P < 0.01). Mineral content (ash fraction) was significantly greater in the compression cortex (P < 0.01), averaging 6.6% greater than in the tension cortex. Strong positive correlations were found between mineral content and section location in both the tension (r² = 0.955) and compression (r² = 0.812) cortices. These correlations may reflect functional adaptations to the linear increases in stress that are known to occur in the distal-to-proximal direction in simple, unidirectionally loaded cantilevered beams. According to engineering principles, the roughly triangular transverse cross-sectional geometries and thicker compression cortex are features consistent with a short cantilevered structure designed to resist unidirectional bending. Conclusions: Known differences in mechanical properties of bone in tension vs. compression suggest that these regional differences in cortical thickness and mineralization may be related to differences in strain mode. These structural/material dissimilarities, however, may be related to regional variations in strain magnitude, since bending and axially directed stresses in a simple cantilevered structure produce greater strain magnitudes in the compression domain. It is possible that the superimposed habitual strain magnitudes enhance strain-mode-specific adaptive responses. We hypothesize that these structural/material differences reflect the capacity of bone to process local information and produce a regionally heterogeneous organization that is appropriate for prevailing loading conditions. © 1994 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public dotmain in the United states of America

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Relationships between in vivo microdamage and the remarkable regional material and strain heterogeneity of cortical bone of adult deer, elk, sheep and horse calcanei

Natural loading of the calcanei of deer, elk, sheep and horses produces marked regional differences in prevalent/predominant strain modes: compression in the dorsal cortex, shear in medial-lateral cortices, and tension/shear in the plantar cortex. This consistent non-uniform strain distribution is useful for investigating mechanisms that mediate the development of the remarkable regional material variations of these bones (e.g. collagen orientation, mineralization, remodeling rates and secondary osteon morphotypes, size and population density). Regional differences in strain-mode-specific microdamage prevalence and/or morphology might evoke and sustain the remodeling that produces this material heterogeneity in accordance with local strain characteristics. Adult calcanei from 11 animals of each species (deer, elk, sheep and horses) were transversely sectioned and examined using light and confocal microscopy. With light microscopy, 20 linear microcracks were identified (deer: 10; elk: six; horse: four; sheep: none), and with confocal microscopy substantially more microdamage with typically non-linear morphology was identified (deer: 45; elk: 24; horse: 15; sheep: none). No clear regional patterns of strain-mode-specific microdamage were found in the three species with microdamage. In these species, the highest overall concentrations occurred in the plantar cortex. This might reflect increased susceptibility of microdamage in habitual tension/shear. Absence of detectable microdamage in sheep calcanei may represent the (presumably) relatively greater physical activity of deer, elk and horses. Absence of differences in microdamage prevalence/morphology between dorsal, medial and lateral cortices of these bones, and the general absence of spatial patterns of strain-mode-specific microdamage, might reflect the prior emergence of non-uniform osteon-mediated adaptations that reduce deleterious concentrations of microdamage by the adult stage of bone development.     

Lower axial skeleton of the musk shrew (Suncus murinus)

Macroscopic structure as well as pre- and postnatal development of the lumbar, sacral, and caudal vertebrae of the musk shrew (Suncus murinus, Insectivora) were observed. The lumbar vertebrae possess two pairs of unusual processes, hyperapophyses and hypapophyses. The hyperapophyses are located on the dorsal surface of the caudal articular processes of all the lumbar vertebrae, whereas the hypapophyses are found on the caudal part of the ventral surface of the bodies in the first few lumbar vertebrae. The former gives attachment to the Mm. rotatores lumborum and the latter to the Mm. psoas major and minor. The articular processes of the lumbar vertebrae are oriented more horizontally compared with those in other mammals. The sacrum is very narrow transversely due to poor development of the ventrolateral wing. The auricular surface includes cranial parts of the wing and of the fused vertebral arches as well as the cranial articular process of the first sacral vertebra. In the caudal vertebrae, chevron bones are H-shaped when viewed ventrally, and give attachment to tendons of the caudal muscles. This report describes the relationships between the structural peculiarities of the lower axial skeleton and the locomotive habits of the musk shrew.     

In vivo surface strain and stereology of the frontal and maxillary bones of sheep: implications for the structural design of the mammalian skull.

Does the skull of the sheep behave as a tube or as a complex of independent bones linked by sutures? Is the architecture within cranial bones optimized to local strain alignment? We attempted to answer these questions for the sheep by recording from rosette strain gauges on each frontal and maxillary bone and from single-axis gauges on each dentary of five sheep while they fed on hay. Bone structure was assessed at each rosette gauge site by stereological analysis of high-resolution radiographs. Structural and strain orientations were tested for statistical agreement. Ranges of strain magnitudes were +/-1200 mu epsilon on the mandible, +/-650 mu epsilon on the frontals, and +/-400 mu epsilon on the maxillae. Each gauge site experienced one strain signal when on the working (chewing) side and a different one when on the balancing (nonchewing) side. The two signals differed in mode, magnitude, and orientation. For example, on the working side, maxillary gauges were under mean compressive strains of -132 mu epsilon (S.D., 73.3 mu epsilon), oriented rostroventrally at 25 degrees -70 degrees to the long axis of the skull. On the balancing side, the same gauges were under mean tensile strains of +319 mu epsilon (S.D., 193.9 mu epsilon), at greater than 65 degrees to the cranial axis. Strain patterns on the frontals are consistent with torsion and bending of the whole skull, indicating some degree of tube-like mechanical behavior. Frontal and maxillary strains also showed a degree of individual loading, resulting from modulation of strains across sutures and local effects of muscle activity. The sheep skull seems to behave as a tube made of a complex of independent bones. Structural orientation was in statistically significant agreement with the orientation of working-side compressive principal strain epsilon 2, even though principal tensile strains may be as large or larger. Cranial bone architecture in sheep is not optimized to both strain signals it experiences.     

The blood supply of the papillary muscles of the left ventricle in the Hereford cattle

The blood supply of the Papillary Muscle (PM) of the left ventricle, cranial (PM) and caudal (PM), in Hereford cattle was studied. The results obtained from the dissection of 50 hearts injected with neoprene latex, indicated that the cranial PM is supplied by collateral of the Ramus descendens paraconalis in all cases, alone or together with vessels from the Ramus circumflexus of the Arteria coronaria sinistra. The caudal PM is supplied from arteries coming mainly from the Ramus circumflexus of the A. coronaria sinistra, and also from the Ramus descendens subsinuosus and Ramus descendens paraconalis. When present, the A. diagonalis supplies the cranial PM and in some cases the caudal PM too. Thus this vessel can be termed "artery of the papillary muscle".     

Sex-specific ontogenetic patterns of cranial morphology,theoretical bite force,and underlying jaw musculature in fishers and American martens

The carnivoran cranium undergoes tremendous growth in size and development of shape to process prey as adults and, importantly, these ontogenetic processes can also differ between the sexes. How these ontogenetic changes in morphology actually relate to the underlying jaw musculature and overall bite performance has rarely been investigated. In this study, I examined sex-specific ontogenetic changes in cranial morphology, jaw adductor muscles, and theoretical bite force between subadults and adults in the fisher (Pekania pennanti) and American marten (Martes americana). I found evidence that cranial size alone does not completely explain ontogenetic increases in bite forces as found in other mammalian species. Instead, cranial shape development also drives ontogenetic increases in relative bite force by broadening the zygomatic arches and enlargement of the sagittal crest, both of which enable relatively larger jaw adductor muscles to attach. In contrast, examination of sexual dimorphism within each age-class revealed that cranial shape dimorphism did not translate to dimorphism in either size-corrected bite forces or size-corrected physiological cross-sectional area of the jaw adductor muscles. These results reveal that morphological size and shape variation can have different influences on bite performance depending on the level of intraspecific variation that is examined (i.e. ontogenetic versus sexual dimorphism).     

Analysis of osteon morphotype scoring schemes for interpreting load history: evaluation in the chimpanzee femur

Osteon morphotype scores (MTSs) allow for quantification of mechanically important collagen/lamellar variations between secondary osteons when viewed in circularly polarized ight (CPL). We recently modified the 6-point MTS method of Martin et al. (Martin RB, Gibson VA, Stover SM, Gibeling JC, Griffin LV (1996a) Osteonal structure in the equine third metacarpus. Bone 19, 165-71) and reported superiority of this modified method in correlating with 'tension' and 'compression' cortices of both chimpanzee proximal femoral diaphyses and diaphyses of other non-anthropoid bones that are loaded in habitual bending (Skedros et al. 2009, 2011). In these studies, the 'tension' and 'compression' cortices differed significantly in predominant collagen fiber orientation (CFO) based on weighted-mean gray levels (CFO/WMGLs) in CPL images. In chimpanzee femora, however, some osteons were difficult to score with the 6-point method; namely, 'hybrids' with peripherally bright 'hoops' and variability in alternating rings within the osteon wall. We hypothesized that some of these hybrids would be more prevalent in regions subject to torsion than bending. In this perspective the present study was aimed at expanding our 6-point scoring method (S-6-MTS) into two 12-point methods with six additional morphotypes that considered these hybrids. Three- and 4-point methods were also evaluated. We hypothesized that at least one of these other methods would out-perform the S-6-MTS in terms of accuracy, reliability, and interpreting torsion vs. bending load histories. Osteon morphotypes were quantified in CPL images from transverse sections of eight adult chimpanzee femora (neck, proximal diaphysis, mid-diaphysis), where the mid-diaphysis and base- and mid-neck locations have relatively more complex loading (e.g. torsion + bending) than the proximal diaphysis, where bending predominates. Correlation coefficients between CFO/WMGL and MTSs showed that the S-6-MTS method was either stronger or equivalent to the 12-point methods, and typically stronger than the 3- and 4-point methods for all load environments. In nearly all instances the S-6-MTS is more reliable and accurate when it is applied to cases where interpreting load history requires distinguishing habitual bending from torsion. Consequently, in studies of osteonal adaptations for these load histories the 3- and 4-point methods are not stronger correlates, and the extra time required to assign additional scores in the 12-point methods is both unnecessary and can be highly unreliable.     

Motor units in cranial and caudal regions of the upper trapezius muscle have different discharge rates during brief static contractions

Aim: To compare the discharge patterns of motor unit populations from different locations within the upper trapezius muscle during brief submaximal constant‐force contractions. Methods: Intramuscular and surface electromyographic (EMG) signals were collected from three sites of the right upper trapezius muscle distributed along the cranial‐caudal direction in 11 volunteers during 10 s shoulder abduction at 25% of the maximum voluntary force. Results: A total of 38 motor units were identified at the cranial location, 36 from the middle location and 17 from the caudal location. Initial discharge rate was greatest at the caudal location (P < 0.05; mean ± SD, cranial: 16.7 ± 3.6 pps, middle: 16.9 ± 4.0 pps, caudal: 19.2 ± 3.3 pps). Discharge rate decreased during the contraction for the most caudal location only (P < 0.05). Initial estimates of surface EMG root mean square values were highest at the most caudal location (P < 0.05; cranial: 32.3 ± 20.9 μV, middle: 41.3 ± 21.0 μV, caudal: 51.6 ± 23.6 μV). Conclusion: This study demonstrates non‐uniformity of motor unit discharge within the upper trapezius muscle during a brief submaximal constant‐force contraction. Location‐dependent modulation of discharge rate may reflect spatial dependency in the control of motor units necessary for the development and maintenance of force output.     

In vivo surface strain and stereology of the frontal and maxillary bones of sheep: Implications for the structural design of the mammalian skull

Does the skull of the sheep behave as a tube or as a complex of independent bones linked by sutures? Is the architecture within cranial bones optimized to local strain alignment? We attempted to answer these questions for the sheep by recording from rosette strain gauges on each frontal and maxillary bone and from single‐axis gauges on each dentary of five sheep while they fed on hay. Bone structure was assessed at each rosette gauge site by stereological analysis of high‐resolution radiographs. Structural and strain orientations were tested for statistical agreement. Ranges of strain magnitudes were ±1200 μ? on the mandible, ±650 μ? on the frontals, and ±400 μ? on the maxillae. Each gauge site experienced one strain signal when on the working (chewing) side and a different one when on the balancing (nonchewing) side. The two signals differed in mode, magnitude, and orientation. For example, on the working side, maxillary gauges were under mean compressive strains of –132 μ? (S.D., 73.3 μ?), oriented rostroventrally at 25°–70° to the long axis of the skull. On the balancing side, the same gauges were under mean tensile strains of +319 μ? (S.D., 193.9 μ?), at greater than 65° to the cranial axis. Strain patterns on the frontals are consistent with torsion and bending of the whole skull, indicating some degree of tube‐like mechanical behavior. Frontal and maxillary strains also showed a degree of individual loading, resulting from modulation of strains across sutures and local effects of muscle activity. The sheep skull seems to behave as a tube made of a complex of independent bones. Structural orientation was in statistically significant agreement with the orientation of working‐side compressive principal strain ?2, even though principal tensile strains may be as large or larger. Cranial bone architecture in sheep is not optimized to both strain signals it experiences. Anat Rec 264:325–338, 2001. © 2001 Wiley‐Liss, Inc.     

Expression of the dlx gene family during formation of the cranial bones in the zebrafish (Danio rerio): differential involvement in the visceral skeleton and braincase.

We have used dlx genes to test the hypothesis of a separate developmental program for dermal and cartilage bones within the neuro- and splanchnocranium by comparing expression patterns of all eight dlx genes during cranial bone formation in zebrafish from 1 day postfertilization (dPF) to 15 dPF. dlx genes are expressed in the visceral skeleton but not during the formation of dermal or cartilage bones of the braincase. The spatiotemporal expression pattern of all the members of the dlx gene family, support the view that dlx genes impart cellular identity to the different arches, required to make arch-specific dermal bones. Expression patterns seemingly associated with cartilage (perichondral) bones of the arches, in contrast, are probably related to ongoing differentiation of the underlying cartilage rather than with differentiation of perichondral bones themselves. Whether dlx genes originally functioned in the visceral skeleton only, and whether their involvement in the formation of neurocranial bones (as in mammals) is secondary, awaits clarification.