Histological analysis of incremental markings and crown growth characteristics in mandibular first molars of the red fox, Vulpes vulpes (Canidae, Mammalia)

The present study analyzed incremental markings in the enamel of eight mandibular first molars (M1) of red foxes (Vulpes vulpes) and a single M1 of a grey wolf (Canis lupus). The most prominent enamel incremental markings in both species were laminations. Based on lamination counts, a mean crown formation time of 102 days (range 99–107) was reconstructed for the red fox M1. This finding fits well with the reported M1 emergence through the gum in this species at between days 105 and 119 after birth, indicating a daily periodicity of the laminations. Based on the nature of laminations as daily growth marks, we reconstructed different crown growth parameters in the red fox molars. Overall, the enamel daily secretion rate (DSR) increased from lowest values near the enamel–dentine junction (EDJ) to highest values near the outer enamel surface. The lowest mean value (6.8 μm/day) was recorded in the inner third of the lingual enamel from the cervical crown region, the highest mean (17.4 μm/day) in the outer third of the buccal enamel from the mid-lateral crown region. While in the inner enamel third, DSR values were similar in buccal and lingual enamel, in the central and outer thirds, mean and median DSR values for buccal always exceeded those for lingual enamel. On the buccal crown side, linear enamel thickness and reconstructed ameloblast secretory lifespan were similar in the three lateral crown regions distinguished (upper, mid, lower) and only dropped in the cervical crown region. By contrast, lingually only the upper lateral crown region showed values similar to those on the buccal side, while the values in more cervical crown regions were always markedly lower than in the corresponding buccal crown regions. Enamel extension rate (EER) was highest in the upper third of the EDJ length, with mean values of 145 μm/day in buccal and 170 μm/day in lingual enamel. EER dropped to lowest values in the cervical third of the EDJ length, averaging 85 μm/day buccally and 97 μm/day lingually. The enamel formation front angle (EFFa) varied between 3.9° and 5.2°, with only slightly higher values in the lower compared with the upper crown portion. EER and the EFFa data indicate a very rapid crown elongation, which is reflected by the very steep inclination of the laminations throughout the enamel. Four to six subdaily growth increments were present between consecutive laminations, whereas long-period enamel incremental markings were not discernible. Our findings indicate that previous studies on canid enamel misidentified the incremental markings by erroneously diagnosing daily laminations as long-period markings (striae of Retzius) and subdaily prism cross-striations as daily markings. In consequence, these studies reported too low DSRs for canid enamel and too long crown formation times for the analyzed teeth.