Teeth reveal juvenile diet, health and neurotoxicant exposure retrospectively: What biological rhythms and chemical records tell us

Inter-rater reliability of stress signatures in exfoliated primary dentition - Improving scientific rigor and reproducibility in histological data collection

Accentuated Lines (ALs) in tooth enamel can reflect metabolic disruptions from physiological or psychological stresses during development. They can therefore serve as a retrospective biomarker of generalized stress exposure in archaeological and clinical research. However, little consensus exists on when ALs are identified and inter-rater reliability is poorly quantified across studies. Here, we sought to address this gap by examining the reliability of accentuated (AL) markings across raters, in terms of both the presence versus absence of ALs and their intensity (HAL= Highly Accentuated, MAL= Mildly Accentuated, RL= Retzius Line). Ratings were made and compared across observers (with different levels of experience) and pairs of raters (who agreed on AL coding through consensus meetings) (N = 15 teeth, eight observers). Results indicated that more experience in AL assessment does not necessarily produce higher reliability between raters. Most disagreements in intensity ratings occurred in categories other than HAL. Furthermore, when AL assessment was performed by pairs of raters, reliability was significantly higher than individual assessments (Gwet's AC1 = 0.28 to 0.56 for line presence assessment; Gwet's AC1 = 0.48 to 0.64 for line intensity assessment). Based on these results, we recommend a workflow called IRRISS (Improving Reliability and Reporting In Scoring of Stress-markers) to increase rigor and reproducibility in histological analysis of dental collections. The introduction of IRRISS is well-timed, given the surge in studies of teeth occurring across anthropological, epidemiological, medical, forensic, and climate research fields.

Synchrotron X-ray Fluorescence Microscopy Reveals Trace Elemental Indicators of Life History in Marsupial Teeth

As teeth develop, their mineralised composition is a bio-recorder of diet, environment, and growth. High-resolution elemental mapping provides a tool to reveal records of life history within teeth. The relative concentrations of a range of trace elements change between in utero development, birth, and weaning in eutherian mammals. Marsupials, however, have a different mode of development: altricial birth and growth within the pouch facilitated by compositional transitions in milk. How these differences alter patterns of elemental mineralisation and become recorded in marsupial teeth is previously unknown. This study analyses the distribution of calcium (major element), zinc (actively incorporated trace element), and strontium (passively incorporated trace element) in the teeth of five species of diprotodontian marsupial using synchrotron X-ray fluorescence microscopy. We find that the diprotodontian lower incisor concatenates elemental variation from across the molariform dentition, preserving a prolonged record of life history in four of the five species. Patterns of elemental incorporation in enamel, dentine, and cementum are presented, with Ca, Zn, and Sr having differing distributions. Zn accretion indicates a role in mineralisation and/or prevention of tooth degradation. Zn also demarcates incremental cementum lines. Sr is shown to be passively incorporated into marsupial teeth, with increasing Sr concentration in milk recorded in dental tissues formed contemporaneously. Older individuals have oscillatory signals in Sr that appear linked to seasonality. These findings highlight some similarities between eutherian and marsupial trace element incorporation, particularly in the distribution of Zn. Sr signals in marsupial teeth record key aspects of life history.

Brief communication: New method for measuring nitrogen isotopes in tooth dentine at high temporal resolution

OBJECTIVES

Nitrogen isotopes (δ15N) are widely used to study human nursing and weaning ages. Conventional methods involve sampling 1-mm thick sections of tooth dentine-producing an averaging effect that integrates months of formation. We introduce a novel protocol for measuring δ15N by multicollector secondary ion mass spectrometry (SIMS).

MATERIALS AND METHODS

We sampled dentine δ15N on a weekly to monthly basis along the developmental axis in two first molars of healthy children from Australia and New Zealand (n = 217 measurements). Nitrogen isotope ratios were determined from measurements of CN- secondary molecular ions in ~35 μm spots. By relating spot position to enamel formation, we identified prenatal dentine, as well as sampling ages over more than 3 years. We also created calcium-normalized barium and strontium maps with laser ablation-inductively coupled plasma-mass spectrometry.

RESULTS

We found rapid postnatal δ15N increases of ~2‰-3‰, during which time the children were exclusively breastfed, followed by declines as the breastfeeding frequency decreased. After weaning, δ15N values remained stable for several months, coinciding with diets that did not include meat or cow's milk; values then varied by ~2‰ starting in the third year of life. Barium did not show an immediate postnatal increase, rising after a few months until ~1-1.5 years of age, and falling until or shortly after the cessation of suckling. Initial strontium trends varied but both individuals peaked months after weaning.

DISCUSSION

Developmentally informed SIMS measurements of δ15N minimize time averaging and can be precisely related to an individual's early dietary history.

Deciduous teeth from the New Hampshire birth cohort study: Early life environmental and dietary predictors of dentin elements

BACKGROUND

Sparse research exists on predictors of element concentrations measured in deciduous teeth.

OBJECTIVE

To estimate associations between maternal/child characteristics, elements measured in home tap water during pregnancy and element concentrations in the dentin of shed deciduous teeth.

METHODS

Our analysis included 152 pregnant person-infant dyads followed from the second trimester through the end of the first postnatal year from the New Hampshire Birth Cohort Study. During pregnancy and early infancy, we collected dietary and sociodemographic information via surveys, measured elements in home tap water, and later collected naturally exfoliated teeth from child participants. We measured longitudinal deposition of elements in dentin using LA-ICP-MS. Multivariable linear mixed models were used to estimate associations between predictors and dentin element concentrations.

RESULTS

We measured 12 elements in dentin including those previously reported (Ba, Mn, Pb, Sr, Zn) and less frequently reported (Al, As, Cd, Cu, Hg, Li, and W). A doubling of Pb or Sr concentrations in water was associated with higher dentin Pb or Sr respectively in prenatally formed [9% (95%CI: 3%, 15%); 3% (1%, 6%)] and postnatally formed [10% (2%, 19%); 6% (2%, 10%)] dentin. Formula feeding from birth to 6 weeks or 6 weeks to 4 months was associated with higher element concentrations in postnatal dentin within the given time period as compared to exclusive human milk feeding: Sr: 6 weeks: 61% (36%, 90%) and 4 months: 85% (54%, 121%); Ba: 6 weeks: 35% (3.3%, 77%) and 4 months: 42% (10%, 83%); and Li: 6 weeks: 61% (33%, 95%) and 4 months: 58% (31%, 90%).

SIGNIFICANCE

These findings offer insights into predictors of dentin elements and potential confounders in exposure-health outcome relationships during critical developmental periods.

Mammalian Life History: Weaning and Tooth Emergence in a Seasonal World

The young of toothed mammals must have teeth to reach feeding independence. How tooth eruption integrates with gestation, birth and weaning is examined in a life-history perspective for 71 species of placental mammals. Questions developed from high-quality primate data are then addressed in the total sample. Rather than correlation, comparisons focus on equivalence, sequence, the relation to absolutes (six months, one year), the distribution of error and adaptive extremes. These mammals differ widely at birth, from no teeth to all deciduous teeth emerging, but commonalities appear when infants transit to independent feeding. Weaning follows completion of the deciduous dentition, closest in time to emergence of the first permanent molars and well before second molars emerge. Another layer of meaning appears when developmental age is counted from conception because the total time to produce young feeding independently comes up against seasonal boundaries that are costly to cross for reproductive fitness. Mammals of a vast range of sizes and taxa, from squirrel monkey to moose, hold conception-to-first molars in just under one year. Integrating tooth emergence into life history gives insight into living mammals and builds a framework for interpreting the fossil record.

Reading children's teeth to reconstruct life history and the evolution of human cooperation and cognition: The role of dental enamel microstructure and chemistry

Studying infants in the past is crucial for understanding the evolution of human life history and the evolution of cooperation, cognition, and communication. An infant's growth, health, and mortality can provide information about the dynamics and structure of a population, their cultural practices, and the adaptive capacity of a community. Skeletal remains provide one way of accessing this information for humans recovered prior to the historical periods. Teeth in particular, are retrospective archives of information that can be accessed through morphological, micromorphological, and biogeochemical methods. This review discusses how the microanatomy and formation of teeth, and particularly enamel, serve as archives of somatic growth, stress, and the environment. Examining their role in the broader context of human evolution, we discuss dental biogeochemistry and emphasize how the incremental growth of tooth microstructure facilitates the reconstruction of temporal data related to health, diet, mobility, and stress in past societies. The review concludes by considering tooth microstructure as a biomarker and the potential clinical applications.

Use of zinc deposited in deciduous teeth as a retrospective measurement of dietary zinc exposure during early development

Purpose

We proposed that zinc (Zn) deposition in deciduous teeth would be a timed record of exposure to this essential micronutrient over very early life. We tested this hypothesis by gathering information on the maternal and child's diet during pregnancy and early infancy and measuring mineral deposition in the dentine at points during deciduous tooth development.

Methods

We developed a short food frequency questionnaire (S-FFQ) to record consumption of food containing Zn during pregnancy and over the first year of life of the child in an Indonesian population. Zn, Sr and Ca were measured by laser ablation ICP-MS in a series of points across the developmental timeline in deciduous teeth extracted from 18 children undergoing the process as part of dental treatment whose mothers completed the SFFQ. Mothers and children were classified into either high Zn or low Zn groups according to calculated daily Zn intake.

Results

The Zn/Sr ratio in dentine deposited over late pregnancy and 0-3 months post-partum was higher (p < 0.001, 2-way ANOVA; p < 0.05 by Holm-Sidak post hoc test) in the teeth of children of mothers classified as high Zn consumers (n = 10) than in children of mothers classified as low Zn consumers (n = 8).

Conclusion

The S-FFQ was validated internally as adequately accurate to measure zinc intake retrospectively during pregnancy and post-partum (∼7 years prior) by virtue of the correlation with measurements of zinc in deciduous teeth. The ratio of Zn/Sr in deciduous teeth appears to be a biomarker of exposure to zinc nutrition during early development and offers promise for use as a record of prior exposure along a timeline for research studies and, potentially, to identify individuals at heightened risk of detrimental impacts of poor early life zinc nutrition on health in later life and to implement preventative interventions.

Brief Communication: Elemental Models of Primate Nursing and Weaning Revisited

Objectives

Intra-tooth patterns of trace elements barium (Ba) and strontium (Sr) have been used to infer human and nonhuman primate nursing histories, including australopithecine and Neanderthal juveniles. Here we contrast the two elemental models in first molars (M1s) of four wild baboons and explore the assumptions that underlie each.

Materials and Methods

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was employed to create comprehensive calcium-normalized barium and strontium (Ba/Ca, Sr/Ca) maps of M1 enamel and dentine at 35 micron resolution.

Results

Postnatal Ba/Ca values were typically high, peaking ~0.5 years of age and then decreasing throughout M1 crown formation; all four individuals showed minimal Ba/Ca values between ~1.2-1.8 years, consistent with field reports of the cessation of suckling. Enamel Sr/Ca did not support patterns of previous LA-ICP-MS spot sampling as the enamel rarely showed discrete Sr/Ca secretory zonation. Increases in Sr/Ca appeared in coronal dentine beginning ~0.3 years, with varied peak value ages (~0.7-2.7 years) and no evidence of a predicted postweaning decline.

Discussion

Inferences of baboon weaning ages from initial Ba/Ca minima are more congruent with behavioral observations than Sr/Ca maxima; this is consistent with studies of captive macaques of known weaning ages. Elemental variation is more apparent in the coronal dentine than the enamel of these baboons, which may relate to its more rapid mineralization and protection from the oral environment. Inferences of nursing histories from enamel Sr/Ca patterns alone should be reconsidered, and elevated values of Ba/Ca and Sr/Ca in teeth formed after weaning require further study.

The origin of placental mammal life histories

After the end-Cretaceous extinction, placental mammals quickly diversified¹, occupied key ecological niches^2,3 and increased in size^4,5, but this last was not true of other therians⁶. The uniquely extended gestation of placental young⁷ may have factored into their success and size increase⁸, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping^9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30-75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.