Lines of Evidence–Incremental Markings in Molar Enamel of Soay Sheep as Revealed by a Fluorochrome Labeling and Backscattered Electron Imaging Study

Abstract

We studied the structural characteristics and periodicities of regular incremental markings in sheep enamel using fluorochrome injections for vital labeling of forming enamel and backscattered electron imaging in the scanning electron microscope. Microscopic analysis of mandibular first molars revealed the presence of incremental markings with a daily periodicity (laminations) that indicated successive positions of the forming front of interprismatic enamel. In addition to the laminations, incremental markings with a sub-daily periodicity were discernible both in interprismatic enamel and in enamel prisms. Five sub-daily increments were present between two consecutive laminations. Backscattered electron imaging revealed that each sub-daily growth increment consisted of a broader and more highly mineralized band and a narrower and less mineralized band (line). The sub-daily markings in the prisms of sheep enamel morphologically resembled the (daily) prisms cross striations seen in primate enamel. Incremental markings with a supra-daily periodicity were not observed in sheep enamel. Based on the periodicity of the incremental markings, maximum mean daily apposition rates of 17.0 µm in buccal enamel and of 13.4 µm in lingual enamel were recorded. Enamel extension rates were also high, with maximum means of 180 µm/day and 217 µm/day in upper crown areas of buccal and lingual enamel, respectively. Values in more cervical crown portions were markedly lower. Our results are in accordance with previous findings in other ungulate species. Using the incremental markings present in primate enamel as a reference could result in a misinterpretation of the incremental markings in ungulate enamel. Thus, the sub-daily growth increments in the prisms of ungulate enamel might be mistaken as prism cross striations with a daily periodicity, and the laminations misidentified as striae of Retzius with a supra-daily periodicity. This would lead to a considerable overestimation of crown formation times in ungulate teeth.

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

“…The high rate of dentine formation recorded in the cuspal crown portions of the sheep molars enables the formation of a hypsodont tooth within a relatively short period of time (< 1 year). Correspondingly, high enamel extension rates have previously been reported for the cuspal crown portions of first molars in sheep and goats (Kierdorf et al, ).…”

“…The sheep of the first experimental group (N = 9) were born between 25 March and 12 May 2009. For six of these animals, data on the periodicity of enamel incremental markings and on enamel growth parameters have previously been published (Kierdorf et al, ). The sheep of the second experimental group (N = 2) were born in mid‐April 2012 (exact dates of birth not known, for all calculations we assumed 15 April as the day of birth).…”

“…The heads of the slaughtered animals were removed and further processed as previously described (Kierdorf et al, ). The mandibular first molars were extracted, photographed and embedded in epoxy resin (Biodur E12, Biodur products, Heidelberg, Germany).…”

“…Subsequently, the teeth were sectioned axiobuccolingually through the tips of the anterior (mesial) and posterior (distal) cusps (Fig. ) using a rotary saw with a water‐cooled diamond‐coated blade (Woco 50, Conrad Apparatebau, Clausthal‐Zellerfeld, Germany), and ground sections of approximately 50 µm thickness were prepared as previously described (Kierdorf et al, ). Measurements were performed on the un‐demineralized ground sections.…”

Characterization of Enamel Incremental Markings and Crown Growth Parameters in Minipig Molars

“…Laminations seem to be the most prominent growth marks in ungulate enamel. In the light microscope, they appear as incremental lines that follow a course corresponding to that of striae of Retzius in primate enamel (Okada and Mimura, ; Iinuma et al, ; Tafforeau et al, ; Jordana and Köhler, ; Kierdorf et al, ).…”