The pattern of mineralization of traumatically-induced developmental defects of sheep enamel assessed by microhardness and microradiography

Stressing out in medieval Denmark: An investigation of dental enamel defects and age at death in two medieval Danish cemeteries

The influence of early life stress on later life experiences has become a major focus of research in medicine and more recently in bioarchaeology. Dental enamel, which preserves a record of childhood stress events, represents an important resource for this investigation when paired with the information from adult skeletal remains, such as age at death. The purpose of this research was to use a life history approach to the exploration of sex differences in the relationship between childhood stress and adult longevity by examining accentuated striae of Retzius (AS). A medieval Danish sample (n=70) drawn from the rural cemetery of Sejet and the urban cemetery of Ole Wormsgade was considered for AS and age at death. The results suggest sex differences in survivorship, with more stress being associated with reduced survivorship in males and increased survivorship in females. A consideration of AS formation time also suggests a difference in the impact of developmental timing between males and females. These results are interpreted in terms of differential frailty and selective mortality, drawing in both biomedical and cultural perspectives.

Enamel biorhythms of humans and great apes: the Havers-Halberg Oscillation hypothesis reconsidered

The Havers-Halberg Oscillation (HHO) hypothesis links evidence for the timing of a biorhythm retained in permanent tooth enamel (Retzius periodicity) to adult body mass and life history traits across mammals. Potentially, these links provide a way to access life history of fossil species from teeth. Recently we assessed intra-specific predictions of the HHO on human children. We reported Retzius periodicity (RP) corresponded with enamel thickness, and cusp formation time, when calculated from isolated deciduous teeth. We proposed the biorhythm might not remain constant within an individual. Here, we test our findings. RP is compared between deciduous second and permanent first molars within the maxillae of four human children. Following this, we report the first RPs for deciduous teeth from modern great apes (n = 4), and compare these with new data for permanent teeth (n = 18) from these species, as well as with previously published values. We also explore RP in teeth that retain hypoplastic defects. Results show RP changed within the maxilla of each child, from thinner to thicker enameled molars, and from one side of a hypoplastic defect to the other. When considered alongside correlations between RP and cusp formation time, these observations provide further evidence that RP is associated with enamel growth processes and does not always remain constant within an individual. RP of 5 days for great ape deciduous teeth lay below the lowermost range of those from permanent teeth of modern orangutan and gorilla, and within the lowermost range of RPs from chimpanzee permanent teeth. Our data suggest associations between RP and enamel growth processes of humans might extend to great apes. These findings provide a new framework from which to develop the HHO hypothesis, which can incorporate enamel growth along with other physiological systems. Applications of the HHO to fossil teeth should avoid transferring RP between deciduous and permanent enamel, or including hypoplastic teeth.

Developmental defects of enamel--historical and present-day perspectives of their pathogenesis

Developmental defects of enamel are visible deviations from the normal translucent appearance of tooth enamel resulting from enamel organ dysfunction. In the past, information about the activities of the ameloblasts has determined the terminology used to describe the lesions. Advances in our understanding of the complicated secretory and maturation phases of amelogenesis have required a re-appraisal of the concepts of defect formation. The phase of ameloblast activity, the duration of the disturbance, and its severity leading to temporary or permanent inactivity of the cells determine the appearance of the three common types of lesions--hypoplasia, and diffuse and demarcated opacities.

Macroscopic and scanning electron microscopic appearance and hardness values of developmental defects in human permanent tooth enamel

Defects present in 12 human permanent teeth were classified on the basis of their macroscopic appearance as hypoplasia (three teeth), diffuse opacities (three teeth), white demarcated opacities (one tooth but two defects), or yellow demarcated opacities (five teeth but six defects). The hardness values and SEM appearance of the defective enamel were determined after the teeth were sectioned through the lesion(s) and were distinctive for each type of defect. The thin enamel of the hypoplastic lesions was either opaque (with reduced hardness values) or translucent (with near-normal hardness values and sometimes a change in prism orientation external to an incremental line). The enamel of the diffuse and demarcated opacities was of normal thickness. The changes in the macroscopic and SEM appearance, and the reduced hardness values of the diffuse patchy opacities, were restricted to the outer 150 microns of the enamel. The demarcated opacities varied in position and depth, and in places had a clearly marked boundary with the adjacent normal enamel. Hardness values were related to color change, with yellow lesions being softer than white. Although prism direction was normal within demarcated opacities, prism outlines were less distinct. The findings suggest that temporary and permanent dysfunction of ameloblasts can occur in both secretory and maturation phases, influencing the final appearance of the lesion.

Histological, macroscopic and microhardness observations of fluoride-induced changes in the enamel organ and enamel of sheep incisor teeth

When aged 8.5 months, 10 sheep born in the same week were given 4 mg fluoride (F)/kg body weight orally for 26 days. Three sheep received no F. Sheep were killed at the end of the treatment period and later at selected stages of tooth development. The macroscopic changes in the enamel of one incisor were related to the cellular changes in the enamel organ of the contralateral tooth. A break in enamel continuity, hypoplasia, was seen on the labial enamel of 9 of the 10 F-treated sheep. Pitting of the enamel was associated with shortening of some ameloblasts and aggregations of cysts affecting cells late in their secretory phase in the first-killed sheep. In sheep killed later, these changes were associated with cells which had progressed into their maturation phase. A more extensive absence of enamel with ledge formation cervically, seen in one sheep, was associated with displacement or death of almost all the cells in their secretory phase during F treatment and consequent retention of the organic matrix. The hypoplastic lesions resulted from secretory-cell reaction during the period of F dosing. Diffuse patchy opacities, characterized by an irregular hypomineralized surface zone, were only apparent in the enamel of the later-killed sheep and were associated in one sheep with abnormal ameloblast regression in the contralateral tooth. These defects possibly resulted from the long-continued release of F stored in the bones during the period of F dosing.