Continuously growing incisors are common to all rodents, which include the Microtus genus of voles. However, unlike many rodents, voles also possess continuously growing molars. Here, we report spontaneous molar defects in a population of Prairie voles (Microtus ochrogaster). We identified bilateral protuberances on the ventral surface of the mandible in several voles in our colony. In some cases, the protuberances broke through the cortical bone. The mandibular molars became exposed and infected, and the maxillary molars entered the cranial vault. Visualisation upon soft tissue removal and microcomputed tomography (microCT) analyses confirmed that the protuberances were caused by the overgrowth of the apical ends of the molar teeth. We speculate that the unrestricted growth of the molars was due to the misregulation of the molar dental stem cell niche. Further study of this molar phenotype may yield additional insight into stem cell regulation and the evolution and development of continuously growing teeth.

] Department of Orofacial Sciences and Program in Craniofacial and Mesenchymal Biology University of California San Francisco San Francisco USA [2] Department of Anthropology and Human Genetics Faculty of Science Charles University Prague Prague Czech Republic

] Department of Psychiatry UCSF San Francisco USA [2] Department of Anatomy UCSF San Francisco USA

Department of Anatomy UCSF San Francisco USA

Department of Orofacial Sciences and Program in Craniofacial and Mesenchymal Biology University of California San Francisco San Francisco USA

Laboratory Animal Resource Center UCSF San Francisco USA

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