Lumbar Vertebral Body Bone Microstructural Scaling in Small to Medium‐Sized Strepsirhines

Abstract

Bone mass, architecture, and tissue mineral density contribute to bone strength. As body mass (BM) increases any one or combination of these properties could change to maintain structural integrity. To better understand the structural origins of vertebral fragility and gain insight into the mechanisms that govern bone adaptation, we conducted an integrative analysis of bone mass and microarchitecture in the last lumbar vertebral body from nine strepsirhine species, ranging in size from 42 g (Microcebus rufus) to 2,440 g (Eulemur macaco). Bone mass and architecture were assessed via mCT for the whole body and spherical volumes of interest (VOI). Allometric equations were estimated and compared with predictions for geometric scaling, assuming axial compression as the dominant loading regime. Bone mass, microarchitectural, and vertebral body geometric variables predominantly scaled isometrically. Among structural variables, the degree of anisotropy (Tb.DA) was the only parameter independent of BM and other trabecular architectural variables. Tb.DA was related to positional behavior. Orthograde primates had higher average Tb.DA (1.60) and more craniocaudally oriented trabeculae while lorisines had the lowest Tb.DA (1.25), as well as variably oriented trabeculae. Finally, lorisines had the highest ratio of trabecular bone volume to cortical shell volume ($3x) and while there appears to be flexibility in this ratio, the total bone volume (trabecular þ cortical) scales isometrically (BM 1.23 , r 2 ¼ 0.93) and appears tightly constrained. The common pattern of isometry in our measurements leaves open the question of how vertebral bodies in strepsirhine species compensate for increased BM. Anat Rec,

Trabecular architecture in the humeral metaphyses of non‐avian reptiles (Crocodylia, Squamata and Testudines): Lifestyle, allometry and phylogeny

“…Trabecular thickness (Tb.Th) and spacing (Tb.Sp) scales with a significant negative allometry in squamates and in turtles. This same significant allometry was also widely found in mammals (Barak, Lieberman, & Hublin, ; Cotter, Simpson, Latimer, & Hernandez, ; Doube et al, ; Fajardo et al, ; Ryan & Shaw, ) and in birds (Doube et al, ).…”

“…BV/TV was also reported to be independent of body size in mammals, through various sampled bones in a wide variety of taxa (Barak, Lieberman, & Hublin, ), but also in more restricted samplings: in thoracic vertebrae of hominoids (Cotter et al, ) or in fore limb epiphyses of xenarthrans (Amson et al, ). However, other analyses have found that BV/TV had a positive allometry in the femoral epiphyses of mammals and birds (Doube et al, ), in the lumbar vertebrae of strepsirrhine primates (Fajardo et al, ), in the humeral and femoral heads of primates (Ryan & Shaw, ) and in the femoral heads of sciuromorphs (Mielke et al, ). For BV/TV, isometry or positive allometry do not seem to be associated to a particular bone (both present in vertebrae and limbs bones) or clade (both present throughout mammals), but this parameter might be correlated to other characteristics, such as locomotor postures.…”

“…Trabecular number (Tb.N) scales with a positive allometry in squamates and in turtles. This parameter has shown a significant positive allometry in mammals (Barak, Lieberman, & Hublin, ; Cotter et al, ; Fajardo et al, ; Ryan & Shaw, ).…”

Connexin 43 Channels Are Essential for Normal Bone Structure and Osteocyte Viability

“…For trabecular bone, a grayscale value of 80 in a set of 8‐bit slices was set as the threshold. After thresholding, the bone volume/total volume (BV/TV; %), trabecular thickness (Tb.Th; mm), trabecular spacing (Tb.Sp; mm), and trabecular number (Tb.N; mm −1 ) were quantified . Cortical bone structure was analyzed over 50 slices centered at the 55% of length (from proximal to distal) position in the tibial diaphysis.…”

A potential mechanism for allometric trabecular bone scaling in terrestrial mammals

“…Primates might follow a different scaling pattern, as another primate study found that trabecular bone scales isometrically in vertebral bodies (Fajardo et al. ). Nevertheless, most of the studies indicate negative allometry in mammalian trabecular bone.…”