2. Histological Characterisation of Dental Diagenesis - a short review

Post-mortem microbial invasion in archaeological teeth was first described in 1864 by Carl Wedl, observing what was subsequently termed Wedl type tunnels (Hackett 1981), attributed in one study to fungal activity (Marchiafava et al. 1974). A century after Wedl, Sognnaes (1950) reported different kinds of tunnels in archaeological teeth found to penetrate both from the pulpal dentinal surface and the external cementum surface. These types of canals are similar to those observed in bone, described by Hackett (1981) as linear longitudinal and budded tunnels and thought to be the result of bacterial activity (Jackes et al. 2001). Microbial borings, fungal and bacterial, are also often described as microscopical focal destruction, or MFD (Hackett 1981). Poole and Tratman (1978) also noted focal destructions in the dentine and attributed this to actinobacteria; mycelium-producing gram-positive bacteria that are common in decaying organic material within both terrestrial and aquatic environments. Although a few more recent authors have noted destructive foci within archaeological teeth (Bell et al. 1991; Hillson 1986; Kierdorf et al. 2009), Turner-Walker (2008) is one of the few to discuss dental histological post-mortem alterations in more detail. He showed that elongated destructive foci dominate in the dentine while the destructive foci found in the cementum are of a larger globular/ampulla-shaped type. As in bone, bacterial penetration is influenced by the micro-architecture of the tissues. The differing orientation of the tunnels to the plane of the section in both bone and teeth, indicates that the bacteria follow the orientation of the collagen fibres and are able to exploit planes of weakness in the tissues (Turner-Walker 2008). After decades of research into microbial alterations in skeletal tissues, it has not yet been possible to link specific organisms to the observed tunnelling. We suspect that in future years, next-generation sequencing will begin to reveal more of the nature of the micro-organisms that colonise ancient teeth and bone. An interesting recent contribution in that respect is the study by Pitre et al. (2013), who characterised biofilms in ancient bones using a combination of SEM-imaging, microbe isolation techniques and DNA sequencing. Likewise, degradation experiments such as that of Turner-Walker (2012) and Fernández-Jalvo et al. (2010), aid in understanding features caused by early stage diagenetic processes, including alterations caused by microbes.

In addition to microbial alteration, other diagenetic features often described in bone thin-sections are expected to occur in dentine. This include generalised destruction (the loss of recognisable microstructure due to mineral dissolution) (Garland 1993), infiltrations, inclusions and micro-cracking (Jans 2005).