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.

Knowledge regarding the steps and mechanisms related to the intra- and interchain cross-linking of collagen and elastin has evolved steadily during the past 30 years. Recently, effort has been directed at identifying the location and types of cross-links that are found in collagen and elastin. There are two major groups of cross-links: those initiated by the enzyme lysyl oxidase and those derived from nonenzymatically glycated lysine and hydroxylysine residues. The formation of enzymatic cross-links depends on specific enzymes, amino acid sequences, and quaternary structural arrangements. The cross-links that are derived nonenzymatically occur more adventitiously and are important to pathobiological processes. Considerable progress has been made in elucidating the pathways of synthesis for several of the enzymatically mediated cross-links, as well as possible mechanisms regulating the specificity of cross-linking. Although less is known about the chemistry of cross-links arising from nonenzymatically glycated residues, recent progress has also been made in understanding possible biosynthetic pathways and control mechanisms. This review focuses on such progress and hopes to underscore the biological importance of collagen and elastin cross-linking.

Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross section of the rat-tail tendon fascicle. The signal intensity depends both on the collagen organization and the backscattering efficiency. This implies that we cannot use intensity measurements alone to characterize collagen structure. However, we can infer structural information from the polarization dependence of the second harmonic signal. Axial and transverse scans for different linear polarization angles of the input beam show that second harmonic generation (SHG) in the rat-tail tendon depends strongly on the polarization of the input laser beam. We develop an analytical model for the SHG as a function of the polarization angle in the rat-tail tendon. We apply this model in determining the orientation of collagen fibrils in the fascicle and the ratio gamma between the two independent elements of the second-order nonlinear susceptibility tensor. There is a good fit between our model and the measured data.

The second-harmonic signal in collagen, even in highly organized samples such as rat tail tendon fascicles, varies significantly with position. Previous studies suggest that this variability may be due to the parallel and antiparallel orientation of neighboring collagen fibrils. We applied high-resolution second-harmonic generation microscopy to confirm this hypothesis. Studies in which the focal spot diameter was varied from approximately 1 to approximately 6 microm strongly suggest that regions in which collagen fibrils have the same orientation in rat tail tendon are likely to be less than approximately 1 microm in diameter. These measurements required accurate determination of the focal spot size achieved by use of different microscope objectives; we developed a technique that uses second-harmonic generation in a quartz reference to measure the focal spot diameter directly. We also used the quartz reference to determine a lower limit (dXXX > 0.4 pm/V) for the magnitude of the second-order nonlinear susceptibility in collagen.

The molecular origins of second-order nonlinear effects in type I collagen fibrils have been identified with sum-frequency generation vibrational spectroscopy. The dominant contributing molecular groups are: 1), the methylene groups associated with a Fermi resonance between the fundamental symmetric stretch and the bending overtone of methylene; and 2), the carbonyl and peptide groups associated with the amide I band. The noncentrosymmetrically aligned methylene groups are characterized by a distinctive tilt relative to the axis perpendicular to the main axis of the collagen fiber, a conformation producing a strong achiral contribution to the second-order nonlinear effect. In contrast, the stretching vibration of the carbonyl groups associated with the amide I band results in a strong chiral contribution to the optical second-order nonlinear effect. The length scale of these chiral effects ranges from the molecular to the supramolecular.

These structural modifications are representative of changes that occur in pathophysiologic conditions such as thermal injury, diabetes, tumor invasion, and abnormal wound healing. The ability to assess these changes rapidly and noninvasively has considerable clinical applicability. SHG analysis might provide a unique tool for monitoring these structural changes of collagen.

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Mice fed a chemically defined diet devoid of pyrroloquinoline quinone (PQQ) grew poorly, failed to reproduce, and became osteolathyritic. Moreover, severely affected mice had friable skin, skin collagen that was readily extractable into neutral salt solutions, and decreased lysyl oxidase. The identification of functional defects in connective tissue and the growth retardation associated with PQQ deprivation suggest that PQQ plays a fundamental role as a growth factor or vitamin.