It has been reported that the fissures of rat molar teeth are apt to cause dental caries. In this study, we observed enamel lamellae in the fissures of caries-free molar teeth of rats, 2 and 4 weeks after birth, by transmitted light microscopy with decalcified sections and by scanning electron microscopy with EDTA-treated samples. Enamel lamellae were always present buccolingually in the fissure enamel. In unerupted teeth, the organic lamellae showing an intermittent structure were composed of the organic matrix, derived from ameloblasts and reduced enamel epithelium, occasionally with these epithelial cells. After eruption, the thin organic membrane of enamel lamellae was probably derived from saliva as well as the organic matrix formed during the enamel formation. The lamellae also contained oral microorganisms and fibrous structures, some of which were formed from microorganisms, under the widely opened fissures containing the deposits of oral microorganisms and diet remnants. Thus, we strongly suggest that such fissure lamellae of rat molar teeth are apt to become the passing point in the way to inducing the dentin caries.
It has been reported that the fissures of rat molar teeth are apt to cause dental caries. In this study, we observed the fissure enamel in caries-free molar teeth of rats, 4 weeks after birth, by scanning electron microscopy with ground and fractured samples. On the fissure floors of first molar teeth, dome-shaped prism ends, which were slightly and clearly elevated, were observed, although the fissure floors of second molar teeth were scattered with incompletely prism-end pits in some dome-shaped prism ends. When the fissure enamel was compared with the other enamel regions, the hypocalcified areae, previously reported, showed a low microhardness and a low resistance to grinding. The enamel prisms, occasionally containing short crystals, had thin crystals in diameter with a smooth surface, and the prism boundaries showed a loose connection. Therefore, it is revealed that the fissure enamel shows a lower crystal density with a porous structure within and among prisms than the other enamel regions. Thus, we strongly suggest that the fissure enamel structure of rat molar teeth is apt to cause enamel caries and to become the passing point in the way to inducing the dentin caries as well as fissure lamellae.