Second-Harmonic Generation Imaging of Self-Assembled Collagen Gels
Cell-seeded collagen hydrogels are particularly amenable to analysis by three-dimensional LSM, including SHG imaging, since the tissues may be imaged nondestructively at any time during in vitro culture [5,24,56-61]. Collagen self-assembly is controlled by polymerization conditions that inuence polymer aggregation, creating gels of varied microstructures and network properties [21,22,62-68]. e cells seeded in or on such hydrogel scaolds create tissue constructs that change dynamically due to force interactions between cells and the surrounding scaold, and by proteolysis and new matrix deposition. Dynamic remodeling of cell-seeded collagen gels may be tracked in four dimensions by assembling z-stacks of SHG image frames from tissues at dierent locations and culture time points. Other optical signals, such as reectance , optical coherence , one-and two-photon uorescence (TPF) [56,71], and coherent anti-Stokes Raman scattering (CARS)  may be imaged simultaneously or nearly on a multimodal platform, increasing the structural, biochemical, and optical information derived from the imaged tissue regions and allowing the study of interactions between the signal-producing species within the tissue. SHG imaging of acellular and cell-seeded self-assembled gels is a powerful technique to address the fundamental questions regarding tissue mechanics and cell behavior within a threedimensional matrix environment.