Tooth enamel represents a durable archive of developmental events in early life. Incremental growth markers laid down during the secretory (matrix formation) stage of enamel formation provide an exact record of the age of onset of mineralization of different areas of enamel. This can be used as an accurate chronological framework that can be related to developmental events such as birth and weaning.

The neonatal line is present in all deciduous teeth in humans and is an accentuated growth increment in enamel and dentine (Schour 1936, Skinner and Dupras 1993). It can be used to identify the portion of enamel that initiated formation before and after birth, and the expression of the neonatal line has been considered an indicator of the severity of neonatal stress during labor and birth. However, this assumption has not been properly tested and the underlying mechanism remains uncertain.

Accentuated striae of Retzius are growth disruptions which reflect a temporary disruption of enamel matrix secretion and have been regarded as a non-specific indicator of systemic stress (FitzGerald et al. 2005). The relationship between the number and distribution of enamel growth disruptions and factors such as socioeconomic status, infant diet, maternal smoking, maternal anxiety, and growth faltering has not yet been tested.

The chemical composition of deciduous tooth enamel is dependant on environmental exposure and physiological influences during the prenatal and postnatal stages of enamel formation. The distribution of trace elements such as strontium, lead and zinc within enamel reflect differences in the age of exposure and in the incorporation of each element into enamel. These patterns can be used to interpret dietary intake and other environmental influences (Humphrey at al 2008 a, b). Analysis of a well documented sample will be used to test assumptions and refine interpretations.

The objectives of the proposed project are to:

1) Evaluate the expression of the neonatal line in relation to mode of delivery (caesarian, spontaneous or instrument assisted delivery), parity status of the mother, maternal anxiety and APGAR score (where available).

2) Evaluate the position of the neonatal line within the enamel crown in relation to birth weight and gestation length.

3) Develop a longitudinal record of enamel growth disruptions in enamel secreted before and after birth for each individual using accentuated striae of Retzius, and relate these records to socioeconomic status, infant diet, maternal smoking, maternal anxiety, and growth faltering measured in the first year of life.

4) Develop spatial maps of the distribution of calcium normalized trace element intensities across each tooth crown, and relate these intensities to mode of feeding at birth, age of introduction of complementary foods, dietary intake, and other environmental exposures (e.g. maternal smoking) and outcomes.

Methods

This project will use naturally shed deciduous teeth donated by approximately 95 children from the core ALSPAC sample. One deciduous canine will be sampled from each child, and we will also select 20 incisors from the study population that have the least amount of wear. High resolution dental casts and micro-CT scans will be produced for each crown prior to sectioning to preserve information on internal and external morphology and ground sections of each tooth used will be returned at the end of the study to extend the usefulness of the collection.

A longitudinal thin section of enamel approximately 100 micrometers thick will be prepared and used for both chemical analysis and a detailed histological study. The neonatal line will be used to identify the portion of enamel that initiated formation before and after birth and also as a baseline for establishing a chronological framework within the enamel crown. Accentuated striae of Retzius (growth disruptions) will be identified following the criteria of FitzGerald et al (2006). Individual chronological records of growth disruptions experienced during the period of tooth crown formation will be established by relating the position of defects to normal incremental growth markers in enamel (Dean et al 1993, Dean 2000, Reid and Dean 2000).

The LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) facility in the Department of Mineralogy at the Natural History Museum will be used to determine calcium normalised trace element intensities at approximately 60 sampling locations in each enamel cusp. LA-ICP-MS combines discrete microanalysis with exceptional spatial resolution and very low detection limits, and is minimally destructive. Data on the distribution of a range of trace elements, including strontium, barium, iron, magnesium, manganese, lead, selenium, and, if possible, cadmium will be collected simultaneously. Geographical Information Systems (GIS) will be used to visually and statistically compare normalised elemental intensities.

We will assess several measures of infant health and diet in relation to deciduous tooth enamel histology and chemical composition (Table 1). Detailed data on a range of attributes and behaviours potentially relevant to health have already been collected in this well-documented sample of children. These include measures of socioeconomic position at birth and in late childhood, parental smoking during pregnancy, birth weight, gestation length and APGAR score and infant diet. This project will address a number of specific and testable hypotheses concerning how these attributes are reflected in enamel structure and composition. The project will also provide baseline data on each of these enamel parameters in a contemporary population. It is also expected that this detailed study will inform appropriate sampling strategies for future epidemiological research using the ALSPAC cohort or other tooth collections.

References

Dean, M.C., Beynon, A.D., Reid, D.J. and Whittaker, D.K. (1993) Longitudinal study of tooth growth in a single individual based on long and short period incremental markings in dentine and enamel. International Journal of Osteoarchaeology. 3: 249-264

Dean, M.C. (2000) Incremental markings in enamel and dentine; what they can tell us about the way teeth grow. In: M.Teaford, M. Meredith Smith and M. Ferguson (eds) Development, Function and Evolution of Teeth. Cambridge University Press. pp. 119-130.

FitzGerald, C. and Saunders, S. (2005) Test of histological methods of determining chronology of accentuated striae in deciduous teeth American Journal of Physical Anthropology 127: 277-290.

FitzGerald, C., Saunders, S., Bondioli, L. and Macchiarelli, R. (2006) Health of infants in an Imperial Roman skeletal sample: Perspective from dental microstructure. American Journal of Physical Anthropology 130: 179-189.

Humphrey, L.T., Dean M.C., Jeffries T.E. and Penn, M. (2008) Unlocking evidence of early diet from tooth enamel. Proceedings of the National Academy of Sciences 105: 6834-6839.

Humphrey, L.T., Jeffries T.E. and Dean M.C. (2008) Micro spatial distributions of lead and zinc in human deciduous tooth enamel. In: J.D. Irish and G.C. Nelson (eds) Technique and Application in Dental Anthropology. Cambridge University Press, pp 87-110.

Reid, D.J. and Dean M.C. (2000) Brief communication; The timing of linear hypoplasias in modern human anterior teeth. American Journal of Physical Anthropology. 113; 135-139.

Schour, I., 1936. Neonatal line in enamel and dentin of human deciduous teeth and first permanent molar. Journal of the American Dental Association. 23: 1946-1955.

Skinner, M. and Dupras, T. (1993) Variation in birth timing and location of the neonatal line in human enamel. Journal of Forensic Sciences, 38: 1383-1390.