A structural biorhythm related to human sexual dimorphism.

Life on earth is regulated by biological rhythms, some of which oscillate with a circadian, monthly or lunar cycle. Recent research suggests that there is a near weekly biorhythm that may exert an influence on human skeletal growth. Evidence for the timing of this biorhythm is retained in tooth enamel as the periodicity of Retzius lines. Studies report that Retzius periodicity (RP) relates to adult human stature and enamel thickness. Adult human stature is sexually dimorphic, and so is enamel thickness of maxillary third molars (M3) but not mandibular M3. Yet, previous studies report sex differences in RP are apparent in some populations but not others, and it is unknown if dimorphism in enamel thickness relates to RP. To further our understanding of this biorhythm we analysed sex-related variation in RP and its relationship with enamel thickness in a sample of M3's (n = 94) from adults in Northern Britain. Results reveal RP was significantly higher in our sample of female molars compared to those of males, which is consistent with the previously reported correlation between the biorhythm and adult stature. The RP of maxillary M3 related to sex differences in enamel thickness, but this relationship was not present in mandibular M3. Our results support previous findings suggesting that this biorhythm is sexually dimorphic and provide the first evidence that RP may be one factor influencing sex differences in enamel thickness. Our study also shows that correlations between RP and enamel thickness appear to be most readily detected for tooth types with sufficiently wide ranges of enamel thickness variation, as is the case for maxillary but not mandibular M3. Achieving a sufficient sample size was critical for detecting a sex difference in periodicity.

Enamel growth rates of anterior teeth in males and females from modern and ancient British populations.

OBJECTIVE: This study explored biological sex differences in the regional daily growth rates of human anterior enamel from modern and ancient populations in Britain. METHODS: Maxillary permanent incisors (n = 80) and canines (n = 69) from Roman, Anglo-Saxon, Medieval, and Modern day populations were analyzed using histological methods. Daily secretion rates (DSRs) were collected for inner, mid, and outer regions of cuspal and lateral enamel. Modern day samples were of known sex, archeological individuals had sex determined using standard osteological methods. Variation in DSRs between the sexes, both between and within populations, was sought using parametric and nonparametric tests. RESULTS: When all samples were pooled, there was no significant difference between males and females. Similarly no significant differences in DSRs were identified between male and females within each population. When DSRs were compared between the populations, DSRs decreased from the more ancient to the more recent populations for males, and for females. More interpopulation differences were observed in males. DISCUSSION: This study presents evidence for the relative consistency of enamel DSRs between male and female groups within each British population. Interpopulation analyses found DSRs slowed significantly between Roman and modern day populations for both sexes, with male DSRs showing the greatest variation between populations.

Enamel thickness and growth rates in modern human permanent first molars over a 2000 year period in Britain.

OBJECTIVES: This study explores variation and trends in first molar enamel thickness and daily enamel secretion rates over a 2000 year period in Britain. METHODS: Permanent first molars (n = 89) from the Roman, Anglo-Saxon, and Medieval periods, as well as modern-day Britain, were analyzed using standard histological methods. Relative enamel thickness (RET) and linear measurements of cuspal and lateral thickness were calculated for mesial cusps. Daily secretion rates (DSRs) were calculated for inner, mid, and outer enamel regions in both cuspal and lateral enamel. Significant differences and trends were identified between samples using nonparametric statistical tests. RESULTS: Enamel thickness differed between some populations, but no temporal trends were identified. Early Anglo-Saxon molars had significantly thinner RET than both Late Anglo-Saxon (p < .00) and Medieval (p < .00) molars. Lateral enamel from the Roman molars was significantly thinner than the modern-day sample (p = .04). In contrast, a significant slowing trend in DSRs was observed across the more ancient to modern-day samples in every measured region except the mid-lateral enamel region. DISCUSSION: This study presents the first evidence for a gradual slowing in the daily rate that enamel is secreted in molars over the past 2000 years in Britain. However, this trend was not matched by consistent or significant positive or negative shifts in enamel thickness. These findings suggest that modern human molars of similar enamel thickness, from different modern and ancient populations, formed at different rates.

Bone histomorphometric measures of physical activity in children from medieval England.

OBJECTIVES: Histomorphometric studies show consistent links between physical activity patterns and the microstructure underlying the size and shape of bone. Here, we adopt a combined bone approach to explore variation in microstructure of ribs and humeri related to physical activity and historical records of manual labor in skeletal samples of children (n = 175) from medieval England. The humerus reflects greater biomechanically induced microstructural variation than the rib which is used here as a control. Variation in microstructure is sought between regions in England (Canterbury, York, Newcastle), and between high- and low-status children from Canterbury. MATERIALS AND METHODS: Thin-sections were prepared from the humerus or rib and features of bone remodeling were recorded using high-resolution microscopy and image analysis software. RESULTS: The density and size of secondary osteons in the humerus differed significantly in children from Canterbury when compared to those from York and Newcastle. Among the older children, secondary osteon circularity and diameter differed significantly between higher and lower status children. DISCUSSION: By applying bone remodeling principles to the histomorphometric data, we infer that medieval children in Canterbury engaged in less physically demanding activities than children from York or Newcastle. Within Canterbury, high-status and low-status children experienced similar biomechanical loading until around 7 years of age. After this age low-status children performed activities that resulted in more habitual loading on their arm bones than the high-status children. This inferred change in physical activity is consistent with historical textual evidence that describes children entering the work force at this age.

The biorhythm of human skeletal growth.

Evidence of a periodic biorhythm is retained in tooth enamel in the form of Retzius lines. The periodicity of Retzius lines (RP) correlates with body mass and the scheduling of life history events when compared between some mammalian species. The correlation has led to the development of the inter-specific Havers-Halberg oscillation (HHO) hypothesis, which holds great potential for studying aspects of a fossil species biology from teeth. Yet, our understanding of if, or how, the HHO relates to human skeletal growth is limited. The goal here is to explore associations between the biorhythm and two hard tissues that form at different times during human ontogeny, within the context of the HHO. First, we investigate the relationship of RP to permanent molar enamel thickness and the underlying daily rate that ameloblasts secrete enamel during childhood. Following this, we develop preliminary research conducted on small samples of adult human bone by testing associations between RP, adult femoral length (as a proxy for attained adult stature) and cortical osteocyte lacunae density (as a proxy for the rate of osteocyte proliferation). Results reveal RP is positively correlated with enamel thickness, negatively correlated with femoral length, but weakly associated with the rate of enamel secretion and osteocyte proliferation. These new data imply that a slower biorhythm predicts thicker enamel for children but shorter stature for adults. Our results develop the intra-specific HHO hypothesis suggesting that there is a common underlying systemic biorhythm that has a role in the final products of human enamel and bone growth.

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.

Biorhythms, deciduous enamel thickness, and primary bone growth: a test of the Havers-Halberg Oscillation hypothesis.

Across mammalian species, the periodicity with which enamel layers form (Retzius periodicity) in permanent teeth corresponds with average body mass and the pace of life history. According to the Havers-Halberg Oscillation hypothesis (HHO), Retzius periodicity (RP) is a manifestation of a biorhythm that is also expressed in lamellar bone. Potentially, these links provide a basis for investigating aspects of a species' biology from fossilized teeth. Here, we tested intra-specific predictions of this hypothesis on skeletal samples of human juveniles. We measured daily enamel growth increments to calculate RP in deciduous molars (n = 25). Correlations were sought between RP, molar average and relative enamel thickness (AET, RET), and the average amount of primary bone growth (n = 7) in humeri of age-matched juveniles. Results show a previously undescribed relationship between RP and enamel thickness. Reduced major axis regression reveals RP is significantly and positively correlated with AET and RET, and scales isometrically. The direction of the correlation was opposite to HHO predictions as currently understood for human adults. Juveniles with higher RPs and thicker enamel had increased primary bone formation, which suggests a coordinating biorhythm. However, the direction of the correspondence was, again, opposite to predictions. Next, we compared RP from deciduous molars with new data for permanent molars, and with previously published values. The lowermost RP of 4 and 5 days in deciduous enamel extends below the lowermost RP of 6 days in permanent enamel. A lowered range of RP values in deciduous enamel implies that the underlying biorhythm might change with age. Our results develop the intra-specific HHO hypothesis.