Blood vessel and osteocyte networks concurrently rearrange during bone maturation and decline during aging in the femur of male mice

While blood vessels and osteocytes have been studied independently, their simultaneous changes with age remain undescribed. Our objective was to investigate the age-related evolution of both osteocyte and blood vessel networks in mouse cortical bone, and to assess the associated effects on osteocyte markers and oxygen intracellular levels. We analyzed femurs of male Flk1-GFP mice from growing, mature, middle-aged, and aged groups with techniques such as laser microdissection followed by RT-qPCR, tissue clearing and 3D fluorescence imaging. In the mature animals - when the cortical bone was thicker than in the growing animals - the osteocyte density, the number of dendrites per osteocyte and the blood vessel density were lower. This was associated with a reduced expression of <i>Pdpn</i> and with a smaller fraction of osteocytes exhibiting low intracellular oxygen. In aged animals - when cortical bone was thinner than in mature animals - the number of dendrites per osteocyte and the blood vessel density were lower. This was associated with a reduced <i>Gja1</i> (Cx43) expression. Our results suggest that changes in the osteocyte network during maturation and aging are led by distinct mechanisms, and that the cortical bone blood vessels are not the main source of oxygen for osteocytes.