Functional Significance of Cortical Bone Microstructure

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OBJECTIVE: The preeminent bone biologist Harold Frost noted that “the skeleton inscribes within itself a record of its genetics, growth, development, use and abuse.” The dynamic yet enduring nature of bone thus provides a window on the life history of organisms both extant and extinct. Despite this potential we have only a rudimentary understanding of the significance of bone microstructure and this is particularly so for cortical bone – the dense outer shell of bones. Indeed, we have just begun to characterize this complex tissue in three dimensions (3D) – an innovation facilitated by high resolution imaging. To address this knowledge gap the overarching goal of my research program is to decipher the information encoded within the microstructure of cortical bone. Are different types of loading reflected in different microstructural patterns? Are these adaptations phylogenetic, 'built-in' during growth through a process known as 'modeling'? Do they arise during the ontogeny of an organism through a process of turnover known as 'remodeling'? How do these processes interact? By answering such questions my research program will shed new light on the adaptive processes of bone and thereby enable more detailed and robust interpretations of life history from the skeleton. APPROACH: My team has been at the forefront of the application of high resolution 3D micro-Computed Tomography (micro-CT) imaging to obtain new insights into cortical bone microstructure. In the current proposal this work is extended and expanded. Two Research Streams will be pursued: STREAM 1: An expanded application of 3D techniques to execute ex vivo comparative studies. A specific example will be the examination of adaptations associated with flight – a mode of locomotion which has been linked to unique modeling-based microstructural patterns in birds. The hypothesis driving this work is that the convergent evolution leading to flight in birds and mammals has resulted in similar cortical bone features. This study will be the first such comparison of birds and flying mammals (bats). STREAM 2: The extension of 3D analysis of cortical bone remodeling into the realm of in vivo longitudinal imaging. In 2013 my group established proof-of-principle for in vivo imaging of cortical bone porosity in the rat. This was achieved utilizing Canada’s national synchrotron facility – the Canadian Light Source (CLS). Building upon this innovation, we will pioneer longitudinal tracking of remodeling events. This will provide a direct means of testing hypotheses related to the spatial regulation of bone turnover. Our first objective will be to examine the postulated relations between loading, microdamage and the induction/progression of remodeling events. Specifically, we will test the hypotheses that remodeling events are aligned by mechanical conditions within the bone and that they are actively ‘steered’ towards microdamage. The further development and application of this novel in vivo platform for the study of the spatial regulation of remodeling will represent a significant advance, providing unique empirical data to a field which has, to date, been largely theoretical. SIGNIFICANCE: Bone exists in 3D and remodels over time – it is a four dimensional (4D) tissue. Two-dimensional analysis is thus limited in its ability to fully characterize this tissue. Through advanced imaging the currently proposed research program will generate new insights into the phylogenetic and ontogenetic adaptation of cortical bone. This is significant for our understanding of bone microstructure in both present and past species. The data generated will thus have wide ranging applications in bone biology and its various sub-disciplines spanning from materials engineering to palaeontology.