Growth- and Sex-Dependent Changes in Bone, Bone Marrow Adipose Tissue, and the Lipid Microenvironment in Healthy C57BL/6 Mice

Bone marrow adipose tissue (BMAT) is increasingly recognized as a metabolically active tissue with direct effects on bone remodelling and hematopoiesis, yet how it develops during periods of rapid skeletal growth, and how sex shapes that development, remains poorly understood. This thesis addresses these gaps through two studies in C57BL/6 mice. The first characterized sex-specific changes in BMAT volume, trabecular bone microarchitecture, and the bone marrow lipidome between 9 and 12 weeks of age. Sex differences in trabecular bone were present at both timepoints, while sex differences in whole bone BMAT and the marrow lipidome were absent at 9 weeks and only emerged by 12 weeks. Notably, whole bone and regulated BMAT (rBMAT) volume diverged in opposite directions between females (expansion) and males (regression) over this growth window. Lipidomic changes with age were broad and driven predominantly by membrane and neutral lipid classes, with sex-specific differences emerging at 12 weeks, mirroring the divergent BMAT trajectories. The second study established sex- and age-specific strain-to-force calibration curves for in vivo tibial compression loading between 6 and 14 weeks of age, assessed at 2-week age intervals. Males required approximately double the force of females to reach target strains at every age tested. While chronological age was not a reliable predictor of required force, body weight showed a moderate positive relationship in males. Together, these findings demonstrate that sex is a critical variable in the study of bone, mechanical loading, and BMAT during growth. Particularly that BMAT dynamics and marrow lipid composition diverge between sexes in ways that must be accounted for in study design. This work provides a foundational characterization of the growing bone marrow niche and the calibration groundwork needed to investigate whether mechanical loading regulates BMAT during this critical developmental window.