Male mice housed in groups engage in frequent fighting and show a lower response to additional bone loading than females or individually housed males that do not fight

Abstract

Experiments to investigate bone's physiological adaptation to mechanical loading frequently employ models that apply dynamic loads to bones in vivo and assess the changes in mass and architecture that result. It is axiomatic that bones will only show an adaptive response if the applied artificial loading environment differs in a significant way from that to which the bones have been habituated by normal functional loading. It is generally assumed that this normal loading is similar between experimental groups. In the study reported here we found that this was not always the case. Male and female 17-week-old C57BL/6 mice were housed in groups of six, and a single episode (40 cycles) of non-invasive axial loading, engendering 2,200 με on the medial surface of the proximal tibiae in sample mice, was applied to right tibiae on alternate days for two weeks. This engendered an adaptive increase in bone mass in females, but not males. Observation revealed the main difference in behaviour between males and females was that males were involved in fights 1.3 times per hour, whereas the females never fought. We therefore housed all mice individually. In females, there was a similar significant osteogenic response to loading in cortical and trabecular bone of both grouped and individual mice. In contrast, in males, adaptive increases in the loaded compared with non-loaded control bones was only apparent in animals housed individually. Our interpretation of these findings is that the frequent vigorous fighting that occurs between young adult males housed in groups could be sufficient to engender peak strains and strain rates that equal or exceed the stimulus derived from artificial loading. This indicates the importance of ensuring that physical activity is consistent between groups. Reducing the background level of the naturally engendered strain environment allows adaptive responses to artificial loading to be demonstrated at lower loads.

Fig. 1

Group housing stimulates an adaptive osteogenic response, masking the effect of artificial loading, in the tibiae of male but not female mice. A: Representative μCT images of trabecular and cortical regions of the left control and right loaded tibiae in male mice. B: Representative 3D reconstruction of the mouse tibia; arrows indicate the direction of artificial loading. C & D: The effect of individual housing on the percentage change [(right–left)/left x 100], due to loading in trabecular (C) and cortical (D) bone compartments in male and female mice. Data represented as mean + SEM. BV/TV = bone volume fraction; Tb.Th = trabecular thickness; Ct.Ar = cortical bone area; Tt.Ar = total cross-sectional area inside the periosteal envelope. Asterisks indicate a significant difference due to housing within each sex: *p < 0.05, ***p < 0.001. The apparent response to loading was reduced in grouped, relative to individual, male mice.