Experimental Animal Research Longitudinally Investigating the Onset Timing of Left-Right Asymmetry in the Femur and Pelvis

Author(s): Shunichi Uetake, Toru Negishi, Yutaro Natsuyama, Mingshou Zhang, Ting Yang, Shuang-Qin Yi

Background: The external appearance of the human body appears to be a bilaterally symmetrical structure, whereas most internal organs are distributed asymmetrically. Although previous studies have demonstrated skeletal asymmetry, few studies have described the causes and timing of left-right differences. The aim of this study was to explore when left-right differences in the femur occur and discuss why they occur.

Methods: SD rats (n = 14) were divided into a complex activity (CA) group and a general activity (GA) group, according to the activity environment. In data collected every 2 weeks from 4 to 16 weeks of age using digital mammography, femoral length was measured at 4 positions and pelvic length at 2 positions. Statistical processing was conducted in 2 groups to investigate left-right differences at each age and the timing of the onset of change.

Results: In terms of femoral length, the left femur was longer than the right in both the CA and GA groups at all ages. However, no difference was found in the timing of left- right differences between the 2 groups, even in the presence of different amounts of exercise. Furthermore, this asymmetry did not widen or diminish halfway through but had already occurred at 4 weeks of age.

Conclusions: This study found that asymmetry of the femur occurred as early as 4 weeks of age in rats, and this asymmetry did not change due to the environment. The extent to which asymmetry affects movement may indicate an opportunity to consider that asymmetry exists and is a natural phenomenon.