ABSTRACT
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3.1 Introduction Tissue engineering involves an attempt to replace natural tissues and organs of the body with synthetic replacements that closely resemble the original tissue. Oen it is hoped that the body will eventually remodel the implanted construct into something that is functionally identical to native tissue, and this means successfully regenerating both the cellular and extracellular components of the tissue. e dominant component of the extracellular matrix (ECM) is the protein collagen. ere are over 20 isoforms of this structural protein, of which types I, II, and IV are most abundant in mammalian tissues. Collagen gives tissues their tensile strength, and the exceptional mechanical properties are closely associated with the ability of this protein to form ordered, semicrystalline domains. Much eort in tissue engineering is thus devoted to inducing the production of various collagen types by cells such as broblasts. Chemical stimuli such as ascorbate (Franceschi et al. 1994) and mechanical stimuli such as applied strain and/ or scaold ber alignment (Lee et al. 2005) are known to upregulate collagen production by broblasts and osteoblasts. To assess the extent to which the cells are manufacturing structurally organized collagen of the correct type, one would ideally like a noninvasive imaging tool that is sensitive to collagen levels and also to the degree of crystallinity of the molecules. Over the past decade, second harmonic generation (SHG) microscopy has seen a resurgence in interest as a tool to study collagen (and some other) ECM molecules. e chief dening features of the technique that account for this are as follows:
1. e technique is based on optical microscopy and so has a spatial resolution measured in hundreds of nanometers.
