Mineral Apposition Rates in Coronal Dentine of Mandibular First Molars in Soay Sheep: Results of a Fluorochrome Labeling Study

A labeling study of dentin formation rates during crown and root growth of porcine mandibular first molars

We used fluorochrome labeling to study spatiotemporal variation of dentin apposition (DAR) and extension (DER) rates during crown and root formation of mandibular first molars from wild boar and domestic pigs. DAR was reconstructed along the course of dentinal tubules in four zones of the crown and in the upper root area. In all five zones, mean DAR increased during the first 30% to 40% of apposition, reaching highest values (22-23 μm/day) in the upper-lateral crown zone. Lowest values were recorded near the dentin-pulp interface (DPI). Typically, DARs in contemporaneously formed dentin areas were higher in more cuspally compared to more cervically/apically located zones. DER was high (>200 μm/day) in early postnatal crown dentin and then decreased markedly in cervical direction, with lowest values in the cervical crown zone. After this nadir, DER sharply increased in the upper 30% to 40% of the root extension, reaching values equaling (wild boar) or even surpassing (domestic pigs) those recorded in the upper lateral crown. After this peak, DER again decreased. While DAR did not differ markedly between wild boar and domestic pigs, the DER showed marked differences, both regarding maximum values (208.1 μm/day in wild boar, 272.2 μm/day in domestic pigs) and the timing of the root growth spurt, which occurred earlier in the domestic pigs. We consider the more rapid recruitment of secretory odontoblasts in domestic pigs (reflected by higher DER) a side effect of selection for rapid body growth during pig domestication.

Formation and Replacement of Bone and Tooth Mineralized Tissues in Green Iguanas (Iguana iguana) Revealed by In-Vivo Fluorescence Marking

Hard tissue formation patterns and rates reveal details of animal physiology, life history, and environment, but are understudied in reptiles. Here, we use fluorescence labels delivered in vivo and laser confocal scanning microscopy to study tooth and bone formation in a managed group of green iguanas (Iguana iguana, Linné 1758) kept for 1.5 years under experimentally controlled conditions and undergoing several dietary switches. We constrain rates of tooth elongation, which we observe to be slow when enamel is initially deposited (c. 9 µm/day), but then increases exponentially in the dentin root, reaching c. 55 µm/day or more after crown completion. We further constrain the total timing of tooth formation to ∼40-60 days, and observe highly variable timings of tooth resorption onset and replacement. Fluorescent labels clearly indicate cohorts of teeth recruited within Zahnreihen replacement waves, with faster sequential tooth recruitment and greater wave sizes posteriorly, where each wave initiates. Fluorescence further reveals enamel maturation after initial deposition. Rates of hard tissue formation in long bones range from 0.4 to 3.4 µm/day, correlating with animal weight gain and cortical bone recording the entire history of the experiment. We suggest additional labeling experiments to study hard tissue formation patterns in other reptiles, and propose strategies for chemical analyses of hard tissues in order to extract temporal information about past environments, behaviors, and diets from reptilian fossils throughout the Phanerozoic.

Labelling experiments in red deer provide a general model for early bone growth dynamics in ruminants

Growth rates importantly determine developmental time and are, therefore, a key variable of a species' life history. A widely used method to reconstruct growth rates and to estimate age at death in extant and particularly in fossil vertebrates is the analysis of bone tissue apposition rates. Lines of arrested growth (LAGs) are of special interest here, as they indicate a halt in bone growth. However, although of great importance, the time intervals between, and particularly the reason of growth arrests remains unknown. Therefore, experiments are increasingly called for to calibrate growth rates with tissue types and life history events, and to provide reliable measurements of the time involved in the formation of LAGs. Based on in vivo bone labelling, we calibrated periods of bone tissue apposition, growth arrest, drift and resorption over the period from birth to post-weaning in a large mammal, the red deer. We found that bone growth rates tightly matched the daily weight gain curve, i.e. decreased with age, with two discrete periods of growth rate disruption that coincided with the life history events birth and weaning, that were visually recognisable in bone tissue as either partial LAGs or annuli. Our study identified for the first time in a large mammal a general pattern for juvenile bone growth rates, including periods of growth arrest. The tight correlation between daily weight gain and bone tissue apposition suggests that the red deer bone growth model is valid for ruminants in general where the daily weight gain curve is comparable.

The pay-off of hypsodonty - timing and dynamics of crown growth and wear in molars of Soay sheep

Background

Several lineages of herbivorous mammals have evolved hypsodont cheek teeth to increase the functional lifespan of their dentition. While the selective drivers of this trend and the developmental processes involved have been studied in greater detail, thus far no quantitative information is available on the relationship between additional investment into tooth growth and the resulting extension of the functional period of these teeth. To achieve this, we performed a detailed analysis of molar crown growth in known-age Soay sheep repeatedly injected with different fluorochromes.

Results

Our study revealed that in sheep molars especially the formation of the crown base portion is prolonged in comparison with other herbivorous artiodactyl species. Our results demonstrate that growth of the crown base accounted for more than half of the total crown formation time (CFT) of the anterior lobes of the first (approx. 220 days of total CFT of 300 days), second (approx. 260 of 460 days) and third (approx. 300 of at least 520 days) molars, and that the formation of this crown portion occurred largely after the teeth had already reached functional occlusion. By combining data on wear-related changes in crown morphology from the literature with the reconstructed additional investment into the crown base portion, it was possible to relate this additional investment to a prolongation of the functional periods of the molars ranging from 4 years in the M₁ to 6 years in the M₃.

Conclusions

Our results allow to establish a quantitative link between an additional investment into molar crown growth of sheep and the extension of the functional period of these teeth. The reported findings enable an assessment of the adaptive value, in terms of increased longevity, of an additional investment into crown elongation in a mammalian herbivore.