Osteocyte Lacuna Population Densities in Sheep, Elk and Horse Calcanei

Abstract

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/mm² 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.

Spatial Distribution of Osteocyte Lacunae in Equine Radii and Third Metacarpals: Considerations for Cellular Communication, Microdamage Detection and Metabolism

“…In view of other investigators' fi ndings that have shown that at least a portion of remodeling is targeted, we presume that other mechanotransduction pathways predominate in the equine radius and MC3. As discussed in our companion study [Skedros 2005], these can include innervation of bone and changes in the dendritic processes of osteocytes within canaliculi that are not associated with osteocyte death and changes in osteocyte lacunar density. However, we were only able to test some hypotheses indirectly; therefore, care must be taken when considering the present data in these contexts.…”

“…However, the MC3s and radii exhibited opposite Ot.Lc.N/B.Ar differences in 'tension' versus 'compression' comparisons. As discussed in a companion study of regional Ot.Lc.N/B.Ar variations in artiodactyl and perissodactyl calcanei [Skedros, 2005], this difference may represent 'adaptations' in the relatively simple bending environment of the radius that are obscured by tissue adaptations of the more complex torsion/bending environment of the MC3.…”

Comparison of structural, architectural and mechanical aspects of cellular and acellular bone in two teleost fish

“…Structurally, the tissues are easily visibly distinguished from one another by the presence and absence of osteocytes, but are more similar to each other in gross, histological morphology than they are to the more-studied cellular bone types of amniotes. In particular, differences in osteocyte presence and associated canalicular density are striking, even for the cellular boned carp where osteocytes are much less densely packed than those of birds and mammals (424 vs 700-1800cellsmm -2 ) (Li et al, 1991;Skedros, 2005). The low osteocyte density, coupled with the apparent relative scarcity of blood vessels in both tilapia and carp bone, make both fish bone types observed here appear, at lower magnifications, as more solid (less porous) blocks of tissue compared with amniote bone, also suggesting they may have a lower metabolic rate.…”

“…elliptical in shape with major and minor diameters of 10 and 5m, respectively (McCreadie et al, 2004;Hannah et al, 2010)] the area fraction taken by the osteocytic lacunae in carp bones is 1.6%. The osteocytic lacunae (black arrows in Fig.3C,E) in the bones of the carp show distinct canalicular networks interconnecting adjacent lacunae (black arrowhead in Fig.3E); however, their distribution is less dense than in mammals (R.S., personal observation) (Skedros, 2005;Li et al, 1991).…”

Are Distributions of Secondary Osteon Variants Useful for Interpreting Load History in Mammalian Bones?

“…The sheep and horses had been kept in large pastures. Calcanei were also obtained from 10 wild-shot skeletally mature male deer used in previous studies [Skedros et al, 1994a, b;Skedros, 2005].…”