Accessing osteocyte lacunar geometrical properties in human jaw bone on the submicron length scale using synchrotron radiation μCT

摘要

The architectural properties of the osteocyte cell network provide a valuable basis for understanding the mechanisms of bone remodelling, mineral homeostasis, ageing and pathologies. Recent advances in synchrotron microtomography enable unprecedented three-dimensional imaging of both the bone lacunar network and the extracellular matrix. Here, we investigate the three-dimensional morphological properties of osteocyte lacunae in human healthy and bisphosphonate-related osteonecrotic jaw bone based on synchrotron X-ray computed tomography images, with a spatial isotropic voxel size of 300 nm. Bisphosphonate-related osteonecrosis of the jaw is a relatively new disease with increasing incidence, which remains poorly understood. A step forward in elucidating this malady is to assess whether, and how, the morphology of the osteocyte lacunar network is modified in the affected jaw tissue. We evaluate thousands of cell lacunae from five specimens of which three originate from patients diagnosed with bisphosphonate-associated osteonecrosis. In this exploratory study, we report three-dimensional quantitative results on lacunar volumes (296-502 μm3), shape (approximated by an ellipsoidal shape with principal axes a b c, such that a = 2.2b and a = 4c) and spatial distribution (i.e., 50% of the mineralized matrix volume is located within 12 μm to the closest lacunar boundary) at submicron resolution on such specimens. We observe that the average lacunar volumes of the bisphosphonate-related osteonecrotic jaw specimens were within the range of volumes found in the two specimens originating from healthy donors and conclude that lacunar volumes are not the key element in the course of bisphosphonate-related osteonecrotic jaw. In three out of five specimens we observe lacunar volume sizes in segmented osteons to be significantly different compared to lacunar volumes in the adjacent tissue regions. Furthermore, we quantify the number of lacunae containing small dense objects (on average 9%). In contrast to lacunar morphology we report the lacunar density (16 000-50 000 per mm3) to be different in jaw bone tissue compared to what has been reported in femoral sites. Lay Description: Under healthy conditions bone tissue undergoes a permanent remodeling process, which allows bones to adapt their structure to the governing loading condition. Remodeling is believed to be regulated by osteocytes, which are the most abundant bone cells located in small cavities of the mineralized matrix. Lacunae are ellipsoidal shaped with their three axis only being about 5, 10, and 20 μm long. Osteocytes are interconnected and communicate via cell processes extending in slender tunnels called canaliculi. Regulation of remodeling is accomplished by the ability of these cells to sense mechanical signals (strain). In this process, the shape, size, and spatial distribution of the lacunae might play an important role. In addition osteocytes have been reported to be able directly involved in the homeostasis metabolism of mineral by actively remodeling bone tissue in their direct environment. Imaging of osteocytes lacunae requires modalities which have high enough resolution to capture the spatial details and which are not limited by the dense mineralized tissue these cavities are embedded in. X-rays-based approaches have been used with success towards this end, but only a few anatomical sites have been investigates so far. In this exploratory study we investigate the lacunar properties of specimens originating from two healthy donors after wisdom tooth removal and three samples originating from patients suffering from necrosis of the jaw as side effect of highly-dosed bisphosphonate treatment. We report the lacunar density to range from about 16 000 per mm3 up to 50 000 per mm3 with no obvious differences for the BRONJ samples. However, these densities are higher than the range reported for the femoral site.