Regional assessment of male murine bone exposes spatial heterogeneity in osteocyte lacunar volume associated with intracortical canals and regulation by VEGF
The porous bone cortex comprises an interconnected network of intracortical vascular canals and osteocyte lacunae, embedded within the bone mineral. Increases in cortical porosity reduce bone strength and increase fracture risk. To date, our understanding of mechanisms coupling the arrangements of the vascular: lacunar network in the bone cortex is poorly understood yet it could be key in establishing regulation of cortical porosity evident with age. Using synchrotron radiation-based computed tomography we develop automated tools to characterise the 3D spatial organisation and morphology of osteocyte lacunae, and the bone vasculature at the tibiofibular junction (TFJ), defining posterior, medial, lateral, and anterior regions in male C57BL/6 mice (n = 3). We also investigate the role of osteoblast-derived VEGF in regulating the 3D spatial arrangement by conditional disruption of VEGF in osteocalcin-expressing cells (OcnVEGFKO versus WT, n = 3). Regional lacunar phenotypes were assessed by 3D distance mapping of lacunar organisation surrounding the vascular compartments, including endosteal and periosteal surfaces, or intracortical canals. Surface-associated lacunae were indistinct in size across posterior, medial, lateral and anterior regions. However, lacunae associated with intracortical canals were significantly larger exclusively within the posterior region. In the absence of VEGF, the increased lacunar volume associated with posterior intracortical canals was lost. Our results suggest that the influence of intracortical canals on lacunar volumes is spatially regulated and sensitive to locally produced growth factors such as osteoblast-derived VEGF.