Coherent x-ray imaging of collagen fibril distributions within intact tendons

Abstract

The characterization of the structure of highly hierarchical biosamples such as collagen-based tissues at the scale of tens of nanometers is essential to correlate the tissue structure with its growth processes. Coherent x-ray Bragg ptychography is an innovative imaging technique that gives high resolution images of the ordered parts of such samples. Herein, we report how we used this method to image the collagen fibrillar ultrastructure of intact rat tail tendons. The images show ordered fibrils extending over 10-20 μm in length, with a quantifiable D-banding spacing variation of 0.2%. Occasional defects in the fibrils distribution have also been observed, likely indicating fibrillar fusion events.

Figure 1

CXD patterns recorded from overlapping regions of a rat tail tendon immersed in buffered solution. They show an approximately one-dimensional speckle pattern in the direction transverse to the momentum transfer. Between frames, the sample is shifted relative to the beam by a fraction of its diameter, precisely 2 μm, resulting in similar, but not identical, diffraction patterns at each location. Intensity in arbitrary units.

Figure 2

Schematic view of the experimental geometry, showing the two pinhole apertures, and the location of the sample, with its major symmetry axis perpendicular to the beam (axis (001) of the fibrils). The insight shows a schematic interpretation of the parallel alignment of the collagen fibrils within the tendon. Within each fibril all the collagen molecules are aligned into D-bands of modulated density, with 67 nm spacing, which diffract x-rays. The D-band position, which offsets between adjacent fibrils within the structure can be represented as phases, indicated by blocks of color. The relative phases of the structure factor of the diffraction signal from each fibril combine together to give a one-dimensionally modulated interference pattern, as seen in the data.

Figure 3

Reconstruction images of the gold lithographed model object and the illumination probe. (a) Scanning electron microscopy image of the sample, where the ladder motifs simulating the collagen d-banding periodicity are clearly shown. (b) Bragg projection ptychography reconstruction of a 17 × 17 μm area of the sample. The image shows the amplitude of the reconstruction (that is, the sample density) normalized to 1. (c) Probe amplitude and phase recovered at the same time as the sample image with the ePIE algorithm. Complex color scale: brightness indicates amplitude, color indicates phase.

Figure 4

(a) Final image of a 25 × 10 μm region of the tendon. Only the collagen contributes to the image, obtained in dark field using the first order Bragg peak. The image is displayed in the complex color scale as indicated in the inset: brightness indicates amplitude, color indicates phase. Phase variations indicate relative shifts of the collagen D-band structure with respect to each other. A shift of one D-band spacing, or 67 nm, maps onto a full 2π rotation of the phase. (b) A zoom in the image. White arrows indicate signatures of fibril fusion events.

Figure 5

Histogram of local lattice parameters (local value of the D-band spacing), calculated by using the phase gradient of the fibrils in the reconstructed image (Fig. 4) over 10 μm and following Eq. 1. $\overrightarrow{Q}$ is the momentum transfer of the Bragg reflection used to measure the CXD pattern, which is the $2\pi /67$ nm⁻¹ here. The maximum occurrence of 3000 events around 67 nm was cropped for display purposes. To see this figure in color, go online.