Collagen structure of the porcine eye revealed

Good, but older news!

The fascia consists of collagen.

"Collagen fibers are found in various forms throughout the body, forming the foundational extracellular structure of tissues.

Despite nearly a century of study, high-resolution views of functional, macromolecular collagen fibers have remained elusive because of compositional heterogeneity and data-processing limitations.

Lou et al. analyzed the relatively uniform fibrils from the vitreous body of the eye and were able to determine a high-resolution structure that reveals layers of individual fibrils wrapped in a fourfold symmetric helix.

Sugar modifications are interspersed at defined positions within the fiber and alter packing density and hydration.

The specialized function of vitreous body collagen as a transparent medium is enabled by specific modifications and a composition distinct from that known for structural collagens."

"... 

Composition and Stoichiometry

Type V/XI core: Acts as the internal nucleation center that dictates initial fibril geometry.
Type II shell: Forms an antiparallel outer layer that expands the overall diameter of the fibril.
Type IX and opticin: Decorate the outer surface to regulate spacing, surface properties, and interaction with the surrounding gel.

Stabilization and Assembly

Glycan modification: Abundant galactose-glucose disaccharides attach to hydroxylysine residues within conserved motifs, stabilizing interhelical packing.
Vitreous-specific design: Unlike cartilage counterparts, the type IX collagen in the vitreous lacks the NC4 domain and instead possesses an elongated chondroitin sulfate chain, which preserves the transparent, hydrated gel matrix.

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From the abstract:

"Collagen, a fundamental constituent of the extracellular matrix, has long remained elusive to high-resolution structural characterization.

Using a tailored system and optimized cryo–electron microscopy processing for long-period filaments, we determined the structure of native collagen fibrils from the porcine vitreous body, with local resolutions extending from 2.6 to 7 angstroms.

Each 67-nanometer periodic unit contains type II, V/XI, and IX collagen triple helices together with opticin, at a stoichiometry of 8:4:4:4, which reveals their detailed higher-order molecular packing.

Abundant galactose-glucose disaccharides modify hydroxylysine residues in conserved -glycine-X-hydroxylysine- motifs, mediating fibril packing and structural stability.

Our structure uncovers the glycan-mediated assembly principle of collagen fibrils and clarifies the structure-function basis of collagens in the vitreous body."

In Science Journals | Science

The molecular architecture of mammalian vitreous body collagen fibrils (no public access)