I. Incremental Structures in Dentin pp 187-197
This unit in Hillson overlaps with Ch 9 Histological methods of age determination. The notes this week will briefly draw together the main points for the reader, and introduce some additional concepts in dental anthropology not normally considered in clinical dentistry.
Dentin formation is revealed by a variety of lines sen by transmitted light microscopy in ground sections. Most familiar are the lines of Owen which are probably the closest analogs to the brown striae in enamel. Unfortunately, they seem to represent intermittent disturbances rather than a convenient regular formation rhythm.
While dentin layering doesn't permit concise chronological counts, systemic disturbances will leave an artifact that can be matched and correlated among different teeth forming at the time of disturbance. You may think of it as analogous to comparing tree rings in ancient timbers in the American southwest.
Schour confirmed a neonatal line in dentin as well as enamel correlating dentin formation with birth.
Interglobular spaces seen in dentin have been shown to correlate with the incidence of hypoplasia in enamel. Thus, their incidence seems inversely related to social and economic status.
Dental sclerosis, the infilling of tubules with mineralized substance, seems to strongly correlate with age. This event is often called root transparency in dental anthropology literature. In ground sections, this feature is easy to observe, it can be assessed with the naked eye. Quantifying the extent of translucency is more difficult
Secondary dentin has also intrigued investigators; however, it is difficult to differentiate primary from secondary dentin.
For a particularly good review of dental ageing in adults, see the Rosing and Kvaal article cited below.
We've said several times that teeth are especially nice for study because they erupt full-sized, can easily be measured at any age after eruption, and preserve well after death.
The obvious surfaces for measurement are the external outlines of the root and crown. There is, however, another intriguing topography for study, the dentinoenamel junction. Authors occasionally will refer to this as the dentin surface.
The thinking goes like this:
(1) Among apes, humans, and human ancestors, dental enamel is classified as thick or thin.
(2) Many additional cuspules, ridges, and fissures appear on the dentin surface but don't appear on the enamel surface. Maybe those features have been 'enameled over'and in these persons. A trait such as Carabelli's cusp can become hidden. (My note: there are also instances where there are enamel features, but not comparable dentin surface feature.
(3) Additional insights into genes and evolutionary history are learned by study of the additional 'dentin surface' traits.
Korenhof, one of the pioneers of this research, focused on the dentin surfaces of upper molars in fossil and living primates, and recent humans. He was intrigued by 'enamel caps' this had survived in soil that destroyed the dentin of teeth, leaving only an enamel cap. Impressions of the inner surface of these enamel caps permits making endocasts when replicate the dentin surface. (My note: I saw the ones that Al Dahlberg had obtained from Korenhof many years ago.)
A related study better known to clinical dentistry is Kraus' and Jordan's The Human Dentition Before Birth.
Deciduous second molars and first permanent molars have been compared. Some traits which appear in deciduous molars don't appear in the permanent molar. In about 10% of cases, it is the reverse. Occasionally, a characteristic found in permanent molars is exaggerated in the deciduous molar.
..... CJ '99
Scott, G. and Turner, C. The Anthropology of Modern Teeth. New York: Cambridge University Press, 1997.
Kraus, B. and Jordan, R. The Human Dentition Before Birth. Philadelphia: Lea and Febiger, 1965.
Rosing, F., and Kvaal, S. "Dental Age in Adults-A Review of Estimation Methods" in Alt, K., Rosing, F., and Teschler-Nicola, M. eds Dental Anthropology Fundamentals, Limits, and Prospects. New York: SpringerWien, 1998.