Stories teeth can tell
Tuesday, Jul 21, 2020, 06:04 AM | Source: Pursuit
By Marija Edinborough
Throughout our lifetimes, our bodies become a record of significant amounts of life history information.
There are scars on broken bones and past infections remembered by our immune system, but our teeth can also provide a fascinating insight into our health, life, and environment – past and present.
As the hardest calcified components of the body, teeth often remain well preserved even over millennia.
When teeth are recovered from archaeological or forensic sites in a controlled and standardised manner, they can create a time capsules of life history, allowing scientists to study the lives of people and populations in great detail.
Tooth and jaw condition can provide data on diseases like caries (cavities) and periodontitis, a severe gum infection. They can also reveal information about the nutrition and hygiene of individuals and entire populations. The size and shape of teeth can shine a light on our origins.
What are teeth made of?
Teeth are complex calcified structures whose primary function is to break down food. A human tooth is formed of three mineralised tissues called enamel, dentine, and cementum, and a soft tissue called dental pulp. The primary mineral component of dental mineralised tissues is calcium hydroxyapatite.
Enamel, which is the hardest dental tissue and the most highly mineralised substance of the body, covers tooth crowns. The roots are coated with a thin layer of bone-like cementum. Underneath these two coating layers lies dentine, which forms the bulk of every tooth.
Dental pulp is contained within the pulp chamber and root canals deep inside the tooth. Dental pulp contains cells, nerves and blood vessels.
Acellular extrinsic fibre cementum (AEFC), one of the five types of cementum, is deposited in a regular annual rhythm around the cervical two thirds of human tooth roots, with varying degrees of mineralisation.
Annual growth of AEFC results in alternating dark and light banding called an incremental line. By counting the number of incremental lines and adding them to the year of the eruption of the tooth observed, we can estimate chronological individual age.
AEFC can also help scientists identify life history parameters like pregnancies, skeletal trauma, and renal disease. To do this, they use optical or light microscopy to observe hypomineralised (meaning low mineralisation) growth layers of AEFC, known as incremental lines.
The development of dental anthropology
As a discipline, dental anthropology dates to the 1930s and 1940s work of Albert A. Dahlberg, a Chicago dentist and anthropologist, and his colleagues.
They examined tooth formation and discovered numerous research possibilities in dentitions – the development of teeth and their arrangement in the mouth.
For 40 years, Dahlberg and his colleagues studied the dentitions of various human populations, including museum collections. He also collected information on his patients, allowing for global comparisons and creating an important resource for scientists and students.
The term 'dental anthropology' has been used since 1958. Today, it involves the study of past and present, human and non-human primate dentitions.
Reading health and life history from teeth
We know that the various tooth tissues – enamel, cementum, and dentine – reflect our genetics, but through growth and ageing they sustain numerous internal and external influences, some of which remain recorded within the tooth (micro)structure and/or composition.
Our international team of researchers wanted to investigate to what extent optical microscopy could be used to understand the visual effects of teeth tissue mineralisation on inferring life history.
To do this, we effectively worked backwards, taking a sample with a known life history – a 66-year-old woman with six full-term pregnancies at known ages. The samples from this patient provided an ideal opportunity to cross-check any resultant pregnancy-cementum mineralisation relationship using multiple microscopic analytical methods, not just optical microscopy.
Her tooth was extracted during routine dental work and analysed with permission using the standard techniques of optical and electron microscopy, but also using with Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS).
ToF-SIMS can analyse the spatial distribution of atoms and molecules, and NASA uses it to investigate cosmic materials such as moon rocks and comet dust.
It was the first time ToF-SIMS, a powerful instrument used to obtain molecule-specific chemical information from sample surfaces, has been used for this purpose.
Identifying a need for caution
Published in the Journal of Analytical Atomic Spectrometry, we found that optical microscopy wasn't reliable for visual detection of the patient's known series of six pregnancies and that electron microscopy wasn't reliable in detecting variations in the degree and distribution of cementum mineralisation at a micrometre level.
This meant accurate estimation of the tooth cementum mineralisation distribution wasn't possible using light and scanning electron microscopy alone.
Using ToF-SIMS, we did detect decreased calcium in the cementum of a patient who had six full-term pregnancies, but the incremental lines' visual effects were unrelated. As a result, individual pregnancies can remain undetected and researchers must be far more cautious in identifying precise numbers.
We were surprised by this finding, as it was derived from an excellent sample.
While ToF-SIMS analysis holds great promise, we concluded that far more caution is required and more work needs to be done before researchers can link observed lines in this tissue to life history causes.
Although optical and electron microscopy can be useful for investigating certain microanatomical aspects of the various tissues, like identifying incremental growth layers within cementum, they must be used more cautiously.
Optical microscopy can potentially help us see the visual effects of certain 'crisis' events, like pregnancies, skeletal fracture or trauma and kidney disease, but without deeper knowledge of the underlying processes of cementum formation, we aren't able to identify or understand the causes of the crisis we are observing.
The use of ToF-SIMS was found to be more suitable for the investigation of elemental and chemical composition of cementum, as it is highly sensitive instrument for this type of analyses.
When used in clinical studies, ToF-SIMS can help us better understand the effects of tissue development and its resulting chemical composition.
This knowledge can be used in dentistry to help choose more suitable clinical treatments and may also be valuable in the field of tissue regeneration and engineering.
Finally, it offers forensic and biological anthropology a new approach for life history parameters detection. For instance, this technique is important forensically when you only have teeth available to identify an individual, and for biological anthropologists who wish to calculate fertility rates in the past.
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