Monitoring dynamic collagen reorganization during skin stretching with fast polarization‐resolved second harmonic generation imaging

Fibril-forming collagens are synthesized in precursor form, procollagens, with N- and C-terminal propeptide extensions. The C-propeptides direct chain association during intracellular assembly of the procollagen molecule from its three constituent polypeptide chains. Following or during secretion into the extracellular matrix, propeptides are cleaved by specific procollagen proteinases, thereby triggering fibril formation. The recent determination of the low-resolution structure of the C-propeptide trimer gives insights into the mechanism of procollagen chain association. In the extracellular matrix, the procollagen C-propeptides ensure procollagen solubility, while persistence of the N-propeptides controls fibril shape. Mechanisms for the control of fibril diameter are reviewed in terms of the radial packing model for collagen fibril structure. Finally, procollagen molecules have recently been shown to undergo liquid crystalline ordering in solution, prior to fibril assembly. This may provide an explanation for the liquid crystal-like suprafibrillar architectures of different connective tissues. (c) 2002 Elsevier Science (USA).

The collagen superfamily is highly complex and shows a remarkable diversity in molecular and supramolecular organization, tissue distribution and function. However, all its members share a common structural feature, the presence of at least one triple-helical domain, which corresponds to a number of (Gly-X-Y)n repeats (X being frequently proline and Y hydroxyproline) in the amino acid sequence. Several sub-families have been determined according to sequence homologies and to similarities in the structural organization and supramolecular assembly. In the present review, we focus on the newly described fibrillar collagens, fibrillar-associated collagens with interrupted triple helix, membrane collagens and multiplexins. Recent advances in the characterization of proteins containing triple-helical domains but not referred to as collagens are also discussed.

Collagen possesses a strong second-order nonlinear susceptibility, a nonlinear optical property characterized by second harmonic generation in the presence of intense laser beams. We present a new technique involving polarization modulation of an ultra-short pulse laser beam that can simultaneously determine collagen fiber orientation and a parameter related to the second-order nonlinear susceptibility. We demonstrate the ability to discriminate among different patterns of fibrillar orientation, as exemplified by tendon, fascia, cornea, and successive lamellar rings in an intervertebral disc. Fiber orientation can be measured as a function of depth with an axial resolution of approximately 10 microm. The parameter related to the second-order nonlinear susceptibility is sensitive to fiber disorganization, oblique incidence of the beam on the sample, and birefringence of the tissue. This parameter represents an aggregate measure of tissue optical properties that could potentially be used for optical imaging in vivo.