Do different locomotor modes during growth modulate trabecular architecture in the murine hind limb?

Vertebrate morphologists often implicate functional adaptations of bone to mechanical milieus when comparing animals with distinct behavioral repertoires. Functional morphologists frequently use comparative osteology and locomotor behavior to construct correlative form-function relationships. While some experimental work has investigated functional adaptations of bone elicited by specific locomotor behaviors, these studies usually manipulate repertoires by introducing artificial situations (e.g., treadmills) or creating differences in the level of activity (i.e., exercise), either of which can compromise extrapolations to free-ranging animals. Here, we present trabecular bone morphology and microarchitecture from an inbred mouse model in which components of naturalistic locomotor repertoires were accentuated. Using inbred mice, we control for genetic variability, further isolating the osteogenic responses to these behaviors. Single female (BALB/cByJ) mice (n = 10 per group) were housed for 8 weeks beginning at 30 days postbirth in custom-designed cages that accentuated either linear quadrupedalism or turning. Concurrently, mice in a control group were housed singly in open cages. The distal femoral metaphysis was scanned by micro-computed tomography at the end of the 8-week experiment protocol. The experimental groups, particularly the "linear" group, differed significantly from the control group (simulated "free-ranging" condition) in several variables: bone volume fraction ("linear" 42% less than controls; "turning" 24% less than controls), trabecular number ("linear" 12% less than controls; "turning" 9% less than controls), connectivity density ("linear" 43% less than controls; "turning" 35% less than controls), and a characterization of trabecular surfaces ("linear" 15% greater than controls; "turning" 11% greater than controls). No differences in the degree of anisotropy were observed among groups, and generally, "linear" and "turning" groups did not differ significantly from one another in any measures of trabecular microarchitecture. Considering the distinct differences in locomotor behaviors between the "linear" quadrupedalism and "turning" groups, these data suggest that comparisons at the distal femoral metaphysis of trabecular microarchitecture or orientation between different groups of animals may be somewhat limited in accurately reconstructing the loading conditions associated with different locomotor modes.