Analyzing the feasibility of discriminating between collagen types I and II using polarization‐resolved second harmonic generation

Abstract

According to previous studies, the nonlinear susceptibility tensor ratio χ /χ obtained from polarization-resolved second harmonic generation (P-SHG) under the assumption of cylindrical symmetry can be used to distinguish between fibrillar collagen types. Discriminating between collagen fibrils of types I and II is important in tissue engineering of cartilage. However, cartilage has a random organization of collagen fibrils, and the assumption of cylindrical symmetry may be incorrect. In this study, we simulated the P-SHG response from different collagen organizations and demonstrated a possible method to exclude areas where cylindrical symmetry is not fulfilled and where fibrils are located in the imaging plane. The χ /χ -ratio for collagen type I in tendon and collagen type II in cartilage was estimated to be 1.33 and 1.36, respectively, using this method. These ratios are now much closer than what has been reported previously in the literature, and the larger reported differences between collagen types can be explained by variation in the structural organization.

Polarization‐dependent second‐harmonic generation for collagen‐based differentiation of breast cancer samples

“…39 Samples that present more organized collagen structures lead to the increment of B values. 40,41 This trend is also observed in studies, where anisotropy coefficient β is calculated and measured. 42 Angle δ in Equation 2 corresponds to the angle between laser's propagation and fiber's axes.…”

“…The calculated B values tend to the unit when σ increases, and this trend is independent of B th. 41 At the same time for σ <20 , R 2 remains close to the unit ( Figure 6B). Therefore, up to that point, PSHG data seem to fit perfectly to the biophysical model.…”

“…For fully isotropic arrangement of fibrils the expected value of the fiber anisotropy parameter is close to the unit, while increasing along with the alignment of the fibrils across the fiber axis. 40,41 For verifying this hypothesis, the effects of angles f and δ orientation disorders on the calculated from the PSHG analysis anisotropy parameter B values were simulated. This procedure takes into account the contribution of each collagen fibril intensity value within the focal volume for the calculation of the total SHG intensity.…”

Three‐dimensional intravital imaging in bone research

“…Since cells and microenvironments are usually the main target of IVI, a notable advantage of IVI is that collagen fibers, the major element of bone matrices, can be conveniently visualized as the background of (sub) cellular events without fluorescent labeling. This is achieved by recording signals from a nonlinear optical process called second‐harmonic generation (SHG) using MPM or harmonic generation microscopy . SHG signals reflect a non‐central symmetry of materials such as extracellular collagen networks in bone, which can absorb the two low energy incident photons and re‐emit them as a single photon with twice the energy at half the wavelength.…”

Super-resolved polarisation-enhanced second harmonic generation for direct imaging of nanoscale changes in collagen architecture

“…Thus, polarisation resolved SHG (p-SHG) is used to acquire information about the orientation and degree of organisation of harmonophores, for example, to calculate the helical pitch angle of collagen fibrils [15] and the tilt angle of the helices relative to the fibril axis [16]. It has also been used to understand the differences between collagen types in tissues [17], and investigate the effect of mechanical stretching [18] on collagen disorder within tendon tissue. Collagen and its deposition/accumulation plays a critical role in many diseases especially fibrotic diseases such as liver fibrosis and idiopathic lung fibrosis [19] [20] [21].…”