The biomechanics of amnion rupture: an X-ray diffraction study

Abstract

Pre-term birth is the leading cause of perinatal and neonatal mortality, 40% of which are attributed to the pre-term premature rupture of amnion. Rupture of amnion is thought to be associated with a corresponding decrease in the extracellular collagen content and/or increase in collagenase activity. However, there is very little information concerning the detailed organisation of fibrillar collagen in amnion and how this might influence rupture. Here we identify a loss of lattice like arrangement in collagen organisation from areas near to the rupture site, and present a 9% increase in fibril spacing and a 50% decrease in fibrillar organisation using quantitative measurements gained by transmission electron microscopy and the novel application of synchrotron X-ray diffraction. These data provide an accurate insight into the biomechanical process of amnion rupture and highlight X-ray diffraction as a new and powerful tool in our understanding of this process.

Figure 1. Transmission electron micrographs from the fibroblastic layer of amnion.

Collagen fibrils appear more regularly aligned, closer together and in more obvious bundles (bd) within the stroma of amnion proximal to the placenta (a), than those shown in the tissue distal to the placenta (b). Scale bar = 1 µm.

Figure 2. High magnification transmission electron micrographs of amnion collagen fibrils in cross section.

No difference in fibril diameter was observed between samples proximal (a) and distal (b) to the placenta. However a marked decrease in fibril packing can be seen in the amnion distal to the placenta. Scale bar = 300 nm.

Figure 3. Radial distribution functions (RDF) calculated from positions of amnion fibrils in cross section from representative images.

The primary RDF peak (*) from amnion distal to the placenta was lower and broader than the corresponding peak from amnion proximal to the placenta indicating less regular fibril spacing. The bulk fibril number density (ρ) was also decreased in distal amniotic tissue (arrows).

Figure 4. Synchrotron X-ray diffraction patterns from amnion.

Representative (false colour) patterns collected from amnion proximal (a) and distal (b) to the placenta. Arrows indicate position of the interfibrillar reflection (magenta) other coloured rings close to the beamstop are background/backscatter. X-ray diffraction patterns displayed an anisotropic arrangement of collagen fibrils within the distal amniotic tissue.

Figure 5. Intensity profiles across representative X-ray diffraction patterns collected from the amnion.

Examination of intensity versus distance plots provided interfibrillar Bragg spacing measurements from amnion proximal (a) and distal (b) to the placenta. The lower broader interference function with a mean peak position of 75 nm from the distal amnion suggests that this region contains a less ordered fibril collagen arrangement and a higher mean interfibrillar Bragg spacing than that shown by proximal amniotic tissue.