Increased Osteocyte Lacunae Density in the Hypermineralized Bone Matrix of Children with Osteogenesis Imperfecta Type I

Abstract

Osteocytes are terminally differentiated osteoblasts embedded within the bone matrix and key orchestrators of bone metabolism. However, they are generally not characterized by conventional bone histomorphometry because of their location and the limited resolution of light microscopy. OI is characterized by disturbed bone homeostasis, matrix abnormalities and elevated bone matrix mineralization density. To gain further insights into osteocyte characteristics and bone metabolism in OI, we evaluated 2D osteocyte lacunae sections (OLS) based on quantitative backscattered electron imaging in transiliac bone biopsy samples from children with OI type I (n = 19) and age-matched controls (n = 24). The OLS characteristics were related to previously obtained, re-visited histomorphometric parameters. Moreover, we present pediatric bone mineralization density distribution reference data in OI type I (n = 19) and controls (n = 50) obtained with a field emission scanning electron microscope. Compared to controls, OI has highly increased OLS density in cortical and trabecular bone (+50.66%, +61.73%; both p < 0.001), whereas OLS area is slightly decreased in trabecular bone (−10.28%; p = 0.015). Correlation analyses show a low to moderate, positive association of OLS density with surface-based bone formation parameters and negative association with indices of osteoblast function. In conclusion, hyperosteocytosis of the hypermineralized OI bone matrix associates with abnormal bone cell metabolism and might further impact the mechanical competence of the bone tissue.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

“…The residual bone sample block can be used for further analyses such as quantitative backscattered electron imaging (qBEI) to gain information on mineralized bone volume, the degree of mineralization of the ECM and two-dimensional (2D) osteocyte lacunae sections (OLS) characteristics (Table 2). (143)(144)(145) This technique utilizes the linear correlation between the intensity of the backscattered signals and the calcium content of the ECM. (143,(146)(147)(148)(149)(150) Other methods to analyze bone tissue properties at the microscale and nanoscale, such as small-angle X-ray scattering to determine size and orientation of the mineral particles, (151)(152)(153)(154)(155) and vibrational spectroscopy (Fourier-transform infrared or Raman) (156,157) to investigate the structure of the organic matrix or a mechanical investigation using, eg, nanoindentation, (158) are reported elsewhere.…”

“…For many of these methods, reference values for healthy individuals (children and adults) are available. (141,143,145,152,(159)(160)(161)(162)(163)(164) To elucidate the variability of bone tissue features in different forms of EOOP and to underscore the considerable amount of information obtained from bone biopsy with different analytic methods, we discuss here bone material properties based on the evaluation of transiliac bone biopsy samples in two representative pediatric patients. One patient has EOOP due to a monoallelic WNT1 variant and the other child has typical OI due to a COL1A1 variant leading to a quantitative defect in type I collagen.…”

“…The results have been compared to reference data from healthy children and adolescents. 141,145,161 potentially deleterious and contributes to bone fragility; hypermineralization stiffens the bone material and makes it more brittle, whereas hypomineralization softens it and makes it weaker. BMDD in the patient with the COL1A1 variant reveals a pronounced hypermineralization (BMDD curve shifted to higher mineralization) corresponding to high-turnover OI, whereas a BMDD shift to lower mineralization indicative of low-turnover EOOP is found in the patient harboring the WNT1 variant (Fig.…”

“…(Right) qBEI of a trabecular feature at high magnification (0.88 μm/pixel) to assess the osteocyte lacunae sections (OLS). Scale bar = 100 μm, published under a creative common attribution license (145). (C) Results from the OLS-analysis.…”

Bone Matrix Mineralization and Response to Burosumab in Adult Patients With X-Linked Hypophosphatemia: Results From the Phase 3, Single-Arm International Trial

“…(42) Briefly, qBEI images were captured at a pixel resolution of 0.88 μm/pixel, as done previously to characterize two-dimensional (2D) osteocyte lacunar sections. (43,44) First, the unmineralized core of osteocyte lacunae (that appear black in the qBEI images) were selected by setting a threshold 1.73 wt% Ca. Then, a second threshold was set to capture areas mineralized below 10 wt% Ca around the core of the osteocyte lacunae.…”