New insights in osteocyte imaging by synchrotron radiation

Bone health depends on the bone mineral density and mechanical strength, characterised quantitatively and inferred through qualitative parameters such as the trabecular and cortical micro-architecture, and other parameters describing the bone cells. Among these cells, the osteocyte has been recognised as the orchestrator of bone remodelling, playing a key role in directing osteoblastic and osteoclastic activities. Conventional optical and electron microscopies have greatly improved our understanding of the cell physiology mechanisms involved in different osteoarticular pathophysiological contexts, especially osteoporosis. More recently, imaging methods exploiting synchrotron radiation, such as X-ray tomography, ptychography, and deep ultraviolet and Fourier transform infrared spectroscopies, have revealed new biochemical, chemical and 3D morphological information about the osteocyte lacuna, the surrounding matrix and the lacuna–canalicular network at spatial length scales spanning microns to tens of nanometres. Here, we review recent results in osteocyte lacuna and lacuna–canalicular network characterisation by synchrotron radiation imaging in human and animal models, contributing to new insights in different physiologic and pathophysiological situations.

Scaling of statically derived osteocyte lacunae in extant birds: implications for palaeophysiological reconstruction

Osteocytes are mature versions of osteoblasts, bone-forming cells that develop in two ways: via ‘static’ osteogenesis, differentiating and ossifying tissue in situ to form a scaffold upon which other bone can form, or ‘dynamic’ osteogenesis, migrating to infill or lay down bone around neurovasculature. A previous study regressed the volume of osteocyte lacunae derived from dynamic osteogenesis (DO) of a broad sample of extant bird species against body mass, the growth rate constant ( k ), mass-specific metabolic rate, genome size, and erythrocyte size. There were significant relationships with body mass, growth rate, metabolic rate, and genome size, with the latter being the strongest. Using the same avian histological dataset, we measured over 3800 osteocyte lacunar axes derived from static osteogenesis (SO) in order to look for differences in the strength of form–function relationships inferred for DO-derived lacunae at the cellular and tissue levels. The relationship between osteocyte lacunar volume and body mass was stronger when measuring SO lacunae, whereas relationships between osteocyte lacunar volume versus growth rate and basal metabolic rate disappeared. The relationship between osteocyte lacuna volume and genome size remained significant and moderately strong when measuring SO lacunae, whereas osteocyte lacuna volume was still unrelated to erythrocyte size. Our results indicate that growth and metabolic rate signals are contained in avian DO but not SO osteocyte lacunae, suggesting that the former should be used in estimating these parameters in extinct animals.