Biology Reference
In-Depth Information
Although they are useful for obtaining developmental standards in extant populations,
assessments based on the “dental age” method are less useful on fragmentary specimens,
as they require information on the entire dentition in order to make comparisons. A combi-
nation of the atlas method and the dental age approach is necessary for analyses of remains
where more than one tooth is present and the teeth are in different stages of eruption. Anal-
ysis of individual teeth must rely largely on histological methods as described below.
An important contribution was recently made by AlQahtani and colleagues, who
provided an updated atlas based on the study of over 500 individuals from a mixed sample
including historic and modern Europeans as well as Bangladeshi immigrants to Great Brit-
ain. In addition to an atlas, they include useful information on alveolar emergence of both
the primary and permanent teeth ( AlQahtani et al., 2010 ).
Histology
Histological techniques are extremely informative in terms of tooth growth and develop-
ment. Similar to the way that the growth rings of a tree record information about the tree's
age and environmental conditions, teeth preserve a record of their development. Although
they are not always reflected in the overall shape of the tooth, stresses affecting the indi-
vidual during development of the teeth can be recorded in the microscopic structure of
a tooth. Furthermore, if the tooth was still growing at the time of death, age assessment
of the individual is possible. Other “stressful” occurrences in an individual's life such as
birth or illness also produce disruptions in the developing enamel that are visible micro-
or macroscopically.
It has long been assumed that the fine cross striations (or discontinuities in the enamel
structure), visible in histological sections of teeth, are indicative of the daily amount of enamel
secretion by ameloblast cells as they appear to divide enamel prisms (the “rod” of enamel that
the ameloblast has formed). These fine striations are in turn separated into groups by brown
striae of Retzius, which manifest on the surface of the tooth as perikymata (wave-like ridges)
of the tooth crown. These features are all visible in Figure 10.5 . In the dentin, similar lines are
formed calledAndresen lines that can be viewed internally. Like the perikymata in the enamel,
these manifest on the root surface as periradicular bands.
The number of cross striae (representing daily incremental lines) between striae of Retzius
(representing longer period features) varies among the primates, both within and between
species fromcounts as fewas 4 e 5 cross striations toasmany as 10 e 12 ( Hillson, 1996 ). Inhumans,
the cross striae count ranges between 8 and 10. This means that if you were to count up the
number of perikymata and periradicular bands on the outside of a growing tooth and then
multiply that number by an average of the daily cross striation count (average of 9 in humans),
you would arrive at a reasonably accurate estimate of the age-at-death of a subadult individual.
Note that this method would not work in older subadults and adults with fully formed teeth.
When a stressful event such as illness or a period of malnutrition occurs during develop-
ment, the event manifests as an accentuated line. This is because at the time of an insult,
although the tooth continues to form, ameloblasts reduce their enamel secretion due to the
stress on the individual. This leads to a defect. A classic example of an accentuated line is
the “neonatal line” that indicates the time surrounding birth. Prolonged insults can manifest
on the surface of the tooth as enamel hypoplasia. Analysis of these types of defects can have
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