The structural origin of second harmonic generation in fascia

Area of Science:

• Biophysics
• Materials Science
• Connective Tissue Research

Background:

• Fascia tissue contains type I collagen, which is imageable using second harmonic generation (SHG) microscopy.
• SHG imaging reveals collagen fibril alignment but the 3D origin of the SHG signal is complex.
• Sub-micron heterogeneity in noncentrosymmetric (piezoelectric) structures is observed in SHG images.

Purpose of the Study:

• To investigate the sub-micron structural organization of collagen fibrils within fascia.
• To elucidate the origin of the second harmonic generation (SHG) signal observed in fascia tissue.
• To understand the relationship between collagen fibril nanostructure and SHG imaging characteristics.

Main Methods:

• Second Harmonic Generation (SHG) microscopy to image collagen fibril alignment.
• Piezoresponse Force Microscopy (PFM) to probe the nanostructure of individual collagen fibrils.
• Analysis of nonlinear optical properties and susceptibility phase in collagenous tissues.

Main Results:

• Individual collagen fibrils exhibit noncentrosymmetric structural organization.
• Collagen fibrils arrange into nano-domains preserving anisotropic axis along the fibrillar direction.
• A 180-degree phase change in nonlinear susceptibility occurs between adjacent nano-domains across collagen sheets.
• This complex nano-domain architecture influences the coherent addition of light and SHG image features.

Conclusions:

• The observed SHG signal in fascia arises from a complex, heterogeneous arrangement of noncentrosymmetric collagen nano-domains.
• Piezoresponse Force Microscopy reveals the intrinsic noncentrosymmetric nature of individual collagen fibrils.
• The specific arrangement and phase variations of these nano-domains dictate the macroscopic SHG imaging outcomes in fascia.