Structural changes in collagen fibrils across a mineralized interface revealed by cryo-TEM

- Collagen fibril structure was examined at tooth-ligament interface in mouse molars.
- D-spacing of collagen in mineralized tissue (cementum) is smaller than in the non-mineralized periodontal ligament.
- This axial contraction of collagen fibrils in cementum is due reduction in the size of the gap zone.
- The major features of the banding pattern are not changed.
- Collagen banding pattern in cryo-TEM was correlated to the X-ray structure for rat tail collagen.

The structure of the mineralized collagen fibril, which is the basic building block of mineralized connective tissues, is critical to its function. We use cryo-TEM to study collagen structure at a well-defined hard-soft tissue interface, across which collagen fibrils are continuous, in order to evaluate changes to collagen upon mineralization. To establish a basis for the analysis of collagen banding, we compared cryo-TEM images of rat-tail tendon collagen to a model based on the X-ray structure. While there is close correspondence of periodicity, differences in band intensity indicate fibril regions with high density but lacking order, providing new insight into collagen fibrillar structure. Across a mineralized interface, we show that mineralization results in an axial contraction of the fibril, concomitant with lateral expansion, and that this contraction occurs only in the more flexible gap region of the fibril. Nevertheless, the major features of the banding pattern are not significantly changed, indicating that the axial arrangement of molecules remains largely intact. These results suggest a mechanism by which collagen fibrils are able to accommodate large amounts of mineral without significant disruption of their molecular packing, leading to synergy of mechanical properties.

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