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‐se...

SummaryProximate mechanisms involved in forming extracellular matrix (ECM) variations within and between bones are not yet clear. Deficiencies in the collective understanding of details required to illuminate the process that forms a highly ordered ECM are exposed when considering that there is still significant debate as to the importance of cellular control in the assembly of the ECM vs. the observation of collagen fibrillar “self-assembly” (i. e., occurring devoid of cells). We examined data and opinions with respect to possible mechanisms involved in the formation of distinctly different ECM patterns of secondary osteon morphotypes (SOMs). Important considerations include: (1) stretch within the osteoid during fibrillogenesis, (2) various mechanotransduction mechanisms, and (3) whether or not the formation of regional variations in osteonal ECMs requires osteo blast alignment and/or rotation and migration. We propose that primary cilia of osteoblasts and osteocytes have an important role in their perception of variant-related (vectorial) stimuli, which is deemed essential in the genesis of distinctive and mechanically relevant ECM patterns of SOMs.

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

SummaryDespite being encased in lacunae, osteocytes are extensively interconnected and have several mechanisms that enable them to physically and chemically appraise their environment and adjust to it. In the perspective that cell-cell and cell-matrix interactions mediate these functions and are critically important during the formation of a mechanically competent bone organ, we focus on several considerations: (1) osteocyte lacunae are not always occupied by living cells and the percent lacuna vacancy can increase with aging, some diseases, and experimental perturbations, (2) the potential for the population density and/or sizes and shapes of osteocytes (or of their lacunae) and of their cell processes (typically seen as the canaliculi in which they reside) in helping investigators interpret the load history of a bone or bone region, and (3) scaling relationships between osteocyte density and various parameters, including animal mass and metabolism. We also point out that all of these considerations are being impacted by high-resolution three-dimensional imaging technologies that allow increased accuracy when quantifying details of lacunar-canalicular geometries.