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The mouse has a total of 16 nondeciduous teeth, one incisor and three molars (cheek teeth) on each side of the jaws. Rodent incisors grow and are worn down continuously; their apical foramina remain open. The rodent incisor is a rootless, hypsodont tooth that contains the incisor enamel organ comprising inner and outer enamel epithelium and enamel pulp (stellate reticulum). The inner enamel epithelial cells differentiate into the tall columnar ameloblasts, which deposit pre-enamel that later calcifies to enamel and forms the outer surface of the incisors. Iron pigments within the enamel layers color the incisors yellow. The inner surface of the incisors is covered with cementum produced by the cells of the periodontal membrane (periodontal ligament), dense connective tissue that connects the tooth to the tooth socket. The inside of the rodent incisor consists of a dental cavity filled with dental pulp (reticular connective tissue, blood vessels, and nerves) and, on the inside surface, odontoblasts. The latter are tall columnar cells that make pre-dentin and calcify this to dentin. All teeth are surrounded by gingiva, a fibrous tissue continuous with the periodontal membrane.
Decalcification of the depicted tissue samples removed the enamel, so enamel is absent in the micrographs. The 4X and 10X micrographs show, in increasing detail, the dental pulp, the adjoining layers of odontoblasts, predentin, dentin, and ameloblasts and the surrounding bone. The incisor enamel organ, consisting of inner and outer enamel epithelium and enamel pulp, is visible in the 20X micrograph. The 40X micrograph displays in detail the outer enamel epithelium, the inner enamel epithelium (with mitosis), and the differentiated ameloblast epithelium.
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