An analytical model for elucidating tendon tissue structure and biomechanical function from in vivo cellular confocal microscopy images

Fibered confocal laser scanning microscopes have given us the ability to image fluorescently labeled biological structures in vivo and at exceptionally high spatial resolutions. By coupling this powerful imaging modality with classic optical elastography methods, we have developed novel techniques that allow us to assess functional mechanical integrity of soft biological tissues by measuring the movements of cells in response to externally applied mechanical loads. Using these methods we can identify minute structural defects, monitor the progression of certain skeletal tissue disease states, and track subsequent healing following therapeutic intervention in the living animal. Development of these methods using a murine Achilles tendon model has revealed that the hierarchical and composite anatomical structure of the tendon presents various technical challenges that can confound a mechanical analysis of local material properties. Specifically, interfascicle gliding can yield complex cellular motions that must be interpreted within the context of an appropriate anatomical model. In this study, we explore the various classes of cellular images that may result from fibered confocal microscopy of the murine Achilles tendon, and introduce a simple two-fascicle model to interpret the images in terms of mechanical strains within the fascicles, as well as the relative gliding between fascicles.