Synergies between microstructure imaging modalities

Light- and electron-microscopy have provided insights into the relationships between the structure and organisation of the extracellular matrix, of its normal physiological functions and of the changes occurring during growth and degeneration. However, current biological questions such as the micro-scale relationships between matrix architecture and mechanics, biomechanical interactions between cells and matrix, and the nature of the interactions between hard and soft tissue expose the limitations of these classical imaging techniques. Recently a number of novel biophotonic imaging modalities have become available and this review will consider emerging biophotonic techniques that have the potential to not only advance fundamental understanding, but also form a basis for the next generation of fast, safe and low cost clinical medical imaging devices. In the visible-light regime, multiphoton techniques such as second harmonic generation (SHG) and two photon fluorescence (TPF) offer sub-micron spatial resolution and sufficient depth penetration to allow the examination of unfixed mm-sized tissue specimens. Imaging the SHG emitted by fibrous proteins such as collagen and myosin allows quantitative analysis of their structure and organisation at the sub-micron resolution. Polarisation resolved SHG has allowed researchers to probe ECM organisation far below the optical resolution limit, and begin to explore the matrix response to mechanical load. TPF provides exquisitely sensitive and specific visualisation of elastin fibres and of a number of intracellular markers of metabolism. Complementing this structural information, a wide range of markers of matrix and cellular biochemistry can be mapped from the spectrum of inelastically scattered light. Raman and Brillouin microscopy provide micron-scale maps of chemical composition and the mechanical stiffness tensor respectively, without the need for exogenously labels or an external force applied to the tissue. Many of these optical techniques can be implemented simultaneously on a single microscope platform and efforts to develop minimally invasive endoscopic probes which would facilitate their application in vivo, are well advanced. In the X ray regime, the ability of small and wide angle scattering to provide structural information on the fibrous proteins from the molecular up to the microscopic scale is now well established. Fourth generation synchrotron sources will provide timely advances in imaging speed and spatial resolution, sufficient to allow studies on relatively large specimens and to observe the changes arising from modifications in the physical environment. Other advances such as 3D phase contrast X ray microtomography are also closing the technological gap between visible and X ray regimes: SAXS tensor tomography already allows resolution of molecular organisation in intact tissue in the form of 3D voxel maps with micron-level spatial resolution. The convergence of pressing biological problems and optical methodologies with which to address them augurs well for the future of fundamental research in our field. In addition, new minimally invasive methodologies such as spatially offset Raman scattering and delivery of this and other modalities using optical fibre technology offer the prospect of measurements in intact joints