3.2 Tissue preservation analysis by means of atomic force microscopy (AFM)
Of the various forms of connective tissue, collagen fibres are among the most important to survive in bog bodies. They preserve the shape of decalcified bones and the general structure of any remaining parts of the intestinal tract and define the body surface (Brothwell & Gill-Robinson 2002, 126).
AFM images revealed collagen fibrils with periodic banding patterns (figs 6, 7, 8) embedded in the tissue matrix. The average D-period, derived from topographic analysis along the longitudinal axis of several fibrils, was 62.8 nm (± 4.2 nm s.d.). The fibrils were unsorted, overlapping at some sites, and formed network-like structures.
Images taken at a greater level of magnification (figs 6, 7, 8) show uninterrupted collagen fibrils, although spherical particles may indicate collagen fragments. The fibril contour structure was very faint. Topographic analysis perpendicular to the longitudinal axis of several fibrils suggested a mean fibril height of 12.2 nm (± 3.6 nm s.d.).
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Figure 6 Collagen fibrils within the histological skin sample. (a) Optical microscope image of Zweeloo Woman tissue (magnification 10x). Figures (b) to (d) show magnifying AFM amplitude images of the outlined areas. Figure (d) shows single fibrils with a periodic structure. |
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Figure 7 Histological skin sample with collagen fibrils. (a) Zweeloo Woman tissue imaged with an optical microscope (magnification 10x). Figures (b) to (d) show magnifying AFM amplitude images of the outlined areas. Figure (d) shows unsorted, overlapping fibrils in a network-like structure. Each fibril features a periodic substructure. |
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Figure 8 AFM topography and amplitude images of collagen found in the histological skin sample of Zweeloo Woman. Figure (left) shows collagen fibrils (measuring 2 x 2 µm) with their characteristic banding pattern. The fibrils form a network-like structure. Some of them overlap one another. The contours of the fibrils are faint. The amplitude image (right) shows the fibril contours in more detail. |
Collagen is extremely durable and may survive in mummified tissue for several millennia (Chang et al. 2006; Janko et al. 2010). This indeed also holds for the structural preservation of the skin collagen of the Zweeloo mummy. As also observed in recent human skin and other mummy skin samples, the collagen fibrils in the skin of Zweeloo Woman were typically arranged in networks or sheet-like structures and showed a periodic banding pattern. Contrary to the results obtained by Stücker et al. (2001), who observed well-preserved collagen bundles in the dermis of six bog bodies, our results indicate moderate decomposition of the Zweeloo Woman collagen. The Zweeloo Woman collagen differs considerably in terms of fibril contour and size from the collagen found in recent human skin and other mummies.
High-resolution images taken with the AFM revealed soft outlines of the collagen fibrils, indicating an inferior degree of collagen preservation. The average characteristic banding pattern is 62.8 ± 4.2 nm, which is less than the value of 67 nm reported in the literature. The value is however still within the range of the error margin. The average fibril height of 12.2 nm (± 3.6 nm s.d.) is significantly shorter than that of recent skin collagen, which has a diameter ranging from 20 to 100 nm (Fleischmajer et al. 1981; Flint et al. 1984). This has also been observed in the case of other mummy skin collagen, such as that of the Iceman, whose fibrils were found to have a diameter of 32 nm (Janko et al. 2010).
The reduction in fibril height may be caused by decomposition, as observed in previous AFM studies (Paige et al. 2002; Bertassoni & Marshall 2009), which revealed the degradation of type I collagen by enzymatic action, e.g. by collagenase or papain-gel. In those studies it was suggested that the enzymes degrade the entire fibrillar structure in a non-specific manner, causing the fibrils to become both shorter and thinner.
An analogous effect may have occurred in the bog and have resulted in slight degradation, hence reduction in size, of the collagen. The differing degree of degradation is most probably due to variations in the composition, in particular the acidity, of the bog governing the preservation and mummification process.